Coulter vs. Pyrex mirrors, Dob feet?

Dear All,

The subject is basically my question: what would you all choose if putting together a truss scope? I know one big difference is temp. adjusting rates, but are they both good optics?

also, what do truss dobs usually have for feet? I mean, under the azimuth ground board?

thanks
Doug

Hi Douglas:

How big of a mirror are you considering? Discovery has some 16" plate mirrors at a heck of a deal right now.

Plate is softer which means it grinds and polishes faster. It also /can/ have a smoother surface than pyrex. Plate expands and contracts with changes in temperture 3 times as much as pyrex. This is not good while the mirror is cooling, but doesn't matter one wit if the mirror is already cooled.

As Jim said, the good one is the mirror with the good figure. Either glass can have a good figure.

Feet?, three wood blocks, teflon right above them.

Steve