Expert: Paul Wagner - 11/14/2004

Question
I can not seem to get much info. on this type of newtonian. So I gather it has a corrective lens with a matching mirror or primary mirror.Is this sort of a cheap man's schmidt cassegrain? Does this increase or double the focal length? is there that much of a difference between a schmidt newtonian and a schmidt cassegrain? thank you,g.phelps

Answer
Dear George:

The real advantage of a schmidt Cassegrain is that it provides high power in a small package.  The primary mirror has a very short focal length, but the secondary mirror has a reverse curvature that extends the focal length of the whole instrument.  It directs the focus back out the bottom of the scope through a whole in the middle of the primary mirror.  THe corrector plate really just corrects spherical aberration so the image is tight.  

A Newtonian version of the same scope doesn't have that reverse curvature, so you have a very short focal length scope to begin with--wide field, but very low power.  And then the secondary mirror directs the focus out the side of the scope.  To get the full field of view, you need a fairly large secondary mirror...and this means that it blocks a lot of the light.  It's just not the best combinaton of magnification and light gathering, which is why not many people want one.

Paul Wagner