Focal reducer.

[The Titan Calibos]

Besides the obvious ..."reducing the focal length of the scope" what does it actually do..eg visually and photographically.What difference can actually be seen?

[rfdesigner]

Visually, it means you need shorter eyepieces to get the same field of view.. or that your longer eyepieces will give a wider field of view.

Photogrphically it means, in arc-seconds, bigger pixels for any CCD. For a given number of pixels, small pixels are cheaper than big ones.

So, take a F10 Schmidt Cassegrain, vs a F5 Newt. The F5 will produce a certain image with 5um pixels, while the F10 Schmidt-Cass will need 10um pixels... and FOUR times the area of chip to produce the same image.

On top of that small pixels can come with low noise and high quantum efficiency, I notice this seems to be less true of larger pixel sensors. With a 10" F5 Newt, a ~6um pixel .. ish... is approximately optimal.. interestingly this is very close to DSLR pixel sizes.

It's only when you come to planetary imaging that schmit-cassegrain scopes really come into their own.. or you can use a newt+barlow.

A Newt + focal reducer ends up being a 'super newt' which effectively gives huge pixels.. which allow very short sub-frames (popular)

Derek

(yes.. I've got 3 newts)