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video astronomy
Making an astro video is as easy as holding a camcorder up to your telescope's eyepiece


As long as people have been gazing at the sky, they've been making records of what they saw. Every medium has been used, from smooth faces of rocks to sketchbooks. Cameras and film revolutionized astronomical record-keeping beginning in the late 1800s, and photography remained king of all media for a century.
You may find this hard to believe, but for recording the Sun, Moon, and planets, these venerable tools all pale in comparison to the video cameras found in millions of today's households. No static drawing or photograph can rival the ineffable sense of reality and the emotional impact of a dynamic video recording. Richard Berry, one of the pioneers of electronic imaging for amateur astronomers, summed it up: "Seeing a video recording isn't experiencing the real thing, but it's the next best thing. Although it is impossible to reproduce the effect of a live video in the pages of a magazine, it is utterly wonderful to see the Moon, the planets, or the Sun on your television looking as good as they ever appear in a high-power eyepiece."
Each minute of videotape contains 1,800 discrete images, a number that dwarfs what even the most nimble photographer can achieve by tripping the shutter and winding the film-advance knob of a conventional camera! And just like with today's ever-capable digital cameras, video avoids the need to wait for film to be developed and printed to see your results.
video of the Moon
The Moon makes a great first target: it's big, bright, and easily recorded with any video camera.
Craig Michael Utter

The image sensor in a modern camcorder is a tiny wafer of silicon called a charge-coupled device (or CCD), an electronic chip far more light sensitive than the crystals of silver salts in photographic film. Unlike the mechanical shutter in a conventional camera, however, the electronics in a camcorder usually limit exposures to 1/60 second or less. Consequently, only the brightest astronomical objects make suitable targets: the Moon, the planets, and the Sun (with proper filtration, of course!). On the other hand, a video camera's short exposures can overcome many of the distorting effects of the Earth's turbulent atmosphere. Video recordings can effortlessly capture details that appear during fleeting moments of good seeing.
The videographer has an insuperable advantage over the still photographer using film to capture fine details on the planets. Recording the colorful cloud belts of Jupiter requires an exposure of 2 or 3 seconds on a relatively slow, fine-grained film, while more distant and dimmer Saturn requires exposures of 4 to 8 seconds. But Earth's turbulent atmosphere limits the duration of perfect images to fleeting moments — typically only fractions of a second. That's why professional astronomers are investing small fortunes to install adaptive optics systems on their telescopes that actually compensate for atmospheric seeing. With its ability to record 30 discrete images every second, video can be considered as "the poor man's adaptive optics."

Just getting started in astrophotography? Learn the basics when you download Sky & Telescope's FREE Astrophotography Primer, perfect for any beginner who wants to shoot the night sky.

Stills of the Night Sky

"The moving finger writes and, having writ, moves on," lamented Persian poet Omar Khayyám. Both the visual observer and the still photographer must maintain an uninterrupted vigil so as not to irretrievably miss the all-too-rare moments when the air steadies and delicate details flash out in what legendary observer Percival Lowell called "revelation peeps." But a video recording captures an entire observing session, preserving those fleeting lucid moments like insects caught in amber.
Unlike 35-mm single-lens reflex (SLR) cameras, only a handful of the most expensive camcorders are equipped with removable lenses that would allow easy attachment to a telescope (using special adapters). Camcorders with permanent lenses require you to use what's called the afocal system. Here's how it works. The rays of light from a very distant object emerge from a telescope's eyepiece in parallel bundles. You bring this parallel light to focus on the camcorder's CCD by holding the camera's lens up to the telescope eyepiece. It's important to first have the telescope accurately focused, and this can be done by eye as long as you either have 20/20 vision or wear glasses that correct any near-or farsightedness to 20/20.
Sound complicated? Don't worry: most people find that focusing a telescope for afocal work is easy, especially since the real-time video image is visible in the camera’s viewfinder.
video of lunar craters
This beautifully detailed close-up of the Moon was captured using a Sony camcorder and a 13.1-i nch Dobsonian reflector. Fine detail of the channels near Hippalus crater emerged after selecting the best video frames and running them through Registax software to create composite images.
Stephen Keene

First Targets for Astro Video

The Moon is the biggest and brightest object in the night sky. Odds are that it was the first object you looked at through your telescope, and it's the best subject for learning the ropes of video astronomy. After you've honed your skills videotaping the Moon for a night or two, you'll be ready to try your luck with the planets. The waxing and waning phases of Venus, the seasonal advance and retreat of the polar caps of Mars, the ever-changing features of Jupiter's cloud belts and four bright Galilean moons, and Saturn's magnificent ring system are all within the grasp of your backyard telescope and a camcorder.
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Combined with typical amateur telescopes, even the most affordable camcorders are capable of capturing dramatic lunar sequences. Examples include the spires of shadows cast by mountains towering above plains of frozen lava and the rims of craters flashing into view as they catch the first rays of the morning Sun.
Set up your telescope on an evening when the Moon is between the crescent and gibbous phases, so that deep shadows throw lunar topography into bold relief near the Moon's day-night terminator. Select a low-power eyepiece (the one with the largest number printed on it), point your telescope at the Moon, and bring the image into focus.

Now manually set the focus of your camcorder's lens to the infinity position. If your camcorder has an autofocus feature, turn it off; otherwise it may continue to hunt for focus while you stand there exasperated. Carefully bring your camcorder up to the eyepiece of the telescope, taking pains to center its lens directly over the eyepiece while keeping it parallel to the emerging beam of light. To avoid jarring the telescope, don't let the camcorder touch the eyepiece. A small gap between the camcorder's lens and the eyepiece will not affect the image.
If the image in the camcorder's viewfinder appears slightly out of focus, tweak the telescope's focusing knob until the lunar landscape snaps into crisp view. Depending on your telescope's eyepiece and the camcorder's lens, the viewfinder may show a diffuse, dark circle surrounding the brightly illuminated lunar image. Since you'll want the image to fill the frame, adjust the zoom setting of the camcorder's lens until this effect — called vignetting — disappears. The zoom lens also provides a very convenient way to increase magnification without changing eyepieces.
If your camcorder is equipped with a digital zoom, don't use it. This dubious "feature" actually creates a lower-resolution picture because the camera uses fewer picture elements (pixels) of the CCD to fill the field. Used in conjunction with an eyepiece of moderately long focal length (15 to 25 mm), the optical zoom of most camcorders will provide plenty of magnification for afocal imaging.
Varying exposures
Shortening exposures using a video camera's shutter speed will darken the view but also boost the image sharpness. This trio of pictures reveals the differences with shutter speeds (left to right) of 1/125, 1/250, and 1/500 second.
Steve Massey

The more you enlarge the image, the dimmer it will become, so experiment with the camcorder's zoom and exposure controls until you achieve an acceptable compromise between magnification and image brightness. Technically speaking, dim images have a poor (or low) signal-to-noise ratio that gives them an objectionably grainy appearance — a look appropriately called "snow." The definition and sharpness of your video recordings will not be determined solely by the optical quality of your telescope and the steadiness of the atmosphere. Older VHS or 8-mm camcorders record only 240 to 250 lines of horizontal video resolution. The S-VHS and Hi8 formats represent a marked improvement of 400 to 420 lines of resolution, while today’s popular digital video (DV) approaches 500 lines. High-definition (HD) cameras have 1,080 lines.
This digital eyepiece from Barska slips directly into the scope's focusing tube and captures 640-by-480-pixel color video.
Kelly Beatty

Another way to get a view of the heavens onto video is to use special electronic eyepieces in which the lens you would normally look through has been replaced with a silcon chip. Meade Instrument’s Electronic Eyepiece ($70) features 320-by-240-pixel, black-and-white video that is fed through a cable directly to the video input of a TV or video recorder. The camera is powered by an internal 9-volt battery and is available from Meade retailers.
Barska makes a similar eyepiece that captures a 640-by-480-pixel color image and sends the signal through a cable that plugs into your computer via a USB connection, which also powers the eyepiece. The Digi-Eyepiece ($65) comes with Windows software to display, edit, and save the movies.
Used in conjunction with a TV, these products are great for showing groups of people the Moon and planets. But with limited ways to change exposure settings manually, they aren’t as capable as other video recorders.
Video camera on tripod
Holding your camcorder by hand can become fatiguing. Mount it on a tripod to stabilize the view.
Dennis di Cicco

