Optical configurations

Figure 7.2 shows six ways to attach a camera to a telescope. One of them, piggybacking, does not take pictures through the telescope at all; instead, the camera

Camera with its ' own lens


Camera body (no lens)

Direct coupling (Prime focus)




Concave lens to increase image size


Camera body (no lens)

Concave lens to increase image size


Positive projection

Negative projection


Convex lens to reduce image size

Figure 7.2. Six ways to attach a camera to a telescope.

takes a long exposure through its own lens while the telescope tracks the stars. The other five methods use the telescope as a giant telephoto lens.

The direct or prime focus method is the simplest. The telescope, without an eyepiece, goes onto the camera, without a lens. Most refractors, Schmidt-Cassegrains, and Maksutov-Cassegrains work well in this configuration, but many Newtonians will not reach focus because the image plane is not far enough from the end of the eyepiece tube.1

The afocal method works with any telescope and any camera. The camera, with its lens, is aimed into the eyepiece of the telescope. This is a simple and

1 The term prime focus is potentially confusing because, on large observatory telescopes, it refers to the focus of the main mirror, as opposed to the Newtonian or Cassegrain focus. Now that some amateur telescopes, such as Celestron's Fastar system, can be used this way, I prefer the term direct coupling or direct method. I thank Lenny Abbey for pointing out the potential for confusion.

foolproof system, especially with digital and video cameras, but I also use it often with 35-mm SLRs.

In afocal coupling, the camera lens is always wide open (at its lowest-numbered f -stop) and focused on infinity. Digital and other autofocus cameras should be locked on infinity focus. The camera can be held in place with a bracket or can stand on its own tripod. When photographing the Moon with a short exposure, you can even handhold the camera.

Positive projection (eyepiece projection) is one way to get greater magnification than with the direct method. The telescope eyepiece is in place, but the camera, behind it, has no lens. Although popular with amateurs, this method, in my experience, does not give very good optical quality; images are usually sharp only at the center, a fact that is often obvious in images of the Moon.

Like afocal coupling and unlike the direct method, positive projection works well even with Newtonians and other telescopes that cannot put the image plane very far past the end of the eyepiece tube.

Negative projection is like the direct method with a Barlow lens (p. 92) or teleconverter added for extra magnification. (A teleconverter is, after all, nothing but a Barlow lens for a camera.) I find it much better than positive projection for imaging the Moon and planets.

Compression is simply the direct method with a focal reducer added to reduce the magnification and brighten the image (p. 93). It is often used in deep-sky work.

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