Buying the Glass for the Scope

But what's extremely important is that you FAX a request to their sales office FIRST for a "glass quote." You cannot buy the glass outright without a quote. And if you send an email asking for a quote, they may not take you seriously. You have to remember that glass for a 6" or 8" achromat is small potatoes to these companies, and they don't have time to waste on you if you don't play by the rules. They aren't interested, from what I can tell, in supplying a bunch of quotes to the randomly curious. They need a sign that you're serious about the purchase. So FAX your quote and if possible call their office and find out who takes such quotes. At Glass Fab the contact person as of summer 2000 was Mr. George Dean.

A quote should look approximately like this:

How much will the glass cost? Prices fluctuate, but as of summer 2000 you could expect to pay about $250-$300 for the two glass 155mm disks. Prices for optical glass seem to have risen over the past decade as the companies struggled to clean up their productions. In particular, my impression is that Schott and Ohara have spent much effort to reduce or banish lead and arsenic from their materials wherever possible. Arsenic is a trace contaminant in glass making, but tons of lead oxide are required to make the various flint glasses. Inside the glass, these compounds don't seem to be dangerous, but particularly the lead oxide could pose a serious environmental risk during production. As a result of this and the general problem of glass itself entering the lungs, etc. it is always wise to work glass wet. It's also the way that glass is ground and polished anyway.

If you have gotten interested enough to read this web page then you may have come into some stray disks of optical glass without any documentation of what it is or some old lenses that you'd like to rework into a telescope. Opticians call such glass "undocumented" for obvious reasons. In order to use such glass, you definitely need to know its type and you will probably need to evolve your own objective design. This can definitely be done: I did it for the first time last fall in producing my successful 6" f/15 Clark-type lens. But you need to exercise caution. I refer you to Appendix 3 for further info on undocumented glass

Not only will you absolutely need to know the glass types which you have, but you'll need to know how good the glass is. There is much worthless optical glass floating around out there, I can tell you from painful personal experience. There's nothing like buying a 12" diameter piece of glass and ending up with a piece that is only big enough to build a 4" telescope with it. If you have acquired stray disks, as I have, and know the glass type for sure, then you'll need to test the homogeneity and residual stress levels (anneal) of your disks before you use them. Toward the end of this webpage, I'll discuss how to do that. It's rather easy for flat disks. Old lenses can probably be assumed safe for use, but usually you won't know the glass types. And it's often utterly hopeless to use disks of an unknown glass type to make a telescope lens. Many, many different types of glass are in use and they have substantially different properties from BK7 and F2. Go to the Schott Glass Technologies website and get their Windows program for their various glasses that they make. Don't frustrate yourself and waste your effort on unknown or inferior optical glass. Use the good stuff that you know is good from the start!

I mentioned the anneal quality of glass a moment ago, and those readers who have some prior knowledge of optical glass working may be surprised that in my sample glass quote I called for using "B" grade, or "fine" annealed glass, instead of "A" grade or "precision" annealed. The reason for this is that nowadays for common glasses like BK7 and F2, there is no substantial difference in quality between fine annealed and precision annealed. The latter is simply inspected more carefully, and the difference in cost reflects the labor of inspection and the guarantee that you get. A well-known optical designer that I recently spoke to told me that he no longer specifies "A" grade, since "B" grade is identical but not inspected. It is also substantially cheaper.

Probably if you order B, and then inspect it yourself using the tests I will later specify, if you find one of your disks substantially defective, the supplier would replace it. But chances are good that this problem of a poor anneal won't happen. The defective disks which I have so far seen have either been very old or of exotic glass types. Schott BK7 and F2 and Ohara S-BSL7 and PBM2 are industry standards and not likely to be defective. Of course, you are free to order A grade if you don't mind paying the extra money which is likely to be 50 to 100% greater than the B grade. You can also ask for two quotes, one for A grade and the other for B grade and take the one whose price you find most attractive.

Finally, it is possible to pay more money to get your disks "curve generated" ("hogged out" in mirror-making parlance) to the curves you desire. This will save you substantial time and trouble during grinding. Just specify clearly in your glass quote what curves you desire on which surfaces. The additional cost should not be large. Don't expect the supplier to nail the curve down perfectly. They will probably guarantee it to within about 3% of what you ask for. That's quite good enough and don't quibble with them. The disks themselves need to be pretty precisely the same diameter (within a few thousandths of an inch), and chances are that they will be, if you ask for them to be edged. If they come with a small but noticeable difference in diameter, you can fix that yourself. Don't complain about it. Problems like a slightly wrong radius of curvature can be easily fixed with a bit of grit and the grinding of the offending surface. The differing diameters of two pieces of glass can also be fixed by grinding the edge before grinding on the surfaces. The manufacturer is more doing this kind of work as a service to you and it's only to help you get towards where you are going. The work is done at a very cheap price compared to the amount of work that you would need to do in order to get to the same place.