 Astro Video Support

After your first videotaping session, fatigued muscles will probably make you wish for another way to keep a camcorder at the eyepiece. Try mounting the camcorder on an ordinary camera tripod. With small equatorially mounted refractors and compound telescopes like Schmidt-Cassegrains and Maksutov-Cassegrains, the movement of the eyepiece as the telescope tracks the sky's motion will be modest relative to a stationary tripod-mounted camcorder.
Supporting the camcorder on a separate tripod is more difficult with larger refractors and Newtonian reflectors because their eyepieces move more as these telescopes pivot. You'll be forced to reposition the camcorder and its tripod frequently, so it's better to attach the camcorder to the telescope and let it go along for the ride. Brackets for this purpose can be purchased from astronomy retailers or fashioned from wood, plastic, or aluminum.
Once you devise a way to attach your camcorder to your telescope, one additional challenge will remain. Despite remarkable advances in miniaturization, a camcorder's videotape mechanism, viewfinder, and battery can weigh several pounds, so you'll need to counterbalance that extra weight carefully. Many amateurs have found a handy solution at their local sporting-goods store in the form of jogger's ankle weights, which can be attached to a telescope using strips of Velcro.
Supporting your camcorder on a tripod or coupling it directly to your telescope will also let you use the remote controls that come with many camcorders. These accessories are a real boon because they let you adjust the camcorder's zoom lens and exposure settings without touching the camera and jiggling the highly magnified image.

Making a Hard Copy of Your Videotapes

To make prints of scenes from your videotapes, you might imagine that simply employing the camcorder's freeze-frame mode while playing back the video on your TV will show a nice image, and perhaps you can take a photograph of your TV screen. Unfortunately, if you're using an analog camera (VHS or Hi8 formats) this will only teach you some harsh lessons about the effects of electronic noise and the physiology of human vision.

When a videotape is played back, 30 discrete images (usually called frames) are displayed every second. Your eye and brain integrate them as a seamless view. At a rate of 30 frames per second, phenomenon called flicker fusion causes the sequence to appear continuous. Your eye-brain combination fills in between the frames and also averages the noise (snow) of individual frames, creating the perception of a vivid moving picture. But in the freeze-frame mode of analog video, the display will invariably suffer from a poor signal-to-noise ratio and have a grainy salt-and-pepper look reminiscent of an overenlarged photograph. To add insult to injury, successive frames will almost invariably exhibit a phenomenon called image excursion — small, erratic displacements caused by atmospheric turbulence. When examined frame by frame, a videotape of a planet that appears stationary and well-defined in the "play" mode will reveal contortions reminiscent of an amoeba under a microscope.

Using a DV-format camcorder avoids some of these problems, because digitally captured video images don't suffer from the "blurry" effects of analog recording. Nevertheless, you’re still at the mercy of our unstable atmosphere. To smooth out the noise and incrementally displaced images, it's necessary to combine dozens or even hundreds of the sharpest frames,a process called "stacking." This was a very laborious, time-consuming task before the advent of software capable of automatically detecting the best frames and precisely superimposing them to create a composite image.
Effects of seeing
These consecutive video frames, recorded at a rate of 30 per second, dramatically illustrate how at mospheric turbulence ("seeing") can vary image quality instant by instant. When making compsite frames, keep the few crisp frames and discard all the blurry ones.
Steve Massey

A very popular program for this is RegiStax, the brainchild of Dutch amateur astronomer Cor Berrevoets. This free software for Windows is accompanied by an excellent tutorial. Macintosh users can perform similar processing with Keith's Image Stacker, shareware by Keith Wiley. And another powerful program is AstroStack ($39) by Robert Stekelenburg, which will work on a variety of computer platforms.
Of course, to use these programs, you must first get the video onto your computer. There are several ways to accomplish this, thanks largely to the prevalence of software to edit home videos and burn DVDs. Many video cameras now come equipped with ports and cables that will send video output to a computer's USB or FireWire input. The movie-making software will then capture the video and save it in a file format for video editing, usually AVI or QuickTime.
If you have an older analog camera, you can install a television card in your computer. These cards have TV tuners that allow you to watch broadcasts on your computer monitor (and sometimes record them to disk). Many also feature inputs to connect VCRs or other video sources, allowing analog signals to be converted to digital video. While intended for people to turn their home videos into DVDs, you can use it to import your astro video and run it through image-stacking software.
The result of stacking will be a single picture that you can save to your computer. Then make a copy on your printer or at a local photo-service center to show your family and friends just how good a planetary imager you are!

More Astro Video Resources

A valuable book that delves into the digital details of astrovideography is the recently updated Video Astronomy by Steve Massey, Thomas A. Dobbins, and Eric J. Douglass ($24.95 from Sky Publishing), which further illustrates the techniques in this article and much more.
Camcorder-to-telescope adapter brackets are available from:

LensPlus
11969 Livonia Lane
Redding, CA 96003
530-549-4257
www.lensadapter.com
Orion Telescope & Binoculars
P.O. Box 1815
Santa Cruz, CA 95061
800-447-1001 or 831-763-7000
www.oriontelescopes.com
As with any realm of consumer electronics, the equipment, tools, and techniques change quickly. Join the VideoAstro e-mail discussion group to remain up to date with the latest trends in video imaging. The site offers valuable infor-mation on video equipment and techniques as well as an excellent gallery of amateur video images.
Capturing planets has become relatively easy, but extracting the most detail from the raw images produced at the telescope takes patience and skill...that's where planetary imaging comes to your aid.


Capturing the planets has become relatively easy, thanks to the webcam. But extracting the most detail from the raw images produced at the telescope takes patience and skill, not to mention the tools available today for your digital darkroom. Author Don Parker created these stunning portraits of Jupiter, Saturn, and Mars using webcams, a 16-inch Newtonian telescope, and the techniques he describes here.

Over the past five years, a tremendous resurgence in amateur planetary astronomy has taken place. This is due in large part to the simple, inexpensive webcam, which has enabled amateurs armed with modest telescopes to produce images of the planets that rival those captured with large professional instruments (S&T: October 2005, page 115). The secret of the webcam’s success is that it can produce images with very short exposure times, perhaps one-tenth of the durations required with conventional cooled CCD cameras. This means that webcams can “beat the seeing” — with planetary imaging and capture sharp images during fleeting moments of atmospheric tranquility.


Unfortunately, there’s a downside. Webcam frames are noisy when compared to those from cooled astronomical CCD cameras. But because signal increases directly with exposure, whereas noise increases only as the square root of exposure, you can stack lots of frames to produce an image with a much higher signal-to-noise ratio than that of any individual frame.


It typically takes hundreds or thousands of webcam images stacked together to achieve an acceptable result. And not every frame you record will be worth keeping, so before you stack images you need to weed out the blurry ones. Sorting and stacking thousands of images manually would be about as much fun as undergoing root-canal surgery! Thankfully, there are computer programs available today that will do this job of planetary imaging automatically.


The primary goal of planetary imaging is to tease out as much fine detail as possible without introducing spurious artifacts. Careful image processing using the tools of the “digital darkroom” is every bit as important as acquiring high-quality webcam frames in the first place if you want to produce highly detailed and scientifically useful images. I frequently employ several different programs and planetary imaging to process my images, because no single program yet contains all the tools I find necessary.

Planetary Imaging 101: Selecting and Stacking

RegiStax 6, available as a free download, is one of the most powerful and widely used programs for planetary imaging techniques including sorting, registering, stacking, and sharpening webcam images. Sean Walker described its key functions in his review of version 3 in the December 2005 issue, page 94. Like him, I find I can produce excellent planetary imaging results by using the program’s default settings but even better results if I exert some manual control.

Using RegiStax's alignment tools can be a daunting task for the uninitiated. 
 
