How to actually measure spherical aberrations?

[Merlin66]

How do you actually measure the SA of your objective??
Do I need to do a focused star image test with the outside annulus, the central region being covered and compare the focus position with a similar test just using the central region??

[Shiraz]

maybe the easiest way is to get Suiter's book and compare star test images with his illustrations - it's not precise by any means, but can at least give you a ballpark idea of what your optics does.
http://www.amazon.com/Star-Testing-Astronomical-Telescopes-Evaluation/dp/0943396905

[bratislav]

Record in and out of focus images of a star with a webcam/planetary camera and run the Roddier. If your objective is not an APO, or is a fast APO, use a green filter (I usually use an OIII). Spherical aberration is Zernike 11 (third order, which is the "main" spherical) and Z22 is higher order spherical (zones). It will also tell you everything else you want to know about your lens (astigmatism, coma, even color correction if you use appropriate filters)

http://www.astrosurf.com/tests/roddier/roddier.htm

(use Google translate).

For example, this is the result of my 102/714 triplet, after collimation and centering.

[Merlin66]

Bratislav,
Thanks for that!!
Sounds like the solution I was searching for.
Much appreciated!

[bratislav]

Ken, there is a yahoo newsgroup dedicated to Roddier testing. It has many useful pointers on how to do the test (bottom line is you need a FITS file, and contrast and sharpening must not be manipulated in any way). There are even English versions of program itself, and latest versions are far more robust to input data variations (the original was painfully intolerant to any extra data in the FITS header, even to the order in the header itself, it required precise amount of defocus in both directions etc. etc. New versions from 2.2 onwards are far more civilised).