If you have a telescope, you can use it with almost any kind of digital camera to shoot close-up photos of the craters on the moon. The telescope doesn't even have to be on an equatorial mounting, or driven with motors or a computer to follow the stars.

The lunar crater in the center of this photo is Clavius. It was taken with a Nikon D1H DSLR camera with a 50mm lens working at f/2.8 held up to the 18mm eyepiece in a 12.5 inch f/6 Newtonian telescope, and exposed for 1/60th of a second at ISO 800.

This image was taken through a non-tracking 12.5 inch f/6 Newtonian reflecting telescope on a Dobsonian mount by afocal projection. A DSLR camera with a 50mm lens on it was simply held up to the eyepiece in the telescope and exposed for 1/60th of a second at ISO 800.

You don't even need a DSLR to take afocal shots, you can use a digital snapshot camera. If your camera does not have a removable lens, the afocal method will be the only way you will be able to take pictures through a telescope.

If you are interested in this kind of astrophotography, you can make it much easier by buying an adapter from Scoptronix or CNC Parts Supply that holds the camera at the eyepiece of the scope, but you don't absolutely have to have one to take afocal pictures.

Method

Afocal photography

Focus the telescope at infinity. Focus the camera lens at infinity.

The camera is held up to the eyepiece and the photograph is made. Care must be taken to hold the camera so that the focal plane is perpendicular to the optical axis and it does not move during the exposure. The camera should be at the focal length of the camera lens away from the eyepiece so that the exit pupil of the scope is located at the iris diaphragm of the lens.

For long focal length eyepieces, it may be a problem making sure the eyepiece is focused on infinity because of the eye's accommodation. To solve this problem, a finder scope or binocular can be focused on infinity first, and then held up to the telescope eyepiece. The telescope eyepiece is then focused at infinity through this auxiliary instrument.

The equivalent focal length of the system is the telescope's magnification multiplied by the focal length of the camera lens.

For example, the picture above was made with a 12.5 inch f/6 telescope. That is 75 inches of focal length, or 1905 millimeters. The telescope eyepiece had 18mm of focal length. The magnification through the eyepiece was 1905/18 = 105x. The equivalent focal length of the entire system: telescope, eyepiece, and camera lens is 105 * 50 = 5,250 millimeters.

The focal ratio of an afocal system is determined in the usual manner, the focal length divided by the diameter of the objective. In this example, the focal length is 5,250 millimeters divided by the diameter of the objective, which is 12.5 inches or 317.5 millimeters. The formula gives us 5,250 / 317.5 = 16.5. So the focal ratio of the system is f/16.5.

Remember that exposures get longer as the focal ratio of the optical system increases. For objects such as the moon, which fill the frame, you can simply take the exposure at the camera's meter reading. For other objects, you can make a guess and take a test exposure and examine it on the LCD on the back of the camera.

Vignetting

Vignetting

With many afocal setups for photography, you may experience vignetting. This will make the image seem like it is being viewed through a tunnel. It is caused by the light cone that reaches the sensor not being fully illuminated.

To reduce vignetting, use an eyepiece with a large lens that is not recessed. Use an eyepiece with large eye relief, such as a Plössl . Try moving the camera lens closer to the eyepiece. If you are using a DSLR with a lens that has a lens shade on it, take it off. If you are using a digital snapshot camera, try zooming it in.

If you can't remove the vignetting and you find it objectionable in the image, you can always crop it out later when you process or print the image.