"Nothing is permanent except change," said the Greek philosopher Heraclitus. Anyone who follows the rapidly evolving digital-imaging world has to agree. Consumer-level digital cameras are becoming ever more powerful, small (if you want), and cheap. They have replaced film cameras as the tool of choice for the snapshooting public. But how good are they for astronomical imaging?
Film still surpasses all but the highest-end digital cameras for resolution and color accuracy. And, unlike dedicated astronomical CCD cameras that use cooled chips, the sensors in consumer cameras generate objectionable "noise" during long exposures, so their useful exposure times are generally limited to a matter of seconds. This is fine for bright subjects such as the Moon and planets, but too short for the deep sky — though you can beat the problem by taking many short exposures and digitally stacking them. Digital cameras generally need a computer to manipulate and print images, but the technology is evolving rapidly, and photo printers that work without a computer are now mass-market items.
Of course, the advantages of going digital are many. You get instant results and can adjust what you do accordingly — no waiting for pictures to come back from the photo lab. You can shoot all you want and throw all but the best pictures away without worrying about film and processing costs. The camera's memory can probably hold many more images than a roll of film.
Since the camera’s output is already digital, the images are ready for computer processing with programs such as Adobe Photoshop, which can correct color imbalances, optimize brightness levels, dynamic range, contrast, and color saturation, and enhance image quality by stacking exposures. You can create a wide-field mosaic by stitching together adjoining frames. The images are Internet-ready — you can e-mail them or share them on the Web. They can be easier to sort and organize and take up less storage space than photo albums or shoeboxes.
Digital cameras are versatile — they can be used to digitize prints and 35-millimeter slides by recopying them. They can also be coupled a spotting scope for wildlife photography. The price of high-resolution cameras is constantly dropping. All these features make digital cameras very attractive for astro imaging. If you already have one, you should certainly see what it can do.
The Afocal Method
Basic, consumer-level digital cameras don’t have removable lenses; you have to go to a more expensive digital SLR camera for that. So the only way to shoot through a telescope with them is by the afocal method, whereby the camera’s lens is aimed directly into the telescope’s eyepiece. You can hold the camera by hand, mount it on a separate tripod, or make or buy a bracket to attach the camera directly to the telescope. (See our
sidebar on the afocal method to learn more about its history.)
Like the old eyepiece-projection technique, the afocal method dramatically increases the image size at the expense of a much slower effective f/ratio (or photographic speed). You may also get some vignetting and image distortion. Vignetting is the darkening of an image's edges that happens when light is cut off by mechanical limitations. It typically happens when the camera is held too far from the eyepiece or when the apparent angular field of the camera's view exceeds that of the eyepiece. This, however, is usually not a problem when you are photographing planets or other small objects that can be kept centered. To reduce vignetting, set the camera as close to and centered on the eyepiece as possible. Also, choose an eyepiece with ample eye relief – some short-focus eyepieces have such limited eye relief that a camera cannot get close enough to image well through them. Find the best camera position by experimentation.
Zooming in narrows the camera’s angular field and thus can minimize or eliminate vignetting (but avoid using "digital zoom," which sacrifices ’s resolution). Zooming also increases magnification, which often makes focusing easier. On the other hand, it makes finding your target tougher.
Image distortion can occur due to various effects in the eyepiece and camera optics. The center of the image may be in focus while the outer areas are not, severely restricting the usable field. Be sure you keep the camera’s image plane centered on and perpendicular to the telescope’s optical axis. Also, keep the eyepiece and camera lenses free from smudges and dust, which degrade contrast.
Mounting the camera on a separate tripod can prevent vibration while avoiding the need to rebalance the telescope for the camera's weight. With such an arrangement, it helps to orient the eyepiece so that it moves directly toward or away from the camera as the telescope follows its subject across the sky. Use a black cloth or cardboard mask to shield the eyepiece and camera lens from stray light.
Although not essential, a telescope motor drive is convenient since it keeps the subject centered in the camera frame as you focus and compose the shot. For scopes without a drive, you can still take good pictures if you keep exposures short. Position your subject at the edge of your frame opposite the direction of drift, then wait for the object to glide near the frame’s center before triggering the shutter.
As with film astrophotography, you need to obtain perfect focus, since any errors will show up prominently. You can begin by focusing the telescope visually using the eyepiece (if you use prescription glasses, wear them) and setting the camera lens to infinity. If the camera doesn’t have a manual override, just use its autofocus mode. The built-in LCD screen can be used for centering and focusing, but some people find it a bit too small and coarse to judge the focus precisely.
