"Pyrex telescope mirrors are better than those made from plate glass."

The primary mirror at the heart of a reflecting telescope is typically made from either Pyrex or ordinary plate glass. Both materials can be fashioned into excellent mirrors. So why is it so commonly held that Pyrex mirrors are inherently superior? There are two main reasons.

First, because plate glass is less expensive, it is presumed inferior — you get what you pay for, right? However, in truth the cost of the substrate material has little to do with the price of the finished mirror. Most of the expense lies in turning that piece of glass into a working optical component.

Second and more to the point, unlike plate glass, Pyrex is a low-expansion material. This means that a mirror made from Pyrex is less prone to warping when it is at a different temperature than the air surrounding it. The ultraprecise curve on the surface of a Pyrex mirror isn't as severely deformed when the telescope is moved from a warm house into the cool night air.

This relative insensitivity to temperature changes also has its advantages when the optician carefully refines (or figures) the curve on the mirror's polished surface. When the glass is worked, a certain amount of heat is generated by friction between the tool and the mirror. Because this heat will deform Pyrex less than plate glass, the optician doesn't have to wait as long to test the mirror and proceed to the next figuring step. Since a commercial firm presumably wants to minimize the time spent on an individual mirror, some amateurs assume that a Pyrex mirror will turn out better than one made from plate glass.

This sounds logical enough, but I have seen no evidence of it in the dozens of mirrors I have tested over the years. In any case, these attributes are more of an issue for the optician than for the mirror user. If you're making a mirror, Pyrex does have its advantages, but if you're simply buying or using one, these characteristics really shouldn't come into play.

"But don't Pyrex mirrors cool faster than plate-glass ones?" Yes, but the difference is not nearly enough to be significant in most amateur-size mirrors. In theory, Pyrex should cool about 20 percent faster than plate glass. But other factors — notably the mirror's thickness and the design of its cell — are far more important.

There's yet another reason why glass type makes little difference: when it comes to thermal problems, your main enemy is the layer of warm air in front of the primary mirror. No mirror, regardless of glass type, will perform adequately until this "boundary layer" goes away. Temperature-related deformations of a mirror's figure are minor compared to the boundary layer's deleterious effects. Venting the primary mirror — ideally with fans, as pictured here — can help you get rid of this problematic air layer.

The bottom line? Pyrex is slightly better than plate glass in theory, but not in practice. Ventilation is far more important.