Quote:
Originally Posted by Satchmo
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).
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While I do agree with you Mark, not enough points can lead to inconsistent output of the interferogram data. I would be brave enough to say that when entering into the high quality optical market, there are few that provide only 100 to 200 point maps. Those that do, you'd surely be foolish to trust given you're probably about to hand over a considerable sum of money and you'd want quantitative evidence of the optical performance.
Star Instruments who are one of the leading manufacturers of Ritchie Chretien and Cassegrain mirror sets perform a 2500+ point test as indicated by Peter. These mirrors are deployed in the RCOS series.
Information on one of their 20" mirror sets can be seen here -
http://www.star-instruments.com/interfero.html - Note fringe analysis sheet - Number of pts.
An interesting statement on the Star Instruments site:
"
STAR INSTRUMENTS guarantees a minimum of 1/4 wave front, 1/20 wave r.m.s. on all systems.
STAR INSTRUMENTS continues to be concerned with the false advertising claims being made by amateur optical suppliers who claim 1/10 to 1/20 wave optics. These claims tend to confuse the amateur astronomer into believing you must have 1/10 wave optics, when in reality there are very few 1/4 wave 8" and larger optical systems. The fact is that Pyrex is not a zero expansion glass and, therefore, cannot keep a figure better than 1/4 wave front."
So I wonder how many commercial instrument manufactures out there that are using Pyrex and making such false claims... With the term "diffraction limited" meaning absolutely anything (well almost), it makes it difficult for a future purchaser to make the right decision.
I've read the book Star Testing Astronomical Telescope by Harold Suiter. Great read for those interesting in optical testing, however for someone after quantitative optical figures it leaves a little to be desired. Is the CCD camera not more discerning than the eye? I'm surprised by your statement that the only way to validate optical quality is the eye.