[QUOTE=Satchmo;864058]Steven
The 36" X 1" thick mirror he made gave apparently fair images for about 5 minutes each night as it passed from under to overcorrected at sunset.
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Hi Mark,
I always do my optical testing in the evening, (outside), to more accurately track the actual observing temperatures the mirrors would experience when pressed into service. In this, I expect performance to be at its best, at full cool down (around 5deg C).
Why so thin? "Budgetary restraint". This being a "raw" experiment,as I have limited finances, but time on hand, I decided to build a large aperture dob with a view to limiting amount spent to around $1500.
First I looked to obtain the mirror blank. My research came up with several options, most not cheap. Some options: 20.5" x1.25" quartz blank A$2000+ with freight extra several hundred $, so out of the question. Another option was a 30" x 2" borofloat blank out of China US$5000
plus freight. Then there was the option of a 600mm(23.6") x25mm plate glass blank out of Oz for around NZ$1000 delivered (and these blanks have very poor anneal-cant even be cut conventionally without shattering), so out of the question.
Finally settled on a 25.2" x 3/4" float glass(origin:Belgium) very good anneal,flat (within 4 waves),no wedge,bubble free, etc. admittedly very thin, but cost to me delivered NZ$186. I already have a very usable 22" using this same substrate so figured on trying to push the "envelope" a little further.
Stephen.