By Clayton Kessler
April 4, 2001
Almost everyone that owns a telescope gets the desire to take some astrophotos at one time or another. This can be a simple task and provide very satisfying results. While taking long exposures tracking the stars can require some equipment and specialized skills, taking colorful "star trail" photographs, or pictures of the northern lights, requires only a camera and tripod. In fact, taking pictures of the moon or the sun may only require a camera and the telescope that you already have (suitably filtered for the sun), no other fancy stuff required!
There are several common methods of using a camera or camera/telescope combination for astrophotography. These are as follows:
Camera on Tripod:
Your camera is mounted on a photo tripod and a "normal" or "wide angle" lens is used to take non-tracked widefield shots of the sky. This is useful in several ways. A "star trail" shot is a long time exposure that allows the stars to move across the film. This results in a very colorful image of the various stars visible at the time. If you point the camera north or south the curved track of the stars is very apparent. You can also take shorter duration exposures (10 to 30 seconds depending on the lens used) and take very nice constellation shots that show bright stars and very little or no star movement. Photos of the northern lights are also wonderful candidates for the "camera on tripod" method. I like 400 to 800 speed film with a 20 to 45 second exposure for the aurora. Remember to use a "cable release" to trip the camera shutter without shaking the setup.
Try this method to get some satisfying shots without lots of equipment.
Camera "Piggyback" on telescope:
Assuming that you have a telescope that tracks the stars in equatorial mode, you can attach your camera to the outside of the scope and use the camera lens to image the sky. This would be a long duration photograph (10 minutes or longer) similar to the "star trails" previously mentioned except the telescope will move the camera to counteract the motion of the earth and the stars will remain pinpoints on the film. A lifetime could be spent taking wonderful astrophotos using this method and various camera lenses. What about those that do not have a tracking telescope? You can build a very simple device called a "Scotch Mount" or a "Barn Door Tracker" and track your camera with this. This is simply two pieces of wood and a sturdy hinge. A small threaded rod is positioned and turned by hand to provide the tracking. The device is mounted on a camera tripod and the hinge is pointed at the north star. This is a very simple device that can be built for about $10.00. There are many sets of plans available on the internet, just visit the "Amateur Telescope Makers" archives (http://astro.umsystem.edu/atm/) and search on "scotch mount" or "barn door mount".
Afocal projection is a method that uses both the camera lens and the telescope eyepiece. The camera is focused on infinity and merely held up to the eyepiece. This can be a wonderful way to take pictures of the moon and sun with film or with that new digital camera that you just bought. If you are taking pictures of the sun you MUST use a full aperture solar filter or you could ruin your camera, your telescope or YOUR EYES!!!
This method is very easy and is especially applicable to those of you who have a dobsonian mount telescope. The exposures of the moon and sun are short so no tracking is necessary.
If you use film to take photos like this I need to stress the necessity to "bracket" your exposures. This means to try different shutter speeds. Many different shutter speeds will make it easier to find the speed that gives you a nicely exposed photo. Don’t be afraid to use up some film here – film is cheap but good astrophotos are priceless!
Here we go! Prime Focus – use that wacking big telescope of yours for a lens! For "prime focus" photography you remove the camera lens and the telescope eyepiece and mechanically couple the camera body to the telescope. Usually a "T" ring and "T" ring adapter is used to attach the camera to the telescope focuser. The telescope must have a robust equatorial mount and a very accurate drive system to be successful with prime focus astrophotography. How much magnification do you get with a telescope? This depends on the telescope focal length but it is easy to figure out. For a 35mm camera a 50mm lens is considered "normal" or 1X. If you divide the telescope focal length in millimeters by 50mm the result is the magnification. As an example, my 4" refractor has a 600mm focal length. Therefore the magnification at prime focus with a camera is 600mm divided by 50mm or 12 times magnification.
Taking prime focus pictures of the planets can be somewhat disappointing. The images that you get are VERY tiny and have poor resolution due to their image scale. To make the image scale larger a special "T" adapter is acquired that allows you to insert an eyepiece into the optical path between the camera and the telescope. This "projects" a larger image onto the film effectively increasing the focal length of the system. There are tradeoffs…. By spreading the available light out over a larger film area the exposure must become much longer and by increasing the system focal length small guiding errors are magnified along with the image. Despite these problems, robust mechanical systems and a lot of practice will allow you to take some very nice planetary photographs.
If you are considering prime focus or eyepiece projection photography you have progressed beyond the scope of this article on "simple" astrophotography. Not that prime focus is "hard" mind you but it requires specialized equipment and much practice. The skills that you develop taking camera on tripod and piggyback astrophotos will provide a firm footing for a foray into prime focus and eyepiece projection astrophotography.
Good reference material for simple astrophotography is available. I suggest the following books:
"Splendors of the Universe" by Terrence Dickinson and Jack Newton
"Astrophotography for the Amateur - 2nd Edition" by Michael A. Covington
"Widefield Astrophotography" by Robert Reeves