Do you believe this

Description: 16" f/4.5

Mirror Thickness 1.625"
Substrate: Pyrex
Focal Length 72.494"
Exact F ratio 4.567

Test Results
RMS Surface Error 1/127.67 wv.
P-V Surface Error 1/34.38 wv.
Strehl Ratio 0.990
Encircled Energy Ratio 0.989
Test Wavelength 635 nm
Analysis Wavelength 550 nm
Design Conic -1.00
Only 1500US advertised on astromart.

I'm extremely skeptical of this being represented as the actual quality of the optics.

I'd be impressed if someone **randomly**(not the ones used) chose say 6 zones tested with a couder screen and it came in at better than 1/6th wave.

I now produce mirrors on a limited scale say 6 a year for local astronomers 6-16 inches in aperture in NZ and only ever claim they are a smooth consistent 1/8th wave on the wavefront.


Interested in what others think. Inwonder if this outfit can really walk on water.

Phil Barker

Phil, you and I both know there "ain't no way in h***!"
Maybe one in every 1000 mirrors will come somewhat close, but I don't how someone could advertise those results as typical.

Walt

If you'd ask them to quote how much it would cost to make such mirror for you (expecting that you could adequately test it), the price would be much higher. And they'd probably let you know that it can't be done, if for no other reason, because this big chunk of (this thin) glass itself is not stable enough to support nothing close to this level of surface accuracy. But for the amatuer market, where such testing is very unlikely, they think can let figures like these fly. I don't see it as clever; rather the opposite.

Btw, the Strehl doesn't match the RMS (or the other way around). The 0.990 Strehl corresponds to 1/63 wave RMS, and the 1/128 wave RMS would result in 0.998 (tiny bit less) Strehl. Even if the Strehl is calculated for the 550nm wavelength (with the RMS being for 635nm), it is still 0.997 for the given RMS figure.

Phil,
You should check out obsidian optics website. Mark Cowan posts mirror specs smoother than 1/20th pvw and even 1/30th.
I won one of his mirrors on ebay almost 17 months ago, but have nothing thus far so I can not give a review one way or the other. Really smooth figures on paper though!


Todd