Quote:
Originally Posted by Peter Ward
Bollocks! 
That is the whole point of the Strehl ratio. Provided sufficient data points are measured (typically 2500+) all aberrations are indeed accounted for and quantified.
The star test is a great *qualitative* test for the optic when it is out of focus...what happens in focus is much more valuable  |
Peter wrote : "Bollocks!
" Well bollocks to you too !
and Happy New year
Only 100 to 200 points ( not 2500+) are typically fitted which is not enough point density at the edge to define problems during the polynomial fitting ( count the number of points fitted on the two interferograms on your web page). Note also that the surface micro roughness in the C8 interferogram is not picked up by the fringe fitting, as it is completely invisible in the 3D wavefronty map, all that are obvious in the wavefront map are primary and higher order Seidal aberrations. .Polynomial fitting to interferograms usually ignors surface roughness and highly sloped edge problems. I can post interferograms to illustrate these issues.
I note that astigmatism has been switched off from both data sets presented . Its rare that consumer interferograms even mention if astigmatism has been subtracted from the data.
Of tests available to the end user , only a star test or ronchi test under excellent seeing will generally show how good the edge is and if the surface is smooth showing nicely defined fresnel rings with the star defoccused. Two mirrors can have equal high Strehl ratio interferograms but only star testing reveals the `gem' which will reveal the better low contrast planetary detail on a superb night. Remember that a Strehl ratio , even in its idealised form, only tells you about the light concentration in the Airy disc, and nothing about the distribution of light that doesn't make it.
I've learnt this through 20 years experience with interferometry and star testing and have tested a number of mirrors that didn't always meet their owners expectatons but looked good `on paper' . As always let your eyes be the final judge of quality .