folded path, plus

Oooh. Now I get it.

I just learned a mystery of catadioptrics.

I've always wondered about this. And I recall chatting briefly with someone about this last summer: how can an 8-inch Celestron Schmidt-Cassegrain have a 2000 mm focal length?! When it is only about 40 cm long?!

Rod Mollise explains this in the early pages of his SCT book...

It is not just that the light path is folded 3 times inside the tube. When you do the rough calculations of that, you get 3 x 400 mm. That's still not enough to accommodate for the quoted amount.

It is because the secondary mirror is a spherical lens. As such, it has a magnifying property. So the focal length is extended by the secondary. That gives the extra amount.

Puzzle solved!

Now if I can only remember who I was taking to about this...