making the secondary diagonal

I'm giving serious thought to making my own secondary. Grinding and figuring it using the 3 blank method.

I have heard that since it's for flats, that using plate glass is adequate.

I have also found out that people will sell just about anything on Craig's List including old glass coffee tables, some with 3/4 inch to 1 inch plate glass on them. I have no idea about the annealing on them, but for $50 for a huge amount of plate glass, I'm wondering if I should take the gamble? Maybe a small enough portion of the glass is ok?

How do you test that? I heard of using polarizers somehow.

What do you all think?

Of course after I make the blank, I have to core it at 45 degrees, using something in the 2.1 to 2.7 inch range. I went to Lowes today and found a hole saw that might work. But if I make my own, how would I do it?

Thanks for your ideas on that.

-Bill Wheaton
Decatur GA

The secondary must be very flat and well made even compared to the primary mirror. Just a little bit of stress in the glass and it will change figure with temperature with very unsettling results.

Hi Bill,

Looks like you've pushed some glass before

There are a couple of ATM books that talk about the 3-plate process for making a flat, I can look it up if nobody else remembers (Allen Thompson?) I recall in the old TM magazines several folks had some luck with it.

If you start with the plate glass already polished, then after you have done your cutout you can use the cross polarizer to check for strain in the glass. Remember LCD screens give off polarized light, so you have one, or a laptop, a white screen will give you 1/2 of your solution. Put your glass to be tested in front of the screen, and look at the glass with the polarizer and roate the polarizer. Try this with other glass, or plastic first to get an idea of what this looks like.

If your glass appears a uniform gray, you should be good to go if the glass is reasonably thick.

Don't forget "Surplus Shed" as a source of glass. They have all sorts of glass blanks and windows/filters and finished flats (for testing) that might work for you.

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Bill Wheaton said:

I'm giving serious thought to making my own secondary. Grinding and figuring it using the 3 blank method.

I have heard that since it's for flats, that using plate glass is adequate.

I have also found out that people will sell just about anything on Craig's List including old glass coffee tables, some with 3/4 inch to 1 inch plate glass on them. I have no idea about the annealing on them, but for $50 for a huge amount of plate glass, I'm wondering if I should take the gamble? Maybe a small enough portion of the glass is ok?

How do you test that? I heard of using polarizers somehow.

What do you all think?

Of course after I make the blank, I have to core it at 45 degrees, using something in the 2.1 to 2.7 inch range. I went to Lowes today and found a hole saw that might work. But if I make my own, how would I do it?

Thanks for your ideas on that.

-Bill Wheaton[/COLOR]
Decatur GA