Figure 1: (Click on picture for a larger image.) Using RegiStax”™s alignment tools can be a daunting task for the uninitiated. By selecting an average–quality frame from an AVI file and increasing the FFT Spectrum radius value, higher accuracy can be achieved during the initial alignment.


Donald C. Parker
After opening an AVI file from my webcam, I first select a reference frame of average quality, as this tends to produce the best alignment compared to using an exceptionally sharp one. If I recorded my target on a night of average seeing, I’ll use an Alignment box that encompasses the entire planet’s globe, selecting a frame near the middle of the movie to minimize rotation artifacts and choosing the Quality Estimate Method Local Contrast. If the seeing was good during the movie clip, I’ll instead choose a smaller Alignment box centered on an interesting planetary feature, such as Jupiter’s Great Red Spot or an albedo feature on Mars, for my registration point. I then select Gradient as the Quality Estimate Method. Once I’ve chosen my alignment point and Quality Estimate Method, a window opens that displays the FFT (Fast Fourier Transform) Spectrum of the registration point. I usually end up with a better stack if I decrease the radius of the FFT filter, so that it displays a larger target area than with the default setting (Figure 1).


RegiStax allows the option of dark-frame and flat-field calibration. If I used a color webcam to record my target, it’s important to convert the flat frame to monochrome before applying it, or else the results will be unusable.
Once the first alignment is completed, the results are displayed as a graph with two lines representing quality and registration. 
 
Figure 2: (Click on picture for a larger image.) Once the first alignment is completed, the results are displayed as a graph with two lines representing quality (red) and registration (blue). If the lines intersect, go back a step and decrease the FFT Spectrum radius, then repeat the alignment process.

Donald C. Parker
At this point, I initiate the Align command. After a few minutes, RegiStax will display the results of the Initial optimizing run, showing two lines in a window: a red one representing image quality, and a blue one displaying the registration difference between each frame. I try to end up with two roughly horizontal lines, though this isn’t always possible (2). If the lines intersect closer to the left of the graph, I may adjust my earlier settings and repeat the alignment routine.

Once I’m satisfied with the initial alignment results, I move the slider at the bottom of the screen toward the left to exclude the poorest frames and select the Limit option, which brings me to the Optimize menu. Here I choose the Reference Frame menu, and change the Frames option from the default of 50 to between 200 and 300. Once I press the Create button, RegiStax combines the best frames within this limited selection to create a smoother reference image for use with the remaining frames. At this point I sharpen this image with the Wavelet filter before reintroducing it to the remainder of the process. I then save the image both with and without the Wavelet sharpening, because occasionally this small stack ends up superior to the final result made with additional frames, especially if the seeing was poor during the original video recording (3).

When choosing the Create Reference Frame option, save the resulting raw stack for future use. 
 
Figure 3: When choosing the Create Reference Frame option, save the resulting raw stack for future use.
Donald C. Parker
I sometimes skip the Optimize command and proceed directly to the Stack tab. Occasionally I’ve found that the post-optimization images have serious artifacts, especially if the seeing was less than favorable. Usually a few frames were grossly misaligned; these appear as a ghost image when the image’s contrast is stretched.


In the Stack menu, I open the Stackgraph tab at the bottom right and exclude any remaining poorly aligned frames by moving the Difference Cutoff slider down and the Quality Cutoff slider toward the left — RegiStax isn’t perfect, so some lower-quality frames usually sneak in. As I adjust these settings, the percentage of these frames decreases. I find that stacking 800 to 900 frames is optimal; a larger stack tends to obscure finer details, while noise begins to dominate if too few frames are combined. Finally, I initiate the Stack command.



After stacking is complete, I click the Wavelets tab. Before adjusting the Wavelet sliders, I first reset them so that no sharpening is applied, then save the image as a 16-bit TIFF file. If I decide to reprocess the image at a later date, I can bring this “raw” stacked file back into RegiStax without having to repeat the alignment and stacking routines. If I used a color camera to record the image, it most likely displays color fringing caused by atmospheric dispersion. Here I utilize the RGBshift function noted in Walker’s review. When a satellite of the subject planet is in the field, I process the entire AVI movie a second time, registering only on the moon, and save this image as a separate file to add later in Adobe Photoshop.


Now that I’m ready to sharpen the image, there are a few self-imposed limits I apply to my processing to ensure I don’t add artificial detail to the picture through oversharpening. I limit the aggressiveness of the Wavelet filter based on various factors such as image quality, number of frames in the stack, and the type of detail I hope to resolve.

Wavelet Sharpening

Wavelet sharpening with RegiStax. A helpful tool for planetary imaging of astrophotographs.
Figure 4: (Click on picture for a larger image.)
Donald C. Parker

The Wavelet filter in RegiStax is controlled by six sliders and three settings. From 1 to 6, the sliders affect detail at lower spatial frequencies, corresponding to features of larger angular size. The first two settings are located above the sliders. Initial Layer raises or lowers the Wavelet filter’s highest frequency. Step Increment increases or decreases the range of frequencies. The final setting is located at the top of the screen, titled Wavelet. When you click this tab, a window with a grid of numbers appears, displaying the setting of the filter itself. By raising the central number, I can achieve better results than with the default setting of 50. Like all my other decisions, this will be dictated by the quality of my stacked image; if the movie was recorded under outstanding conditions, I may start by raising the center frequency to 1,200, and then see how this affects my image by raising the first slider. Lower numbers translate into lower frequencies and smaller steps between the 6 layers (4).

Another example of wavelet sharpening in RegiStax.
Figure 5: (Click on picture for a larger image.)
Donald C. Parker

When I’m comfortable with all these settings, I move the slider of the first wavelet layer as far as possible to the right before noise becomes objectionable. Then I move on to the second and perhaps the third wavelet, until I’m satisfied with the results. I only use the lower frequencies when the seeing is particularly poor, or if my target was very small, such as Uranus, Neptune, or perhaps Mars far from opposition. During these steps I use the Gamma and Histogram functions to enhance contrast, avoiding clipping the high pixel values. I find the Brightness and Contrast functions tend to clip the brightest and darkest areas of the image, so I avoid using them (5). Finally, I save the sharpened result as a 16-bit TIFF file.

Deconvolution

Although I may be finished with RegiStax, additional planetary imaging processing often can improve my image further. I next open the wavelet-sharpened image in MaxIm DL. Here I rotate and resample the image, as well as apply a few iterations of deconvolution. I prefer to perform the rotation and resampling here rather than in RegiStax, because MaxIm uses a powerful bicubic algorithm that interpolates neighboring pixel information to create a smooth resized image. I find these modifications should be done before further processing, because resampling at the end of my routine usually results in a softer final picture. I usually resample my images to about 150% of their original size, which seems to produce a smoother final image and helps me to avoid overprocessing.

Deconvolution in RegiStax can help define your astrophotographs even further
Figure 6
Donald C. Parker
If I used a color camera, I split the image into its red, green, and blue (RGB) components, then manually realign them on a surface feature rather than on the limb using the Process > Align pull-down menu and align using the Overlay option (6). In addition, I sometimes use one of the monochrome color channels that displays the most detail and use it as a luminance image. This is often the red channel, so the resulting color combination will actually produce an "RRGB" image. If I used a monochrome camera, I convert the individual R, G, and B images to monochrome (RegiStax saves all TIFF files as color images), align them on a surface feature, and color combine them.
At this point, I again save the image as a 16-bit TIFF and make a duplicate for further processing with a deconvolution filter. I generally use the Lucy-Richardson filter in MaxIm DL (Filter > Deconvolve), choosing the Extract by Mouse Click option in the Noise Extraction Tools and selecting 16 background points to map the noise level of the image. My next step is to manually input a PSF (point spread function) Radius to properly apply the deconvolution (7).


