|Astronomy with a Digital Camera
by: Richard Taylor,
A general purpose digital camera can be used for many other purposes than astronomy. Since the price of either camera is fairly steep, it's easier to convince the rest of the family to buy a toy that they can all play with.
Although the initial cost is high, the incremental cost of each picture is very low (as long as you use rechargeable batteries). When you realize that each picture is no longer costing you 50 cents, your picture-taking habits will change dramatically, and the quality of your pictures will almost certainly improve (delete the bad ones!!!).
General-purpose cameras are designed for ease of use. You can start almost immediately and learn new features gradually over time. They are small and portable. They are self-contained (built in memory card and display screen).
Consumer pressure and competition are quickly bringing improvements. The market for general-purpose cameras is very large compared with the market for astronomical CCDs. This means that the features and resolution of these cameras is increasing rapidly and at the same time the price is dropping.
My decision: Nikon Coolpix 800.
Features to look for: ability to align separate images by selecting control points in each image, ability to add and subtract images.
|So you've got a new camera and want to try it out right away. What better target than a bright, sunlit scene: the moon. Since the moon is lit by the same amount of sunlight as we receive on earth, the lighting conditions are similar to a sunny day at the beach. General-purpose camera designers understand these conditions, so the camera will probably perform quite well with standard settings. Aim your telescope at the moon, point the camera into the eyepiece, and shoot.||Well... maybe not so simple. It can be a bit tricky aiming the camera directly into an eyepiece and holding it steady while you press the shutter. The lighting is very high contrast, the automatic exposure settings may not be correct, and there can be focussing problems. Strangely enough, using a Schmidt-Cassegrain telescope, some focus settings can be a problem. I have had a number of shots ruined by a slightly out-of-focus image of the central obstruction superimposed on the image of the moon.|
|Another easy way to start is to mount the camera on a steady tripod, aim it at the sky and take the longest exposure possible. The maximum sensitivity of modern commercial CCD chips is equivalent to ISO 400 film, which means that the brighter stars can be captured with an exposure of just a few seconds. And with such a short exposure with limited magnification, there is no need for tracking.|
|Of course to aim the camera into the telescope eyepiece, adjust the focus and take the picture requires about four hands. It would be really convenient if the camera could be attached to the telescope somehow. A few pieces of scrap wood, a drill and a glue gun soon solves that problem.|
|The telescope magnifies the objects we are observing, but also magnifies the effects of
the earth's rotation. You can get adequate pictures of bright objects like the moon with
very short exposures, but for anything fainter, it is essential to have the telescope track
the motion. Even for the moon, it is much more convenient to have it tracked while you adust
the focus and composition.
So for any astrophotography with a telescope, you do need an equatorial mount with a motor drive. Since long exposures are not possible, the polar alignment and drive accuracy are not as critical. In fact, I suspect that a computer-driven alt-azimuth mount would also work quite well. Usually such mounts are not recommended for photographic work since the field of view gradually rotates. But again, for short exposures, the effects would be small.
|Focussing is the biggest difficulty I have had with any kind of astrophotography. The
objects I want to photograph are usually faint and fuzzy. They don't show up very well
in the "viewfinder" (display screen) of the digital camera, and it's hard to tell on this
small screen if they are in focus or not. I have a similar problem with the ground glass
screen of a single lens reflex camera.
The best solution I have found so far is to move the telescope to a nearby bright star, and use the maximum optical and digital zoom of the camera to magnify the image. Then, with the camera focus fixed at "infinity", I adjust the telescope's focus to make the image of the star as small as possible. But zoom back out before taking the picture.
