Stress within Glass can exist due to the glass solidifying and cooling from a melt too quickly. The inside will still be amorphous after the surface has solidified, and when the inside finally solidifies and shrinks the outside is pulled in and is now stressed. To save this glass it can be annealed. Annealing is a process by which the glass is heated to proper temperatures and cooled stepwise through proper temperatures for proper times. Each glass, volume and configuration may have specific needs for these temps, times and steps.
We at the SAS ATM do not anneal nor have the concept and wisdom to do so, however Greg Wilhite [www.meniscusmirrors.com], a local mirror blank supplier, does have the kiln and the ability to do this and has done so very successfully.
An indicator that the mirror is under stressed is the illusion of a Maltese Cross within the glass when viewed through polarized light with a polarization filter. Here are some images of glass blanks in a variety of stress levels taken by us using an LCD TV set as a rear illumination and shot through a polarization filter. Many ATMers use the sky as a polarized light source and look at the glass through a polaroid filter. It works, but the intensity and contrast of using an LCD TV is higher. Also, the photos have been enhanced for contrast to increase the view.
This is a 12” f/8 mirror finish in the 1950s by Dr. Clarence Custer of the Stockton Astronomical Society. The figure on this mirror is zoned, under-corrected, rough and unacceptable. We will rework the mirror regardless of the failing stress test shown here. This mirror served Dr. Custer for decades and was used in his famous M31 photograph featured in Sky and Telescope magazine.
James Marino donated this thick and heavy 18” Pyrex blank to be used in the SAS’ Bruce Orvis Memorial Telescope, an 18” Classical Cassegrain with a Naysmith focus to be used for wheelchair-bound individuals. The mirror will first be cored, then ground and figured to an f/1.875 Hindle Sphere for testing the hyperboloidal secondary mirror, then ground and figured again to its final f/3.75 configuration.
Jeff Baldwin’s 6” blank with weird stuff in it. The mirror will be figured mostly as an experiment to see if this type of internal freak-show does anything negative to the stability of the glass.+
Jeff bought this blank of unknown material from Jan Bentz and finally tried to make a mirror out of it. After working the glass three or four times and failing due to astigmatism each time [I mean serious astig] it was annealed at Greg’s shop. This is the strangest internal structure we’ve seen so far.
After the first anneal the main components of the cross are gone. The strange striations in the glass are still present.
After a second anneal, the striations are still present. We decided to work the glass anyways, and this time the astig was prevented. The glass has sat for months, and will be pulled out again to see if there was any change in the figure specifically as a surface of revolution that may be the result of released stress.
Jeff worked 3 12” quartz flats together in 2012, and these are images of flats #2 and #3. The fused quartz were purchased from RECO Labs and are ¾” thick.
Dr. Mike Lavieri obtained a 31.5” quartz blank that Jeff will work probably around 2016. It’s bigger than the TV, but we can still see the stress.
Mike also purchased a 32” blank of borosilicate from Newport Glass. There is more Maltese Cross present than in the locally annealed meniscus mirrors. We’re not sure Newport anneals their glass or if their supplier does. We’re sure the glass will perform excellently as a telescope mirror.
James Schucknecht sent Jeff a 13.1” Coulter mirror to refigure. After many attempts the astigmatism would not go away. The mirror was set aside. We recently tested in for stress even though the back was ground and it is difficult to see through it. Even with a whole lot of contrast enhancement, there isn’t much of a stress problem. So the mirror will be ground flat on the back and retreid. You can see that the mirror was ground flat on the back but there is a bit on the left side left unground. Is that all it took to keep this form working out? Perhaps. The mirror is now really thin, which doesn’t help much.
Buck Turgis from San Francisco brought one of John Dobson’s 32” blank by the shop. It was originally 7” thick, but he had it sawn into two 32” blanks 3.5” thick. One blank was broken and “glued” together, so we denied working on that. However, this blank, even with severe chips, was put in front of the TV for checking for stress. The cross is obvious. Between the cross and the chips, we decided it wouldn’t be a blank to work in the shop. There are interesting stress flow patterns in this blank. The blank may be worked by somebody somewhere, a large fun telescope ought to be made by somebody. This blank was really heavy to lift up onto a stool top 5 feet above the floor to get in front of the TV. It’s a miracle nobody died in this crazy day. What ATMers will do. . .