Second step in deconvolution in RegiStax
Figure 7
Donald C. Parker

Third step in Deconvolution in RegiStax.
Figure 8
Donald C. Parker

Most deconvolution algorithms require a point source to be sampled directly from the image to get an accurate reading, but photographs of the planets are exposed for too short a duration to record stars. I specifically use MaxIm DL because it allows me to input my own PSF radius and experiment until I find one that works well. Generally I select a PSF radius between 1.0 and 2.0 pixels, then apply two iterations of deconvolution (8). The Lucy-Richardson algorithm has an advantage over unsharp-mask sharpening in that it can bring out fine details while suppressing noise. This routine should be gently applied, however. I often achieve good results by repeating the process a second time using a different PSF radius.


Once I’m satisfied with these planetary imaging results, I save the file (again as a 16-bit TIFF) with a new name and open it in Adobe Photoshop CS2 for final adjustments such as color balance, saturation, and noise reduction if needed. I generally touch up the image’s brightness, contrast, and color balance by making a Curves layer. Again, I try to avoid clipping the histogram. Finally, I carefully inspect the image for detail and grain. If necessary, I’ll apply a mild high-pass filter to add slightly more contrast, or a median filter to reduce any unwanted residual noise. Now I consider my work done, so I save the image in Photoshop Document (PSD) format. Before I can share the image via e-mail or on a website, I must flatten the layers (Layer > Flatten Image) and convert the image to 8-bit data (Image > Mode > 8 Bits/Channel), finally saving it in JPEG format.


Remember that this advice on planetary imaging is based on my own telescope, camera, and seeing conditions, so my preferred settings may not apply perfectly to your situation. Experiment with these settings to find a routine that works best for you.


The planets are always changing, so planetary imaging these bodies can be a very rewarding experience. While capturing and conducting planetary imaging techniques can be time consuming, the result has scientific value. Even in this exciting age of solar-system exploration by spacecraft, amateurs still can make significant contributions to planetary science. With today’s steady improvements in both cameras and software, I’m sure that the amateur’s place in planetary astronomy is secure for many years to come.


Donald C. Parker has been photographing the "major" planets for more than 30 years, including one that became a dwarf.
1882 transit of Venus (left); 2003 transit of Mercury (right)
Left: Witnessed only five times since the invention of the telescope, Venus’s solar crossing is an eagerly anticipated event. This photograph of the planet’s last transit on December 6, 1882, was captured by a US Naval Observatory expedition. (The grid and spots are artifacts of the telescope system and emulsion.) Compare it with the composite view of the transit of Mercury (right) on the right obtained on May 7, 2003, by Dominique Dierick from Ghent, Belgium, with a 155-millimeter f/7 Astro-Physics refractor, a Nikon D100 digital SLR camera, and a Baader AstroSolar filter. With Venus nearly 2½ times larger than Mercury, and only half as far away form Earth, its silhouette will appear almost 5 times bigger this coming June 8th (58 arcseconds versus 12).



For the transit of Venus across the Sun on June 8th, amateur astronomers today have at their disposal a wide array of recording media, from modern high-speed ultrafine-grain film to high-resolution webcams and digital, CCD, and video cameras — high-tech, off-the-shelf equipment that 19th-century astrophotographers could only dream of. While we don’t expect any new scientific breakthroughs from this event, its appeal to amateur astronomers has not been diminished by this fact. Here are some tips and pointers on how to capture this rare and historic event for posterity.



Basic Requirements
Size of Sun and Venus on 35-mm film
Use this guide to see what telescope focal length (FL) you’ll need to photograph this year’s transit of Venus. On June 8th the Sun and Venus will measure 31.51-arcminutes and 0.97-arcminutes across, respectively. Here they're shown at their actual sizes relative to the 35-mm film format (24 by 36 mm). To get the size of the Sun in millimeters with your equipment, divide the effective focal length of your lens or scope by 109.1; for Venus, divide it by 3,544. For example, a refractor with a focal length of 4,000 mm will produce images of the Sun and Venus 36.7 and 1.1 mm across, respectively, at the film plane.
Sky & Telescope: Gregg Dinderman.

Photographing the transit is much like photographing sunspots. But first you have to decide what you want to record — the whole disk of the Sun or close-ups of Venus’s ingress (entrance) and egress (exit) along the solar limb to try and capture the so-called “black-drop” effect. In this curiuos phenomenon, as Venus’s silhouette makes contact with the limb, it seems to draw a thread of blackness that distorts the silhouette’s shape. It’s an elusive effect that was first reported by astronomers during the 1761 transit.

You’ll likewise need a proper, visually safe solar filter to cut down the Sun’s intense brightness and heat. Make sure your filter is securely mounted on the front of the telephoto lens or telescope objective. (Polarizing or photographic neutral-density filters are not safe for visual use.) For a list of filter manufacturers and dealers, see "Solar Filter Suppliers."

The focal length of the camera setup you should use depends mainly on what you want to capture. A standard 50-millimeter lens gives only a minuscule image (0.5 mm in diameter) of the Sun on film. To show the Sun’s disk (and that of Venus) reasonably large on film, you’ll need a telephoto lens or telescope with a focal length from 1,000 to 2,000 mm or more (see the diagram above), as well as a solid tripod or mounting.

For any telescope of a given focal length, the diameter of the Sun’s image is roughly equal to focal length (in mm) divided by 109. A standard 8-inch f/10 Scmidt-Cassegrain telescope with a 2,000-mm focal length yields a solar image 18 mm across, which nearly fills the frame of a standard 35-mm film. For close-up shots of the transit, you’ll want a telescope with about 4,000-mm effective focal length or more.


Be sure to test your equipment and practice your procedure well ahead of the actual event. You can begin by letting the camera’s light meter determine the exposure. Then try a variety of other exposures on either side of it (a technique known as “bracketing”). Keep careful notes and see which one comes out best. Such dry runs can also reveal potential problems with focus and vibration, as well as internal reflections and vignetting in your setup. Try to do your testing around the same time the transit will occur to determine the best exposure to use.


Film Considerations

In general, color-negative emulsions (those used for prints) offer greater exposure latitude than transparency (slide) films do; that is, they record features over a wider range of brightness with a single exposure. When choosing the film’s speed (ISO rating), bear in mind that the faster the film, the shorter the exposure. Short exposures tend to minimize blurring due to vibrations and tracking errors. But fast films are relatively grainier than slow emulsions.
For those traveling overseas to see the transit, bear in mind that newer, more powerful security X-ray machines are now in use at many airports. These units can damage film, especially high-speed emulsions. If possible, ask that your films be hand-inspected; never pack your film in your checked luggage.


Focusing Issues
Mercury near the Sun's limb
This image showing Mercury about to exit the Sun’s face on May 7, 2003, was made by Enrico Perissinotto of Talmassons, Italy, using a 130-mm Astro-Physics refractor working at f/12, a Baader filter, a Herschel wedge prism, and a Philips ToUcam Pro webcam. This image scale is ideal for capturing the so-called black-drop effect.
Courtesy Enrico Perissinotto.

Focusing is especially critical when you use telescopes and superlong telephoto lenses that don’t have a fixed infinity setting. In some 35-mm cameras you can replace the viewfinder with a magnifier to aid in focusing. Once you achieve optimum focus, place a piece of adhesive tape on your lens’s focus ring or your telescope’s focus knob to prevent it from accidentally being moved during the transit. The same technique also applies when you set zoom lenses, which can slip without warning, especially when aimed high in the sky.
If the Sun’s image fills the frame, focus to make the solar limb look sharp where it will actually fall on the film (near the edge of the frame); don’t move the limb to the field’s center to focus on. Schmidt-Cassegrain telescopes in particular focus a little differently at the center of the field than they do at the edge. Be sure to recheck your focus as the transit progresses since changing temperature can cause the focus to shift slightly.


 Mountings

Whether you’re traveling by land, sea, or air to your observing site, try to keep your mount as portable, light, and easy to assemble and operate as possible. Portability is especially essential if you need to relocate in a hurry to escape clouds.
In central Europe and the Middle East the Sun’s altitude will be up to 76° at the end of the transit — make sure your camera tripod can be aimed this high. A geared head allows for slow-motion control and is ideal for manually tracking the Sun, which moves across the sky by its own diameter every two minutes or so.
To improve a tripod’s stability, hang some weights under its center post. You can also set the tripod legs on rubberized footpads to dampen vibrations. The mirror slap in single-lens reflex (SLR) cameras can blur images, especially at slow shutter speeds. To reduce camera shake, operate the shutter button with a long cable release or use the camera’s delay timer. Lock the viewfinder mirror up beforehand if possible. Last, choose your site so it’s shielded from direct breeze; erect a windbreak, if needed.

