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Hi Jase , Thanks for your thoughtful comments. Regards to fringe point numbers I didn't say less points leads to inconsistant results, I maintain that edge data often doesn't make it into final consideration at all due to lack of sampling density.I used the example interferograms from Peters site becasue re fringe point #, that is what is provided in the interferometry demonstration on that site.
The link to Star Instruments shows a fringes that have been analysed with phase shifting interferometer..which is why there are so many points: companies that amateurs typically buy from use standard fringe analysis . This is still only slightly more than one data point per square cm of mirror area on a 20" which is only going to be able to mathematically model gross surface roughness if present, not compare mirrors with medium and fine scale roughness.
RCOS / Star may well be right that large equatorially mounted Cassegrains using thick 6:1 ratio blanks in Pyrex might find it hard to settle within 1/4 wave if the temperature is moving around. Users of large altazimuth scopes using Pyrex in much thinner ratios of computer designed cells don't have to contend with such thermal issues. In the workshop enviroment at least , I have no trouble stabalising 12:1 ratio Pyrex mirrors to an aspheric correction of 1/10 wavefront which is the point in th eprocess I stop quantitative measurements of residual spherical aberation shadows and concentrate on achieving an excellent optical Null.
My point about the validity of evaluating good mirrors from excellent mirrors, by eye, is that the more subtle features such as edges and surface smoothness are just not necessarily getting picked up by interferometry and reflected in the calculated Strehl ratio, but will be obvious in a star test on a good night. Traditional methods of knife edge and eyepiece under optical Null are ample to give quantitative and qualitative measure of a mirrors quality. Interferometry IMHO is icing on the cake for any company that knows what it is doing and been around for along while.
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