I'll repeat the question - how come that many of big (and presumably heavy) mirrors Rohr still tested at 0.98+ Strehl ? Apparently, large mirrors can
be held in vertical (or nearly vertical) position and not show astigmatism. After all, that is what most Dobs do, day after day (in fact they can't even be used with mirror in horizontal position. It's called Dobson's hole).
I have a lot of respect for Roland, but in this case he's just plain wrong. [/quote]
Here is what has to say about Rohr's testing:
"I would be leery of tests done by Wolfgand Rohr. His test setup is not
ideal for testing telescope optics since he tests everything in a horizontal
position. There is also a question in my mind about the components of his
interferometer, since they seem to be home made and lack any kind of
certification. I know that for my own interferometer I initially used a
splitter which introduced mild astigmatism into the measurements, and I had
to replace it with a certified 1/30 wave hand crafted beam splitter from a
company in NY that specializes in interferometer optics.
Personally I have not seen any astigmatism caused by storing oiled optics
Since he does not post on CN, I would recommend that if you disagree with him, you give him a chance to defend himself on his AP Yahoo group.
Tom Mack [/quote]
Since Roland didn't post it here either, it would be totally wrong to jump over to ap_ug and hit him over the head with it. But look at Rohr's test of an AP StarFire 155 EDT: 1/10.4 wave and 1/55 wave (0.987 Strehl).
For comparison here's what A-P promises:
[quote]A lens is finished when it displays a wavefront accuracy of l/50 RMS (Strehl Ratio of 98.4%).[/quote]
So do you still think Rohr is a mere amateur?
He doesn't only provide interferometry results but Foucault-grams, Ronchi grams, star test images and Lyot-tests (to measure surface roughness). Hats off to him for dispelling a lot of hype and smoke.