Types of coupling

Figure 5.2 shows, from an optical viewpoint, how to couple a camera to a telescope. Optical details and calculations are given in Astrophotography for the Amateur and in Table 5.1.

Camera with its own lens

Piggybacking

Direct coupling

(Prime focus)

Eyepiece

Eyepiece

Direct Astrophoto Focus
Concave lens to increase image size

Afocal coupling

Positive projection

(Eyepiece projection)

Negative projection

Compression

(Focal reducer)

Convex lens to reduce image size

Figure 5.2. Ways of coupling cameras to telescopes. Piggybacking, direct coupling, and compression are main modes for deep-sky work.

Not all of these modes work equally well, for several reasons. First, DSLRs excel at deep-sky work, not lunar and planetary imaging. Accordingly, we want a bright, wide-field image. That means we normally leave the focal length and f-ratio of the telescope unchanged (with direct coupling) or reduce them (with compression). The modes that magnify the image and make it dimmer -positive and negative projection and, usually, afocal coupling - are of less interest.

Table 5.1 Basic calculations for camera-to-telescope coupling.

Direct coupling

Focal length of system = focal length of telescope f -ratio of system = f -ratio of telescope

Afocal coupling focal length of camera lens

focal length of eyepiece

Focal length of system = focal length of telescope x projection magnification f -ratio of system = f -ratio of telescope x projection magnification

Positive projection, negative projection, and compression

If you get negative numbers, treat them as positive.

A = distance from projection lens to sensor or film F _ focal length of projection lens

(as a positive number even with a negative lens) Projection magnification = Focal length of system = focal length of telescope x projection magnification f -ratio of system = f -ratio of telescope x projection magnification

Figure 5.3. Why some Newtonians won't work direct-coupled to a camera. The solution is to use positive projection, or else modify telescope by moving the mirror forward in the tube.

Second, if you do want to increase the focal length, positive projection (eyepiece projection) is seldom the best way to do it. Positive projection increases the field curvature that is already our primary optical problem. Negative projection, with a Barlow lens in the telescope or a teleconverter on the camera, works much better. The appeal of positive projection is that, like afocal coupling, it works with any telescope that will take an eyepiece; you don't have to worry about the position of the focal plane (Figure 5.3). Indeed, positive projection with a 32- or 40-mm eyepiece can give a projection magnification near or below 1.0, equivalent to direct coupling or compression.

Regarding negative projection, note two things. First, a teleconverter on the camera has a convenient, known, magnification, and the optical quality can be superb, but the DSLR may refuse to open the shutter if the electrical system in the teleconverter isn't connected to a camera lens. The cure is to use thin tape to cover the contacts connecting the teleconverter to the camera body.

Second, Barlow lenses make excellent negative projection lenses, but the magnification is not the same as with an eyepiece; it is greater, and the best way to measure it is to experiment. The reason is the depth of the camera body, putting the sensor appreciably farther from the Barlow lens than an eyepiece would be.

Compression is the opposite of negative projection. Meade and Celestron make focal reducers (compressors) designed to work with their Schmidt-Cassegrain telescopes. These devices work even better with DSLRs than with film SLRs, and they are very popular with DSLR astrophotographers. As well as making the image smaller and brighter, they help to flatten the field.

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