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Hi Rugo,
I agree with Jase. Mirror quality documents really are an absolute lucky dip and don't tell you as much as some would have you believe. They are more a tool for the optician making the mirror rather than the user IMO.
Some mirror makers issue a certificate quoting a "theoretical strehl ratio" based on zonal focault testing, others issue a certificate quoting Peak to Valley accuracy and a few, who have access to very expensive test equipment, issue certification based on full interferometer testing. Others use a Ross Null Test (Steve Swayze in the USA is one) to test the quality of the mirror without producing certification.
All of the above test methods can be used to make outstanding mirrors. However, they can't be used for comparing the quality of mirrors calculated with different test methods. Even the same type of testing conducted under different test conditions by a different optician can produce different results, particularly with regard to larger mirrors which can suffer flexure during testing. For example Galaxy Optics and OMI Torus, the suppliers of the mirrors for Obsession Telescopes are large companies and use full interferometer testing. An interferometer measures for many different types of aberrations and across the full face of the mirror, using "hundreds" of data sampling points. It is the "toughest" test of any mirror. Any mirror having a strehl over .90 or an RMS wavefront error less than 1/20th wave RMS, will be excellent, provided it is smooth.
On the other hand zonal focault testing measures for spherical aberration only, over a handfull of data points, usually less than 10. The optician then uses this data to estrapolate theoretical strehl and RMS values. This method will ALWAYS give higher Strehl and lower RMS than an interferometer because it assumes rotational symetry of the mirror which never happens in practice.
You could have a mirror with a "theoretical" strehl of .98 calculated with zonal focault testing that may only have a strehl of .90 when tested on an interferometer. Hence its pretty pointless comparing numbers from one mirror maker to the next. If the mirror maker has a good reputation, it's likely your friend will get a very good mirror, regardless of the test methods and certification used.
What is the size and F-Ratio of the mirror and who is making it ?
CS-John B
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