I enquired about the quality of these mirrors, as I was interested in the 6" f/6 myself, and this is the answer I got:
These are garden variety Guan Sheng mirrors, so the optical quality varies and we don't test them. We occasionally take one and re-figure it to near perfection for use in an Astrograph, but at that bargain price, it is just hit or miss. Right now we are only doing strip and recoat services. No polishing, re-figuring, testing, etc. We are trying to crank out our own telescopes".
Hope this helps in deciding.
Wow. So I understand, is the term "blank" used to refer to untouched glass or a finished mirror? Seems they are being a bit deceptive. I mean I suppose they don't mention polishing and figuring, but they also don't mention that they are just recoating hit or miss GSO mirrors.
THIS WAS NOT THE CASE IN MY EXPERIENCE.
Perhaps there was some confusion of items, etc.
I do not think GSO mirrors are pyrex, probably bk7. The OWL/OSI product I received is thicker than the GSO mirror also.
I have a gso 6in f/6 newtonian ota, and ordered a set of the OWL/OSI mirrors a year or so ago. I talked to them on the phone and they said the mirrors were .95 strehl or better. Stock was being depleted and I got the last 6in f/6 set, I believe as after my order, set was listed as out of stock. When I checked a couple of months ago all of the pyrex sets were out of stock I believe. The primary furnished by OWL is thicker than the oem gso primary. Thick enough to require longer retaining screws on the mirror cell. Came with a interferometer test certificate at .986 strehl and 1/12 wave peak to valley. The mirror they furnished me is definitely NOT anything GSO.
I suggest you call them directly about the Pyrex Parabolic Mirror/sets.
Edited by dmgriff, 28 February 2021 - 12:22 PM.