Pyrex from Plate Glass

Is there a way to tell plate glass from pyrex by looking at it?
My brother has a 8" discovery mirror and it has a very good figure but just won't keep up with my 8" meade starfinder.
It appears to be a heat problem you can see the heat in the image. The discovery tube is smaller than the meade less air flow around the mirror. The other possibility is the mirror is plate glass and more unstable. :S

Matt:

One way to tell is to accurately measure the density of the mirror. I believe Pyrex is about 2.18 grams/cc and Plate Glass/BK-7 is around 2.4 gm/cc or so. (Best to look it up)

Plate glass has quite a green tinge to it but BK-7 is clear. Pyrex is reputed to have a slight green tinge but is quite clear.

What model Discovery is it. If it is the standard model, then it most like has a Plate Glass mirror, at least that's my understanding, Pyrex was an upgrade.

But in any event, whatever your brothers mirror is made from, if it has a good figure then when it is cooled down, it should provide nice views. A simple fan is a big help in cooling and stablizing a scope. Any mirror needs to cool down and an 8 inch plate glass mirror should not take too long if it is done correctly.

Also note that cooling down is mostly about convection so the mirror material is of little importance...

(By the way, Floyd Blue's fans are quite simple, nicely made and very effective in getting a scope cooled down quickly and are free from vibration. I bought two recently, one for my 10 inch GSO/Hardin and one for my 12.5 inch Discovery, both really do the job.)

I am assuming your Starfinder is an EQ mounted scope because the 8 inch DOBs had serious thermal problems...

jon

Matt,
Most likely the problems is heat plumes. The scope is just not cooling down. A fan system would probably improve the images considerably.