The Digital Alternative
Nikon Coolpix with adapters
For digital cameras with threaded lens barrels, such as the Nikon Coolpix 990 shown here, you can purchase various lens adapters to attach the camera to the telescope. Since you get instant results, digital cameras, like video camcorders with their autofocus and autoexposure features, can take the guesswork out of imaging the transit — what you see is what you get.
Sky & Telescope:Craig Michael Utter.
Digital cameras operate basically the same as a conventional camera except that they use a CCD (charge-coupled device) or CMOS (complementary metal-oxide semiconductor) chip instead of film for capturing images. A digital camera’s resolution is measured by the number of pixels in its image (expressed in millions of pixels, or megapixels). The more pixels an image has, the better quality the image will be, so get the model with the most megapixels that you can afford.
Digital cameras range from the consumer-level point-and-shoot cameras costing between $200 and $1,000 to "prosumer" digital SLR cameras (about $1,000 and up), which look and feel like 35-mm SLRs and feature fully manual controls and detachable lenses. They use removable memory cards to store images. High-resolution pictures require more memory, so try to buy cards with the largest capacity in order to avoid running out of memory at a critical time. Use your camera’s built-in preview screen so you can save memory by deleting the bad images.

Transit Videography

Nothing offers instant gratification in the field better than a video. All of today’s camcorders use highly sensitive CCD or CMOS detectors. The Mini-DV or Digital 8 format offers the highest resolution compared to 8-mm, VHS, VHS-C, Hi-8, or S-VHS formats. The compact size and light weight of 8-mm camcorders make them ideal for travel. There are dozens of models and prices from which to choose, with features such as flip-out color LCD viewfinders and image-stabilized optics.


Some camcorders now have zoom lenses with up to 32x optical and 64x (or more) digital magnification. Optical zoom is more important since it increases image scale on the detector. (Digital zoom simply enlarges the pixels.) The easiest way to determine the actual size of the Sun in your camcorder is to shoot brief footage of it, zooming in to the highest power. If your camcorder doesn’t have enough magnification, consider adding a high-quality teleconverter (3x or more) to the front of the lens or shooting through a telescope with a wide-field, long-eye-relief eyepiece.


As with still cameras, you need a proper solar filter over your camcorder or scope when recording the transit. Keeping the Sun centered in the field of view at high magnifications will be a lot easier if the telescope is on a motor-driven, polar-aligned equatorial mount.


Take 2- to 3-second clips every two to five minutes to produce a time-lapse sequence that compresses the 6.2-hour-long transit into just minutes. High-end camcorders have manual controls for adjusting the gain, f/stop, and shutter speed so you don’t overexpose the Sun’s disk and cause blooming (streaking) of the image. Again, it's best to test your setup well in advance. On transit day, be sure to use a freshly charged battery. Keep a spare one as backup.

Mounting the camcorder directly to the telescope requires that you use a beefy mount and rebalance the scope; holding it with your hands can create shaky footage. A better alternative would be to attach a small black-and-white video camera, such as Orion's Electronic Imaging Eyepiece or Meade's Electronic Eyepiece to the telescope’s 1¼-inch eyepiece holder. These cameras are compact, lightweight, and inexpensive (around $70), and run on a single 9-volt battery. You record the video output separately with either your camcorder or a portable VCR.

After the transit, be sure to remove the tape from the camcorder for safekeeping; don’t forget to label the videocassette and “lock” it or break its tab so you won’t accidentally erase your recording.


Webcams and CCDs
Webcam attached to a telescope
To use a webcam for solar imaging, unscrew its supplied lens and attach an adapter that mates the camera to a properly filtered telescope; use a Barlow lens to increase the magnification. You can either machine your own adapter or buy one commercially.
Sky & Telescope: Craig Michael Utter.

Webcams and astronomical CCD cameras are also ideal for capturing transit images, though they do require a computer to operate and download images. This entails an additional power requirement,
which can be an issue during the hours-long transit.

As with digital cameras, the output of these devices is already in digital format, so anyone in the field who has Internet access can e-mail images anywhere in the world or post them on a Web site. In addition to still images, webcams can send live streaming (continuous) video of the transit in real time over the Internet or another network. For more information about webcam astro imaging, see "Shooting the Planets with Webcams" in Sky & Telescope: June 2003, page 117.


Owing to their sensitivity, filtered solar images from CCD and video cameras can still be greatly overexposed. When this happens, you have to add a neutral-density or variable polarizing filter in front of the camera to further cut down the brightness.

Image Processing

With the advent of powerful desktop computers and commercial image-processing software, astrophotographers can now take a range of exposures, select the best ones, and use popular graphic-arts programs such as Adobe Photoshop or Paint Shop Pro to “stack” (combine) them into a single image or stitch them together to create a movie.

With just a few mouse clicks you can easily retouch any film defects (dust specks, scratches, or fingerprints) and adjust the image contrast, brightness, and hue. About a dozen or more images can stacked to improve image quality and produce a smooth composite, which can be printed on a high-quality photo printer or sent to a photo lab to produce prints or slides. The potential for imaging is only limited by your imagination.


If you miss this transit, don’t despair. Remember that transits of Venus come in pairs, so you don’t have to wait more than a century for the next one — you’ll get another chance on June 5–6, 2012.
Veil Nebula
Digital cameras are not well known for their wide-field imaging abilities. Nevertheless, when a chip’s pixel size is properly matched to a telescope’s focal length, some of today’s CCDs can cover a considerable amount of sky. David Hanon of Ringgold, Georgia, captured this 11/4°-tall view of the eastern Veil Nebula with an SBIG ST-8 camera equipped with a KAF-1600 CCD. His 20-minute exposure was with a 7-inch Astro-Physics refractor operating at f/6 with a focal reducer.


The scene played with the predictability of a well-rehearsed script. On a half dozen occasions visitors stopped by while I was testing two high-end digital cameras during the late 1990s. Each knew about the Kodak KAF-1600 and KAF-1000 CCDs in these cameras, but none had seen them firsthand. Handing each person the first camera, I would click the computer’s mouse to snap open the shutter and reveal the Chiclet-size KAF-1600. With almost 20 times the imaging area of chips in early cameras marketed to amateur astronomers, this CCD impressed everyone.


Nevertheless, when the shutter clicked open on the KAF-1000 camera, jaws dropped. "Now that’s a CCD!" exclaimed one guest. Measuring 1 inch square, this chip offers only slightly less imaging area than a frame of 35-millimeter film. While everyone was predictably fascinated by this expensive bit of silicon real estate, blank stares followed my comment that, at a given resolution, I could capture more sky with the KAF-1600 despite its substantially smaller size.


How can this be? Even a quick glance reveals the KAF-1000 to be considerably larger — 4.65 times, to be precise — than the KAF-1600. The key to this paradox, however, was my qualifying statement that at a given resolution the KAF-1600 covers more sky.


Most of us photographers never think much about resolution. Today’s emulsions have relatively fine grain, and we use the same film with telescopes big and small. As such, the larger the piece of film, the more sky will be captured up to the point where optical or mechanical considerations limit the field of view.


CCDs, however, are a different story. Pixels — the individual, light-sensitive picture elements that make up the checkerboard array of a chip’s imaging area — come in many sizes. The detectors found in today’s popular cameras have square or slightly rectangular pixels ranging from about 7 to almost 30 microns (thousandths of a millimeter) across. The best results occur when a pixel’s size is matched to a telescope’s resolution under a given set of observing conditions. For example, conventional wisdom suggests that the astronomical seeing conditions experienced by a typical backyard observer will produce excellent deep-sky images with pixels that cover about 2 arcseconds (2") of sky.


