Key concepts

The first thing to understand is that a focal reducer makes the image smaller -it doesn't make the field wider. It doesn't turn your telescope into a wider-field instrument than it was originally.

It's true that the field of view increases when the image becomes smaller, because more of the image fits on the sensor. But that is only true of the image that the telescope captured in the first place. A focal reducer will not make the telescope see things that were outside its field altogether.

That is one reason some vignetting is almost inevitable with a focal reducer. (Roughly, with a x 0.63 reducer, you'll get the same vignetting on a DSLR sensor that you got on film without it.) Another cause of vignetting is the limited diameter of the reducer itself.

Second, there is always some loss of image quality because, just like any other lens, the focal reducer introduces aberrations. In the best case, when the focal reducer is well matched to the telescope, the resulting image can be quite good. But focal reducers do not automatically produce sharp, aberration-free images. There are always some compromises.

Telescope objective

Telescope objective

Typically 100 to 200 mm m vu

Compressor lens position may be deep within tube

Camera body may not allow lens close enough to sensor

Typically 100 to 200 mm

Normal eyepiece or camera sensor position

Figure 6.1. Not all telescopes can accommodate a focal reducer.

Third, a focal reducer gives its rated magnification only at a specific distance from the sensor. If you put a x 0.63 focal reducer the wrong distance from the sensor, it may come out x 0.5, or x 0.8, or it may fail to form an image at all. After you install a focal reducer, I recommend that you measure how much it actually compresses the image with your setup.

Fourth, a focal reducer requires a lot of back focus. It has to intercept the converging light rays at a position far forward of where an eyepiece or Barlow lens would go (Figure 6.1).

That is why you generally can't use a focal reducer with a Newtonian telescope; it would have to go deep within the tube. The same is true of some refractors. Schmidt-Cassegrains and other catadioptric telescopes work well with focal reducers because they focus by shifting the primary mirror forward, and the shift is greatly magnified by the secondary mirror (Figure 6.2).

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