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16" mirror blank

Started by BABOafrica, 02/07/2012 10:14PM
Posted 02/07/2012 10:14PM | Edited 02/07/2012 10:16PM Opening Post
My next goal is a bit far fetched but I'd like to make a dob with a 16" mirror. Getting the mirror, of course, will be difficult, given the (severe) restraints in my budget.

However, I noticed the following mirror blank for sale on ebay:

item number: 250989389442

What would it take, assuming I had a mirror blank like this one, to figure it and get coated?


In lumine tuo videbimus lumen.

8O Home-made 10” Dob / Home-made 4” refractor

EPs: Konig 32mm (1.25") / Zhumell WF 30mm (2") / Nagler 13mm T1 / Orion Sirius Plossls 25 & 10mm / Zhumell Plossl 9 mm / Meade MA 9mm
Posted 02/08/2012 12:37AM | Edited 02/08/2012 12:37AM #1
My girlfriend and I took a class at the local college. The instructor told us at the beginning that the easiest and fastest way to make a larger mirror is to make a 6" f-8/f-9 mirror first. Once you have developed skills and gained experience it will be easier for you to understand how to make a larger faster f-ratio mirror. This worked very well for us when I made my 12" f-5 and she made her 10" f-6 mirror. They both came out great. Another guy made his 6" f-9 mirror and the next semester went on to make a 16" f-4.5 mirror that also came out great. (Any other input on this from other ATM'rs?) Good luck with your project. Gerry
Posted 02/08/2012 05:57AM #2
Other people have made mirrors from glass like that. In doing so, you must be aware that it is ultra thin.

This is good because it is relatively light. So, your telescope as a whole will be much lighter. On the other hand, many people use that extra weight at the bottom to help balance the top.

Because the glass is thinner, it will not have the thermal mass of thicker glass, and will thus cool more quickly.

THe ultra thinness of the blank will require extra care in building a mirror cell that can hold it flat in whatever position. Ultra thin glass can flex more than its thicker counterpart.

Working an ultra thin mirror can present its own difficulties. When pressing on an area of the glass to shape it, the glass itself can flex. In effect, pressure you think you are putting on the blank is in fact being transferred to the supporting surface behind it. This is something you have to take into account. Also, testing with the Foucault tester must be done rather carefully, since the thin glass will flex into a potato chip figure unless supported well in the cell.

It is made of plate glass instead of pyrex or some other material. Plate glass is fairly easy to work (shapes relatively easily with normal grinding and polishing materials). On the other hand, is is not as thermally stable as other materials. This means that it will change its shape more in the course of an evening as the temperature changes. This will affect the image. On the other hand, it may not be as big a deal where the ambient temperature remains fairly steady as the day turns into night, and through the evening (as in the tropics).

And, as the other poster has pointed out----it is tough to start on a sixteen incher. Better to start on a smaller mirror and learn what you are doing before taking on a larger one.

At sixteen inches, you will need a fairly short focal length/F ratio. These are relativley more difficult to get right, and more demanding overall. It is good to have practice on the smaller blanks first. Make a mistake on a sixteen incher, and it takes some work to get it back where it belongs. Make that same mistake on an eight and you can fix it quickly. Best to practice the basics on the little ones.

Posted 02/09/2012 07:48AM | Edited 02/09/2012 07:48AM #3
Joe, join the ATM email list and take a look at the Stellafane website. You Tube has some videos as well. I'm currently fine grinding a 12.5 inch quartz blank (f/3.7). Got Grit is a nice place to get grinding material. That 16 incher is a bit thin for its diameter, but with proper support it would work for visual use. Mel Bartels and others have made thin mirrors, and it can be done. f/8 would probably be best for this blank, but that would mean a 108 inch focal length, requiring a small step ladder to view near the zenith. I say go for it, at that price you really can't go wrong.