L.J.

Answer #1 is correct. You can purchase kovar seals commercially. Also, you can purchase tubular graded seals in which a series of glasses are sealed, so the differences in thermal expansion from one to the next is always low enough to avoid cracking or shattering. At least, all the above was true back in my chemistry days.

--JMM

Sorry to be vague. But This is definitely rarified area. I saw a Nature / Science ? program about the casting of a very large parabolic reflector (by the University of Arizona?) It was pyrex, with hexagonal mineral foam cores to make the thermal mass minimal. They needed minimal thermal expansion. And succeeded.

Look into it.

You are not being vague Leveles and your response is appropriate to the context. Thank you.

However, I am only attempting mechanical fixturing.

The material I am using in casting these blanks is not a borosilicate or other low expansion glass but common plate glass. I can get away with using this material.

The mirror is so light for it's size and has such low thermal mass, that it reaches thermal equilibrium quickly and stays close to ambient temperatures easily.

The optical figure is not impacted under changing temperature conditions and is usable most all the time.

L.J.

Kovar (NiFeCo) is often used. In order to make a gas tight seal to borosilicate glass, the kovar must be oxidized. There are volumes written about proper oxidation of kovar. There are also methods for making gas tight seals (housekeeper seals) that don't require expansion matching.

This is correct, the kovar must have an oxide layer if you are sealing by melting the glass. If you use a pyroceramic seal or frit then the kovar needs to be clean but will oxidise when the pyro is air baked to cure it. Worth noting that kovar is the common trade name for this controlled expansion alloy but it is still also known as telcoseal 2 or (I think) nilo 42.

Any time when we do metal to glass seal we do one of the following

1. Have glass which is really a mix of glass and metal like lead boroate glass which is still very common for Kovar, invar and ally 42 sealing

2. If we have glass chemistry based on silicate then we have to go through control oxidation that is commonly done at 40 to 50% humidity at 850oC temperature with no dual. This is one of the common practice since most of the thickfilm furnaces run at that temperature and I was lazy to change it and controlled with ramp rate

Masyood wrote: "If we have glass chemistry based on silicate then we have to go through control oxidation that is commonly done at 40 to 50% humidity at 850oC"

Been there, done that!

My first experience with Kovar was in the design of hermetically sealed. nitrogen filled housings which had INGAS arrays for IR and Near-IR imaging sensors. The optical window was a borosilicate and was soldered to a Kovar aperture plate.

I could not remember the material and couldn't locate it using conventional search engines.

Hence this inquiry.

Thanks

L.J.

Alloy 42, invar, kovar, Moly and list goes on will be compatible with Borosilicate glass and pyrex

It was not my intention to cast glass around the metal.

I am looking for an easy method of providing redundant security for a large telescope; something that would constrain a 55 pound, 24 inch, honeycombed mirror and prevent damage in the event that the primary fixturing failed. (A solid disk of the same approximate geometry would weigh in excess of 125 pounds!)

Conventional mechanical clips introduce too many problems.

I considered bonding an array of narrow straps to the back of the mirror, each with an appropriate adhesive, and using the strips to provide anchors for "Hail Mary" tethers, each made from a loosely fitted, multi-stranded stainless steel cable (fishing leader?)

My concern for thermal expansion developed when I tried to anticipate the effects of bonding a 3 inch long by 1 inch wide metal strip to a vitrified substrate with a different C of TE.

Would the different rates of thermal expansion generate astigmatism in the optical figure?

When you consider that the optical surface is accurate to better than 1/30th of a wave of light in the visible spectrum, my concern at preventing stress in the glass is more easily appreciated.

However, once you gentlemen successfully identified the material for me (Thank you!) I went and priced it. Talk about a cold shower!

Big as this mirror is, I'm not building another Hubble and do not have NASA's budget! (In fact even NASA doesn't have NASA's budget any more!)

I had bad experiences working with Molybdenum (too brittle) but Tungsten might work.

Thanks, all of you, for your support.

"Time to bake the donuts!"

Laughing Jaguar.

Remember, 1) that anything you attach to the mirror is only matched over a limited temperature range and 2) that the expansion match is good only when the two parts (glass and metal) change temperature at the same rate. Since you require a safety harness, I suggest you design something that doesn't contact the mirror unless the primary support fails.

an air cushion type suspension maybe; pulling answer out of i think i think out of well where is that fortune cookie calculator.....?