[ausastronomer (John Bambury)]

Originally Posted by casstony

Thanks John . Clear as mud Alex .

John, I'm wondering if you're able to confirm something else for me - that the curved field of commercial SCT's is of opposite sign to Newts?
Here's my thinking: the wavefront leaves the primary mirror curved in the same direction as a Newt; the secondary has a shorter radius of curvature and therefore produces stronger curvature than the primary; since the secondary is convex it changes the wavefront to the opposite sign.

Hi Tony,

My apologies for the delay in replying. I wanted to check some of my textbooks before I replied. Unfortunately they haven't given me all the answers we are seeking. As Wavytone has pointed out it gets pretty complicated. Also remember I have a Commerce Law degree and run a large Club. I have been an amateur astronomer for many years with an "interest" in optical design. This is really getting out of my league into the league of professional opticians and mathematicians.

Here is a good starting point you can read on telescope field curvature

http://www.telescope-optics.net/curvature.htm

Here is a further section on Schmidt Cassegrains and there own inherent aberrations (off axis)

http://www.telescope-optics.net/SCT_...berrations.htm

Essentially, for a each curved surface of the telescope you will have +ve field curvature from a concave reflecting surface and -ve field curvature for a convex reflecting surface

On face value your assumption seems correct looking at the formulas in the links above. As the secondary (convex) in a SCT has a shorter Radius of Curvature (ROC) than the primary (concave), one could reasonably assume the secondary would apply more -ve field curvature than the +ve field curvature induced by the primary, with the net result being -ve field curvature, which is the opposite to a newtonian with it's single concave reflecting surface. I wasn't satisfied with this because my own field testing of the 14mm and 20mm Pentax XW's in several different SCT's indicates that the +ve field curvature of those eyepieces is "compounded" by the telescope or at best, not really improved. In fact it gets further complicated with the field curvature induced by the aplanatic Schmidt corrector. You cannot determine the field curvature of the corrector without knowing the curves used in it's design. I don't know where those numbers lie for the commercial SCT's running around today.

So the short answer is, in theory you might be right. In practice that's not what my eyes tell me and you would need to know the curves of the aplanatic Schmidt corrector and the ROC of both mirrors to work it out.

I am going to give Mark Suchting a call and ask him to have a look at this thread. If anyone on IIS knows the answers, Mark will.

Cheers,
John B