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Six inch f6 Mirror

Started by Kevin Barker, 02/22/2005 10:57AM
Posted 02/22/2005 10:57AM | Edited 02/22/2005 10:58AM Opening Post
Hi all,
Attached is a Jpeg of a test on a six inch f-4 mirror from an RFT I own.

I am not sure how you read these so please comment.

Many thanks

Kevin Barker

Attached Image:

Kevin Barker's attachment for post 22429
Posted 02/22/2005 02:56PM #1
Hi Kevin,

The Ronchi is not really accurate at the r.o.c. of fast parabolas. The reason is that they suffer from significant spherical aberration with objects placed so close. In fact, a parabola at r.o.c. has as much of wavefront error s.a. as a comparable sphere at infinity focus (only of opposite sign). This causes significant distortion of Ronchi lines with even perfect parabolas, and relatively small additional deformation caused by comparatively much smaller deviation from perfect is often times hard to measure with sufficient accuracy.

This particular mirror, if perfect parabola, would have 2.1 wave p-v of overcorrection at the r.o.c. (from 0.89D/F^3, for D in mm). If you want to try to figure out possible errors from the Ronchigram, you can use one of available softwares at

If you go with Peter Smith's software (simple and user friendly), just remember that this parabola, if perfect, should have 2.1 wave of overcorrection
at the r.o.c. So if you get, for instance, 1.9 wave, that indicates figure error resulting in 0.2 wave of s.a. (undercorrection).

Still, probably more accurate result could be obtained with star testing the mirror.


Posted 02/22/2005 03:33PM #2
Just wondering which it is. (f/6 or f/4) Also, you need to mention is this from inside the RoC or outside? My hope is this is from inside the Radius of Curvature. If so the mirror does have some correction. Too much or two little, it is hard to say.