Even a glance reveals the dramatic difference in physical size between the Kodak KAF-1600 (left) and KAF-1000 chips. But, as explained in the text, at a given resolution the KAF-1600’s 1.6 million pixels can cover 60 percent more sky despite having only about one-fifth the area of the KAF-1000.
Sky & Telescope / Chuck Baker

With this criterion established, the paradox is quickly resolved. If you adopt a given pixel scale such as 2" for deep-sky imaging, then you need only remember that the more pixels a chip has the more sky it will cover regardless of the chip’s physical size.


Consider the CCDs mentioned above. The KAF-1000 has 1 million 24-micron pixels arranged in an array measuring 1,024 pixels on a side. At 2" per pixel, the detector covers a field 2,048" (about 34') square. The KAF-1600, on the other hand, has 1.6 million 9-micron pixels assembled in a 1,552-by-1,032-pixel array. At the same scale, this chip covers a field measuring 3,104" by 2,064" (about 52' by 34'). The KAF-1600 has 60 percent more pixels than the KAF-1000 and should therefore cover 60 percent more sky.


There is a catch, however. Obtaining the same 2"-per-pixel scale for these detectors necessitates very different effective focal lengths. Indeed, the KAF-1000’s larger pixels require an effective focal length of 2,475 mm (about 97 inches), while the smaller KAF-1600 pixels need only 928 mm (about 37 inches). The nomogram on the next page makes simple work of determining the relationships between pixel size, focal length, and a pixel’s image scale.




Four CCD images with insets
Image scale, not pixel size, controls the resolution of digital images. To illustrate this point, the author made these pairs of 3-minute exposures of the edge-on spiral galaxy NGC 981 in Andromeda with a Meade 16-inch LX200 Schmidt-Cassegrain telescope and SBIG ST-7 camera equipped with a KAF-0400 CCD. By changing focal reducers and binning pixels, roughly similar pixel scales were obtained at focal lengths of 1,303 and 2,365 millimeters (f/3.21 and f/5.85, respectively). Note that the resolution at a given scale is independent of pixel size. The shorter focal length covered about four times more sky than the longer one. Insets: A 5x enlargement of the double star to the lower right (southwest) of the galaxy’s nucleus. The magnitude-161/2 components are separated by 5.8'.
Sky & Telescope / Dennis di Cicco

Pixel Binning

You might think that these parameters would be fixed for a given telescope and CCD camera. However, it is usually possible to vary both the focal length and pixel size within some limits.
Most cameras sold today offer what are called binning modes -- the ability to electronically combine the signal collected by several adjacent pixels such that it appears to come from a single, larger pixel. There are several advantages of binning, including faster image readout, smaller file sizes, and greater CCD sensitivity for a given optical system. This technique is often used with long-focal-length systems, which deliver generous images scales. Unfortunately, binning also reduces a CCD’s effective number of pixels.


Nomogram
Roger W. Sinnott developed this nomogram to show the relationship between image scale, effective focal length, and pixel size. A straight line connecting any two values passes through the third. For example, in order to have a 9- micron pixel cover 11/2' of sky requires a focal length of about 50 inches. While experience ultimately dictates the best image scale for given conditions, conventional wisdom suggests that scales of 11/2' to 2' are good for general deep-sky imaging, while lunar and planetary work can benefit from scales as small as 1/2' with apertures large enough to allow short exposures that "freeze" the astronomical seeing.
Sky & Telescope diagram.

Consider the example of 3x3 binning with the KAF-1600. The resulting 27-micron-square pixels are similar in size to those of the KAF-1000, and both chips will provide similar resolution when coupled to the same telescope. This binning, however, reduces the KAF-1600’s effective number of pixels from 1.6 million to about 178,000, which is roughly one-fifth the number available with the KAF-1000 chip. In this situation the sky coverage of the KAF-1600 will be about one-fifth that of the KAF-1000, which is exactly what common sense tells us. When placed on the same telescope, the KAF-1000 covers about five times more sky than the KAF-1600 since physically it has about five times more area. Changing the binning mode of the KAF-1600 will change the resolution but not the total sky coverage.


If we want the greatest sky coverage from a given CCD, we should operate the chip in its full-resolution (unbinned) mode and select a focal length to produce the desired image scale. As mentioned earlier, for 9-micron pixels, a scale of 2" requires an effective focal length of about 37 inches. Traditionally such short focal lengths have been the domain of small apertures. While CCDs can deliver remarkably big performance with small telescopes, it’s still desirable to use large apertures for deep-sky imaging. Besides, you probably want to work with your existing telescope. So from a practical standpoint the question becomes, what can be done to adjust its focal length? Fortunately, you can do a lot.




Focal reducers
Focal reducers come in all shapes and sizes. The author feels they are one of the most important accessories for digital imaging since they are ideal for adjusting a telescope’s effective focal length to a CCD’s pixel size.
Sky & Telescope / Chuck Baker

Focal Reducers
During the past 20 years numerous focal reducers have appeared on the market. Most observers think of these in terms of decreasing a telescope’s f/number to make it "faster" photographically. But, as the name implies, these accessories work by reducing a telescope’s effective focal length. They are excellent for helping match image scale and pixel size. This is especially useful for Schmidt-Cassegrain telescopes.


In the past the challenge was to design a system with high-quality images across a large field. But since CCDs are relatively small this tolerance can be relaxed, and many focal reducers suitable for digital imaging can be made from simple achromatic lenses such as those scavenged from a old pair of binoculars. (An excellent source of information about the design and function of focal reducers is an article by the late Alan Gee on page 367 of this magazine’s April 1984 issue.)
Today, however, designing a custom focal reducer is necessary only in unusual situations. Commercial units, particularly those for Schmidt-Cassegrain telescopes, offer many options -- especially when the resulting focal length is tweaked by adjusting the spacing between the reducer and CCD.


Diagram
The compression factor of popular focal reducers can be varied somewhat by adjusting the spacing between the reducer’s back mounting surface and the CCD. The author derived this graph using Celestron and Meade f/6.3 reducers, which are designed for a spacing of 105 mm.
Sky & Telescope diagram.

Popular f/6.3 reducers sold by Celestron and Meade for their f/10 Schmidt-Cassegrains are designed to be used with a 105-mm separation between the back surface of the reducer and the detector (be it film, CCD, or whatever). As the accompanying graph indicates, altering this spacing changes the compression factor — great for fine-tuning a CCD system. Increasing the separation increases the amount of compression and thus reduces the effective focal length. Ideally we could increase the separation enough to accommodate small pixels. In practice, however, either image quality or, more likely, mechanical restrictions imposed by the telescope’s focusing system will limit the amount of compression that can be obtained.
There is one notable exception. Optec’s MAXfield unit is specifically designed to compress the field of an f/10 Schmidt-Cassegrain to a remarkable f/3.3 for CCD work. The spacing between the reducer and chip is critical, however, and changing it by even a millimeter degrades images. Also, the reducer has a maximum usable field about 11 mm across, too small for large chips.
SCHMIDT-CASSEGRAIN FOCAL LENGTHS
Instrument
Focal length (millimeters)
  Nominal f/3.3 reducer f/6.3 reducer
8" f/10 2,032 670 1,050—1,400
8" f/6.3 1,280 -- 800
9.25" f/10 2,350 775 1,200—1,650
10" f/10 2,540 838 1,350—1,800
10" f/6.3 1,600 -- 1000
11" f/10 2,800 924 1,500—1,950
12" f/10 3,050 1,005 1,600—2,100
14" f/11 3,910 1,290 2,050—2,700
16" f/10 4,060 1,340 2,100—2,850
 
   
There are also a few caveats for observers planning to use focal reducers with Meade’s 8- and 10-inch f/6.3 Schmidt-Cassegrain telescopes. I have found that the MAXfield focal reducer, while in theory yielding an f/2 system when attached to these instruments, will not give satisfactory star images — it works only with f/10 telescopes. The f/6.3 focal reducers, on the other hand, will compress the f/6.3 telescopes to about f/4 with very acceptable results. But experience suggests that changing the spacing to obtain other compression ratios is not recommended and is the reason for the single focal-length entries in the table at left.
Of the many comments I’ve heard about focal reducers, no one has ever mentioned their cost-saving benefit. Consider this example. I do much of my deep-sky imaging with a Meade 16-inch LX200 Schmidt-Cassegrain. The telescope’s nominal f/10 (4,000-mm) focal length is long even for large pixels. Adding a f/6.3 focal reducer drops the effective focal length to about 2,500 mm — a good match for the 18-micron pixels available with a KAF-1600 chip binned 2x2. Such a setup would yield an image scale of 1.49" per pixel and a field of view measuring roughly 19 by 13 arcminutes.


