Most common mirror getting refigured?
Posted 05 July 2020 - 10:51 PM
Mostly ATM who gave up, or professional?
Mostly mass produced, or "premium" owners with high standards?
What size, thickness, and f# is most commonly getting refigured?
Do people typically think it is cheaper to get a good mirror by buying another new one? Someone with a custom size would have to refigure.
Posted 06 July 2020 - 05:47 AM
From my experience: mostly mass produced ones that perform really badly. Unfortunately, I've personally star-tested a couple of 12" and 16" Meade LightBridges that were really horrible (really unpleasant even at 100x) and were a nightmare to refigure (in both cases they are re-annealed and so saddle shaped after re-annealing that they had to be refigured from scratch; one of them even had to have its back made flat again).
But even some supposedly less "mass produced" mirrors get refigured. Both my Oldham Optical (Strehl ratio of 0.57 before refigure, due to a raised centre and some nasty astigmatism that was caused by stresses in the blank) and my Orion Optics "standard grade" (Strehl ratio almost acceptable, but extremely rough and slightly astigmatic, and I absolutely hate that when focusing) got refigured.
The refigurer of the OO liked a challenge and did not re-anneal the OO but he spent ages removing some four lobed astigmatism close to the edge until he was satisfied. The refigurer for the Oldham Optical flatly refused to work the mirror as is after having looked at the stresses in the blank (he values his sanity more).
Surprisingly, it's my personal "mass produced" mirrors that are not refigured -- a 130mm Skywatcher that is fairly good except for a raised centre that is completely in the shadow of the (albeit fairly large) central obstruction. I also had two StarBlasts that were quite good, at least after combined with an eyepiece combo that didn't generate psherical aberration at f/4.
Of course, I should mention that I've also just sent back one mass produced mirror too (a cheaper Celestron Astromaster 130mm tube that had a mirror that was so undercorrected it could just as well have been spherical).
Posted 06 July 2020 - 08:55 AM
Ever since Corning stopped making Pyrex blanks I've wondered about the quality of blanks now being sold. It's been decades since I last ground, polished and figured my own mirrors.
So, judging from your experience, I suppose it would be wise to keep my stash of decades old fine annealed Corning Pyrex blanks if I ever hope to make another decent mirror. Got it! Thank you.
Posted 06 July 2020 - 09:29 AM
Oh, you can still get good blanks. But Suprax blanks and BK7-like Chinese blanks are iffy, unless the mirror maker (or blank supplier) was smart enough not to cut corners and re-annealed them properly. As for Corninc Pyrex, there's nothing inherently better or worse about that material than e.g. Schott Supremax (or borofloat if you can get away and want to work with thinner blanks).
Plus, re-annealing is not the end of the world (at least not here, where we have someone bending over backwards to help ATMers and is very good at it).
There are large differences here,
-from the "time is money" mirror makers (the BK7 blanks are cut from sheets and all have stresses; it's a wonder that the GSO supplier manages to make any mirror without gobs of astigmatism, but they do manage at least some of the time ;-) ),
-to the "we'll just assume the blanks are OK" manufacturers (Oldham Optical and Orion Optics seems to be in that vein, and the assumptions don't always hold; my experience tells me you really don't want to buy an Orion Optics "standard grade" as a result),
-to the "I'll assume it's OK but I'll check for stresses and/or if the blank gives me trouble I'll catch it and re-anneal" manufacturers (John Nichol seems to work that way, at least looking at the finshed products made out of Suprax),
-to the "I'll buy from a source that only gives good blanks" manufacturers,
-to those that simply re-anneal even every high quality Supremax Schott blank because they want to take NO risks and they only trust themselves.
Edited by sixela, 06 July 2020 - 09:34 AM.
Posted 06 July 2020 - 09:50 AM
"-to those that simply re-anneal even every high quality Supremax Schott blank because they want to take NO risks and they only trust themselves."
That's the type of manufacturer that I'd prefer to buy from.
Edited by Richard O'Neill, 06 July 2020 - 10:04 AM.