Do you need an interferogram purchasing a star dia

I just have heard to my dismay this morning from a premier diagonal glass optical manufacturer how my (originally very expensive) 2" enhanced silver mirror I sent for recoating (it developed areas of marring) did on an interferometric test. So called 1/20th mirror had only the sweet spot of 1/4th wave, the rest was worse. He suggested to dump it. I might look for a glass only replacement...I will send an anonymized copy of the interferogram to the original manufacturer for his comments but do not expect much based on previous contacts with him.

This confirms the results Vic Maris obtained when he measured interferometrically comparable market offerings during development of his SV diagonal.
There are now many options on the star diagonal market, more than 1-2 years ago. But nobody to my knowledge will guarantee you the stated optical performance with an interferogram.

David

Vernonscope quartz diagonals are individually tested. I don't think I got a copy of the test, but I did get the test results. Mine was 1/26 wave.

David,
That sucks! I can say that this is not too uncommon with many mirrors period. A friend has a shop that makes elliptical flats. He bought a high dollar scope that I helped him find. He tested the secondary elliptical flat just to see what it was like. It was 1/7 wave! Now, that is not too bad, but it was not anywhere near as good as he and I assumed these mirrors should be. He replaced his with a 1/30 wave quartz and sent me a 1/30 wave for my scope too, just to be sure of the quality. I did not have mine tested, I just put the new one in the scope.
Perhaps you could buy a good mirror? Maybe one of the good makers will sell a replacement, if they are standard sized that is.
If not, how about contacting the vendor that you purchased it from and asking if they can help you. Maybe they will send you a new one?

My INTES 2" diagonal did come with an interferogram - I just sold it here on A-Mart - 1/25 wave flat. I believe INTES offered this through APM and probably ITE back when they were still around.

That said, I can see NO difference visually between my typical 1/4 wave (guestimate) Orion one that is going on 15-18 years old, my 1/10 wave WO Dielectric, or the old INTES. I would guess because the light cone is so narrow by the time it hits the diagonal it has a much less effect visually.

Hello:
By 2" diagonal, I assume you mean the kind used in cassegrains and refractors, not newtonians. If so, 1/4 wave can be just fine. I suspect that such a diagonal can be off by a full wave or more and still give very good image quality. The reason is that the mirror is typically used in slow systems (refractors and cassegrains are typically f/8 and slower) and close to the focus, so the beam is only about 1/2" to 1/4" in diameter. If a 2" mirror is 1/4 wave over it's whole surface, over the area that's acutally used by a star it's more like 1/16 to 1/32 wave. Serious optical problems with that mirror will look more like distortion in the field of view rather than as bad image quality of a point source. As long as the mirror is smooth, a crappy surface figure can be okay. So, I wouldn't dump the mirror. From reading your post, it sounds like you were quite happy with it's performance before you hear the news. The tolerances of an optic strongly depend on where it is used in the system.

Cheers
Mike Connelley