By switching to the f/3.3 focal reducer, however, I can get nearly identical sky coverage and imaging performance from an unbinned KAF-0400 detector. This chip has the KAF-1600’s same 9-micron pixels but is only one-quarter as large, with a 768-by-512-pixel array. What is really attractive about this arrangement, however, is that cameras equipped with the smaller chip cost about half as much as those with the KAF-1600, amounting to a savings of $2,500 to $3,000 depending on make and model! Similar results are possible with today’s 12- and 14-inch Schmidt-Cassegrain telescopes.




CCD image of NGC 2903
Small telescopes can deliver big performance when properly coupled to today’s CCDs with small pixels. This 10-minute exposure of the spiral galaxy NGC 2903 in Leo was made with a Celestron 5-inch Schmidt-Cassegrain and a focal reducer, yielding a effective focal length of 898 mm (about 35 inches). The camera’s 9-micron pixels each covered 2.1' of sky, and the field is nearly 1/2° wide with north up.
Sky & Telescope / Dennis di Cicco

More Thoughts

The previous discussion only highlights ways to maximize the field of view for deep-sky imaging with today’s popular CCDs. There are many other considerations when it comes to matching telescopes and detectors. First, nowhere is it chiseled into stone that you must have an image scale of 2" per pixel. Anyone doing lunar and planetary imaging will get superior results with scales of 1/2" or less per pixel. Even for deep-sky imaging, any site with good seeing will benefit from scales of less than 2". Some image-processing techniques, especially those involving resolution-enhancing algorithms like maximum-entropy deconvolution, work better with images that have large image scales (so-called oversampled images).


Conversely, excellent deep-sky imaging has also been obtained with pixel scales of 4" or more, especially in the case of large, bright objects. Indeed, many stunning images are produced with conventional camera lenses attached to CCDs. The resulting image scales (tens or even hundreds of arcseconds per pixel) may not yield the best-looking stars, but they can render remarkable views of huge nebulae.


Another consideration is that some desirable features are found only on large-pixel chips. Take, for example, the back-illuminated SITe CCDs that are currently available in cameras manufactured by companies such as Apogee Instruments. Having 24-micron-square pixels in arrays with 512 and 1,024 pixels on a side, these chips have exceptional sensitivity, especially to blue light, compared to their front-illuminated cousins. The blue sensitivity alone makes these detectors very attractive to people who are interested in photometry and tricolor imaging.


The number and size of pixels in a detector are only two considerations when you are planning the purchase of a CCD camera. In the coming months we’ll look at other important issues involved with getting the best performance from today’s state-of-the art digital-imaging equipment.

Specifications for Popular CCDs
Manufacturer CCD Imaging area
(millimeters)
Array format
(pixels)
Pixel size
(microns)
Total pixels
Kodak KAF-0400 6.9 x 4.6 768 x 512 9 x 9 390,000
Kodak KAF-1000 24.6 x 24.6 1,024 x 1,024 24 x 24 1,000,000
Kodak KAF-1300 20.5 x 16.4 1,280 x 1,024 16 x 16 1,310,000
Kodak KAF-1600 14.0 x 9.3 1,552 x 1,032 9 x 9 1,600,000
Phillips FT-12 7.7 x 7.7 512 x 512 15 x 15 260,000
SITe SI502A 12.3 x 12.3 512 x 512 24 x 24 260,000
SITe SI003A 24.6 x 24.6 1,024 x 1,024 24 x 24 1,000,000
Sony ICX027BLA* 6.4 x 4.3 500 x 256 12.7 x 16.6 13,000
Sony ICX055AL* 4.9 x 3.6 500 x 256 9.8 x 12.6 145,000
Texas Instruments TC-211 2.5 x 2.5 192 x 165 13.75 x 16 32,000
Texas Instruments TC-215 12.3 x 12.3 1,024 x 1,024 12x12 1,000,000
Texas Instruments TC-241* 8.6 x 6.5 375 x 242 23x27 91,000
Texas Instruments TC-245* 6.4 x 4.8 378 x 242 17x19.75 91,000
Texas Instruments TC-255 3.2 x 2.4 320 x 240 10x10 77,000
*An asterisk indicates the size and number of pixels as generally configured for astronomical use, since these chips actually have smaller, highly rectangular pixels originally intended for video applications.
There's no more seductive image in astronomy than a picture of the night sky filled with stars and colorful glowing clouds of interstellar gas. You know, the kind you see in books and magazines. These images usually come from multimillion-dollar telescopes, not from simple astrophotography setups. But it doesn't take a mountaintop observatory to capture panoramic deep-sky vistas. They're within reach of anyone with modest stargazing equipment and access to dark, rural skies. The technique is called piggybacking photography, and today's high-speed color films and digital cameras have made the process easier than ever before.


Piggybacking astrophotography involves using a telescope's mount to track the sky, but the camera shoots through its own lens, not through the telescope.
In piggyback photography, you use a telescope's mount to track the sky, but the camera shoots through its own lens, not through the telescope.
Night Sky: Craig Michael Utter

Simple Astrophotography: What is Piggybacking?

The richest star fields in the sky are found along the Milky Way, formed by the spiral arms of our home galaxy and well-placed during late summer. The exposure times needed to record the Milky Way typically last a few minutes. For this, you'll need a camera with a B ("bulb") setting on the shutter to permit long exposures and a telescope capable of tracking stars as they move from east to west. But you don't shoot through the telescope itself — it's the mount you want, to serve as a stable, motorized platform. You attach your camera securely to the telescope's tube assembly. It then rides along, similar to piggybacking, as stars are recorded by the camera's lens.

Without motorized tracking, Earth's rotation would cause stars to trail across a camera frame, an effect noticeable with even a 30-second exposure. Star trails can make for great astrophotos, but for rich, star-studded time exposures, accurate tracking is the key.

The beauty of piggybacking is that if the mount is set up correctly and tracks well, all you need to do is lock the camera's shutter open and walk away. You don't need specialized and expensive guiding gear. While the camera and mount do their work unattended, you can sit back, relax, and enjoy the night sky.

The Power of Piggybacking

The power of piggybacking while shooting the night sky results in some magnificent The Milky Way
A "fisheye" lens on a film SLR can capture the sky from horizon to zenith. This shot required a 1-hour exposure on Kodak Ekatchrome E200 Pro slide film.
Alan Dyer
A camera with a wide-angle lens (18 to 35 mm in focal length) or a standard lens (50 to 55 mm, assuming a 35-mm-frame format) — the same one you use to take daytime landscape or family-vacation snapshots — is ideal for capturing large swaths of the summer Milky Way. These common lenses can reveal the glowing star clouds that traverse the constellation Cygnus and run down to the galaxy's dense central region in Sagittarius and Scorpius, which are prominent in July and August. While to the unaided eye the Milky Way looks like a dim, grayish band, a 5-minute exposure with off-the-shelf equipment will show that it's made up of countless stars laced with delicate wreaths of red nebulosity — the luminous clouds of hydrogen gas that old stars cast off into space and out of which new stars form. With the power of piggybacking photography you can create your own atlas of the Milky Way's bright clouds and dark dust lanes or compile a portfolio of constellation portraits.


Switching to a modest telephoto lens brings a new realm of targets within reach. You don't need very long focal lengths — something in the 85- to 135-mm range is ideal for framing large nebulas, bright star clusters, and the Milky Way star clouds. (Besides, that monster telephoto in your closet may be too heavy to mount securely on the telescope.) The North America Nebula in Cygnus, the Double Cluster in Perseus, the Pleiades star cluster in Taurus, and the Andromeda Galaxy are all suitable telephoto subjects. As a rule of thumb, any object you can see well in binoculars is a good target for a piggy-backed telephoto lens.