After focussing, the next problem is to re-locate the object I want to photograph. Again, the display screen is too faint for most objects of interest, so I remove the camera and eyepiece combination, put in an eyepiece and find the object by eye. This will only work if you are lucky enough to have another eyepiece and tube combination that is at almost the same focal position as the eyepiece and camera. Otherwise, you'll have to use the telescope's finderscope or a computerized "GOTO" system and hope they are accurate enough.
|My first attempts at enhancing my astrophotograhs were simply to "stretch" the brightness. But I was appalled at the results - there were horrible yellow streaks all across the picture, and quite a few additional "stars" in garish purple and green colours.|
|Fortunately, I remembered hearing about subtracting a "dark field" image when doing CCD astrophotography. Now I understand what it is all about. The CCD chip that is used to capture the image as an array of thousands (or even millions) of tiny electronic devices that change light into electricity. It's impossible to make them all behave in exactly the same way, and sometimes they have a tendency to react to heat as well as light. To compensate for these differences, it is fairly easy to take a picture of complete darkness (just put the lens cap on) and then subtract that picture of darkness from all other pictures taken that evening. Thinking mathematically, a completely black part of the picture should have a brightness level of zero. If the CCD chip components produce a non-zero value for real darkness, then that value has to be subtracted from its output all the time. This has quite dramatic results.|
Fortunately computer processing comes to the rescue by allowing us to add many short exposures to get the equivalent of a long exposure. For this to work, you need software that allows you to "register" two or more images by identifying matching points. Stars make ideal matching points, but this means that you can only add up pictures in which some stars are visible even before processing. This can be a big problem when trying to photograph "faint fuzzies" in a dark, starless field. The pictures below demonstrate the adding process. In fact, four images were added to produce the last one.
|Even adding several images may not be enough to make faint objects bright enough for a good picture. Digital images generally have 256 levels of brightness (for each of the three primary colours, red, green and blue), but the computer screen and eye are not sensitive to all levels. However, it is quite easy to scale or stretch the range of brightness values so that faint objects become brighter, and/or the differences in brightness are amplified.|
|The moon is an excellent target for a digital camera on a telescope. It's possible to use fast exposures and all levels of magnification. Sometimes turbulence in the air causes the image to blur, but another advantage of the digital camera is that you can take lots of pictures and select the best.|
|I only bought my digital camera in February, and there haven't been any planets in a good position for observing since then. Planets are difficult to photograph because they are small and more weakly lit than the moon. Greater magnification tends to spread out the light and to amplify atmospheric distortions. If you have a telescope that gives good views of the planets, then you may be able to get good photos, but expect to take lots of bad ones for each good one.|
|Star clusters are good targets because the stars are point sources of light and do not get spread out by higher magnification. Using the image adding technique described above, it is possible to get good pictures of many of the brighter globular clusters and open clusters.|
|In the July 2000 issue of Sky and Telescope, I noticed an article about measuring the separation and position angle of double stars. The article was based on visual measurements of double stars using a special eyepiece. "My eyes are dim, I cannot see..." but I have found that my digital camera and computer will often substitute for my poor eyesight. In this case, I soon came up with a simple method of using my digital camera to measure double stars. The method at the telescope is extremely simple: aim and focus at a double star, turn the telescope's tracking drive off, and expose for 8 seconds. The result is a pair of star trails as you see here. Later, on the computer, it is also fairly easy to measure the pixel positions of the endpoints of each of the trails. The hard part was translating the mathematics from the visual method into a spreadsheet for the digital method. However, I was very pleased with the results. On my first tries I measured separations within 1 arcsecond (3%) of the published separations and position angles within half a degree!|
|Nebulae are at the limit of what the digital camera and telescope can see. The brightest nebulae do show up using the technique of image adding, but the fainter ones are just too faint to be captured.|
|Most galaxies are also beyond the capability of the digital camera. The image here is an attempt at M81 in Ursa Major. I expect M31, the Andromeda galaxy will be possible but very few others.|
|Astronomers are generally interested in atmospheric phenomena, too.|
|Once I got the idea of aiming the camera into an eyepiece, it seemed logical to try the microscope.|
This page last modified: August 12, 2000