Interpreting the Optical Quality Tests

[John]

This piece has recently been posted on the "Cloudynights" forum which provides an interesting summary on interpreting optical test data:

https://www.cloudynights.com/articles/cat/articles/reading-optical-measurement-data-r3116

Of course the best test is with your own eyes under the stars

[DirkSteele]

Thanks for sharing that. Very succinct yet comprehensive summary of a test certificate.

[Tim]

Thanks John.

Perhaps worth pointing out that a seemingly great or even near perfect Zygo report might actually bear no relation to the quality of the views provided by a given mirror. The report on my mirror before it was refigured claimed it had no astigmatism, well that was just pure fiction

 As you say, a star test at high power with your own eyes is the only really useful measure of quality.

[John]

2 minutes ago, Tim said:

Thanks John.

Perhaps worth pointing out that a seemingly great or even near perfect Zygo report might actually bear no relation to the quality of the views provided by a given mirror. The report on my mirror before it was refigured claimed it had no astigmatism, well that was just pure fiction

 As you say, a star test at high power with your own eyes is the only really useful measure of quality.

I agree Tim. I believe some of the best mirror manufacturers in the world (eg: Carl Zambuto) don't supply optical test reports - they rely on their reputation

 

[John]

I suppose this does beg the question though, are the reports that are supplied with some scopes of any accuracy or value at all ?

 

[Macavity]

I think I learned some useful stuff simply by reading through a LOT of results
on a BLOG devoted to the optical testing of certain well known telescopes!

Even the BEST of manufacturers could produce the odd "dog" sometimes. 
Also things can "slip a bit"... Achromatic Triplets can be tweaked (Egads!) ...
Among similar-spec scopes, some can be quite a LOT better than others etc.
If I were in the market for top class optics, I would rather LIKE a report!

Of course, the best optics can be compromised by poor mechanics / setup.
Perhaps the reason why I spent (far too much!) time tweaking things? I'm
quite a fan of experimental measurements though! But as someone once
said of "science students"... Most suffer from "delusions of accuracy". lol 

[niallk]

Wouldn't it be kinda interesting if in addition to an interferometry report, data from a proper artificial star test was also included (with say 1/30~ reference secondary)?  Of course, then you are into the more involved topic of the correct interpretation of a star test...!  Cue many a heated debate on CN

Guaranteeing 50x per inch is also an appealing, tangible metric... though personally I've never observed with my 15" at 750x!!  One reads of 1000x being used in Florida for example on Jupiter.  The most I've used is 560x with the image holding up once, 330x more usually the max on a very good night tbh, and cooling and collimation are big factors if the atmosphere is sufficiently stable.

Reading around and getting a sense of consensus reputation is probably a pragmatic approach.  Mr Rohr's site is quite interesting too.

[John]

I use 300x - 400x with my 12" dob when the target and seeing make it worthwhile. The optics seem to hold up well on such occasions. The highest I've been is 795x on Uranus when trying to tease the uranian moons from the background sky. Oh boy do you need to be on the ball tracking with a 50 degree AFoV at that power ! - the Earth seems to be spinning so fast !!!

[joelshort]

Thanks for posting the link!  I just received a new CFF 250 RC and of course it came with a test report...now I know how to read it.  And it looks good! 

[Timebandit]

24 minutes ago, joelshort said:

Thanks for posting the link!  I just received a new CFF 250 RC and of course it came with a test report...now I know how to read it.  And it looks good! 

 

 

From what seems to becoming apparent, a report can be a informative and reassuring document. But the ultimate test of the optical glass/mirror quality is a star test and your personal experience of the optical quality on views received from observation session in the chosen scope.