Drawbacks of Piggybacking

What piggyback photography can't handle are extreme close-ups of small "deep-sky" objects such as the Ring Nebula in Lyra, the Crab Nebula in Taurus, or the beautiful globular star cluster in Hercules. For these targets you'll need the light-gathering and magnifying power of a telescope (serving as your camera's lens) to obtain good views. That's beyond the scope of this article. Nevertheless, there's still a wide selection of subjects to capture with your simple piggy-back setup.

Film Versus Digital

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The film camera of choice is the traditional 35-mm SLR (single-lens reflex) model for simple astrophotography. If you've purchased such a camera in the last few years, chances are it uses battery power to keep the shutter open. Even a couple of 15-minute exposures in the cold night air can quickly drain the battery, shutting down the camera. In the past, astrophotographers opted for low-cost, no-frills SLRs with mechanical shutters that worked without batteries (for sky shooting you don't need the autoexposure and autofocus features). Classic mechanical models such as the Pentax K1000, Canon F-1, Nikon F2, Minolta SRT-101, and Olympus OM-1 are still available at used-camera stores and through online auctions such as eBay.

On the other hand, digicams are taking over just about every sector of celestial imaging. For piggybacking simple astrophotography, the best choices are today's digital SLRs. These "prosumer" cameras offer 6 to 11 million pixels (megapixels) of resolution and inter-changeable lenses, usually the same lenses that fit on older film SLRs. (Most compact point-and-shoot digicams, the kind with nonremovable lenses, generate too much electronic noise during exposures lasting more than a minute.) While they depend on batteries, digital SLRs can deliver one to two hours of long-exposure shooting before you need to recharge the battery.

Film die-hards scoff at digital SLRs — until they see the results appear on the camera's built-in LCD viewing screen as soon as the exposure is completed. Then they sell off their aging film gear. I know I did! Digicams are wonderful for on-the-spot checking of the framing, focusing, exposure time, and tracking. A lot can go wrong during even a 5-minute exposure. Having instant feedback allows instant fixes, thereby improving your best-to-blooper ratio. Admittedly, digital SLRs are still pricey (starting at $700 to $1,000), but their cost has been coming down steadily.

Getting Lined Up

Aligning an equatorial mount on Polaris
For a good initial polar alignment, point the tube parallel to an equatorial mount's polar axis, look through the finderscope, and fine tune the altitude and azimuth to center Polaris.
Night Sky: Craig Michael Utter

Trailed stars are the number one flaw in piggybacked shots. This is caused by poor alignment of the telescope mount with the north celestial pole, the point in the sky near Polaris, the North Star, about which stars appear to rotate as Earth spins on its axis. To track stars accurately, you must use an equatorial-type mount and align it so that its polar axis aims as close to the celestial pole as possible.
While computerized "Go To" telescopes can also follow the stars, they do so by constantly moving two axes. Piggybacking a camera onto one of those scopes can result in badly trailed stars, since the field will rotate during the exposure. Optional wedges or tripods for popular Go To models, such as those from Meade and Celestron, allow them to be tipped over and polar aligned, which is essential for piggyback shooting.


How accurate does this polar alignment have to be for simple astrophotography? It depends on the focal length of the lens you're using. Simply aiming along the mount's polar axis by eye on Polaris, nearly 1° from the north celestial pole, might be good enough. Wide-angle lenses, which cover relatively large areas of the sky, can be forgiving of errors in polar alignment. Long telephoto lenses, on the other hand, magnify everything, including any tracking errors. So they demand greater accuracy when aligning the telescope's polar axis, to within a fraction of a degree of the true pole. Some equatorial mounts have polar-axis finderscopes built in, and these make accurate alignment much easier than with fork-mounted telescopes.


Even with precise polar alignment, however, stars can still appear to trail. The culprit here is the telescope's motor-drive mechanism, which might not be tracking smoothly or simply is not turning at the correct speed. In that case, the solution is to manually "guide" the telescope with the aid of a high-power eyepiece fitted with illuminated crosshairs. The trick is to select a moderately bright star to guide on and keep that star perfectly centered on the crosshairs throughout the exposure. Dual-axis drives (with a motor and push-button speed control on each telescope axis) greatly ease the guiding process and keep your shaky hands off the mount.

What Exposure Should You Use?

Orion Nebula at different exposures
On a digital camera at f/4 and ISO 400, a 1-minute exposure (left) reveals the Orion Nebula, but 3- and 6-minute exposures record fainter details.
Alan Dyer
Piggybacking shots of the Milky Way demands dark, moonless skies. To record the most stars and nebulosity, the best exposure is usually the longest one you can take before skyglow from city lights starts to wash out the details. There's more to setting the exposure, however, than just opening the camera shutter. The lens's aperture (opening) also controls the amount of light reaching the film or digicam sensor. Opening the lens to its widest aperture (that is, the smallest f/number, typically f/2) lets in more light and keeps the exposure time to a minimum, while still picking up stars as faint and as numerous as your sky will allow. Opening the lens by one f/stop, from f/4 to f/2.8 or from f/2.8 to f/2, cuts your exposure time in half.


Another option for shortening exposure time is to use a "fast" (sensitive) film or to switch your digicam to a high ISO setting. A film rating or digicam setting of ISO 400 will require only half the exposure time of ISO 200. If you're shooting with slide film, you can request that your photo lab "push" the film when it's developed — for example, to ISO 800 if the film is ISO 400. This will increase the picture's contrast, but at the cost of a slight increase in grain (coarseness).

A computer can work wonders on astrophotos. The raw image at left shows the low contrast typical of mediocre sky conditions. At right, image processing has boosted the contrast and corrected the colors.
Alan Dyer
Short exposure times may record less detail, but they'll minimize the streaking of stars due to poor or imperfect tracking and avoid aircraft flying through your field of view. So is a high ISO and a wide-open lens the best combination? Not necessarily. Wide-open apertures can reveal optical flaws in the lens, producing bloated and distorted star images, especially at the corners of the frame. Stopping down the lens (decreasing its opening) by one full f/stop to f/2.8 or f/4 reduces lens flaws and sharpens star images, at the expense of longer exposures.


Faster ISO speeds also have their drawback. They introduce grain with film or electronic noise with digicams. Fuji's Provia 400F offers remarkably fine grain with fast speed and is my choice for all-around piggybacking film shoots. With digital cameras, try settings of ISO 400 or 800. Their electronic noise decreases if the ambient air temperature is chilly, such as on winter nights. It gets worse after the camera has been used to take several images and has warmed up, especially during summer. If that happens, turn off the camera for a few minutes to let it cool before resuming shooting.
Finally, film loses sensitivity over long exposures — a 20-minute exposure does not record twice as much light as a 10-minute exposure. By comparison, digicams maintain their full recording ability throughout an exposure. That's why a digital SLR can pick up in 3 minutes what it might take a film camera 9 to 12 minutes to record. That can make all the difference between images that are trailed and ones that aren't.

Secrets of Success

Don't be afraid to experiment with these simple astrophotography tips. Try a variety of exposure settings (a technique known as "bracketing"), and keep a record to help you determine the best combination of film or ISO rating, exposure time, and f/stop for your particular site.

Unlike shooting through a telescope, focusing a camera lens is usually easy — just turn the focus ring to the infinity setting. But many autofocus lenses can actually focus past infinity. Determining the lens's sharpest focus may be a trial-and-error process, involving shooting several frames at different focus settings. The instant feedback of digital cameras makes it easy to find the best focus, and it's worth taking the time to do so. I've found that even a slight shift of focus, by no more than the width of an index mark, can make the difference between stars appearing as pinpoints or as fuzzy blobs.

When things go wrong . . .
Everything went wrong with this astrophoto. The motor drive stalled and trailed the stars; the sky background was overexposed; the lens frosted over; and high clouds rolled in, making the bright stars hazy.
Alan Dyer

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