If your camera has a video output, you can connect it to a larger monitor (say a 13-inch portable TV) to help you adjust the fine focus while watching the TV screen. This real-time video display is also useful when good seeing is intermittent – you can use the video to judge the best time to take the images.
Imaging the Sun through the telescope requires a proper, safe solar filter (click here for a list of filter vendors). Be sure to cover the front of the camera’s viewfinder. Focusing with the LCD screen in bright daylight can be tricky, so shade the screen or use an external, shaded TV monitor. (Some people attach a loupe to the LCD screen with rubber bands or Velcro for added accuracy when focusing and to assist in viewing the screen in bright daylight.)
As with film astrophotography, a good-quality finder is helpful for aiming the telescope. A larger, higher-power guidescope, if precisely aligned with the main scope, can help in getting small targets such as planets into the camera's view.
Taking the Shot
Unlike single-lens reflex (SLR) cameras, digital point-and-shoots have no "mirror slap" that can cause vibration when taking a picture. But you can still cause shaking when pressing the shutter. To prevent this, use the camera’s self-timer or wireless remote release if it has one. Otherwise, you have to press the shutter yourself (very gently!) without shaking the setup. Some people have fashioned homemade brackets to hold the end of an old-fashioned cable release over the shutter button.
Many popular cameras have no manual override for choosing exposure (you cannot set your own shutter speed and aperture), so you have to rely on the autoexposure function. This works best with large, bright, uniformly lit subjects such as close-ups of the Moon. But the camera’s light meter may overexpose or underexpose the crescent Moon or planets, so you need to manually correct using the camera’s exposure-compensation capabilities (usually +2 to —2 stops). Remember to bracket your exposures. Preview the results on the LCD screen, and save the best. Don’t be afraid to experiment with exposure compensation — try one or two stops brighter or darker than normal, and note the settings that come out best.
For the planets, trial and error is needed to find the correct exposure. A good starting point for Jupiter and Saturn is ¼ to ½ second for typical Schmidt-Cassegrain telescopes. Keep exposures short to minimize blurring due to atmospheric turbulence. (The best modern planetary imaging, however, is done not by taking single shots with a camera at all, but by stacking hundreds of video frames taken with a digital webcam.) Use your camera’s highest image-quality setting when saving the image. This means using the camera’s full resolution and applying the least image compression. Most popular consumer cameras save images using JPEG compression to reduce file size and get more images into the available memory. The more you compress an image, however, the more it begins to show artifacts such as degraded color and, when magnified, squarish blotchy patterns: "jpegging." Use the lowest compression possible, or save the image in an uncompressed format such as RAW or TIFF if your (high-end) camera allows.
Don’t be surprised if you can store more astronomical than daytime images in the same amount of memory. The dark background of a typical astronomical scene compresses far more than conventional images. You can also experiment with the black-and-white mode if your camera offers this feature. Because of the way color CCDs work, images shot in black and white can appear sharper than color images.
Digital cameras are battery eaters, especially if you use the LCD screen continuously. You can avoid the problem if you have an AC adapter and access to a wall outlet. Fresh batteries can last less than a half hour in some cameras. Also, on cold nights batteries lose their capacity more quickly. Keeping a supply of fresh alkaline or lithium batteries can be very expensive, so the best option is to buy rechargeable nickel—metal hydride (NiMH) batteries. Keep one or two spare sets charged and handy for long sessions.
To prevent damage to sensitive electronics, protect the camera from excessive dew. Cover the camera when not in use, and on dewy nights, keep it powered up to maintain some internal warmth.
Despite their shortcomings for some applications, handy, mass-market digital cameras are here to stay ٳ and are sure to get better. No doubt they will play a growing role in amateur astronomy. So why not start now?
|Selected Manufacturers and Dealers|
Here’s a partial list of companies that sell digital-camera accessories that astro
|Adirondack Video Astronomy|
26 Graves Street
Glens Falls, NY 12801
42 West 18th Street
New York, NY 10011
BrightStar Optical & Photo
29325 Hwy 178
Onyx, CA 93255
23 Graywood Dr.
Orangeburg, NY 10962
11969 Livona Lane
Redding, CA 96003
Oceanside Photo & Telescope
1024 Mission Avenue
Oceanside, CA 92054
|Orion Telescopes & Binoculars|
P.O. Box 1815
Santa Cruz, CA 95061
21272 Chiquita Way
Saratoga, CA 95070-4259
1423 SE 10th St., Unit 1A
Cape Coral, FL 33990
Tau Ceti Company
P.O. Box 1101
Conroe, TX 77305
Tele Vue Optics
32 Elkay Drive
Chester, NY 10918