Quote:
Originally Posted by Mark Suchting
BK7 bends its shape *3X* more than Pyrex for a given residual difference between mirror and air. Effect not so prominate on a lightweight 10" that is only a slight asphere..effect will be much more obvious on a larger slab of glass whose mass is goin up by the cube root of the aperture.
Mark
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Mark,
Thats actually not quite correct. It's just over twice.
7740 Pyrex has a thermal co-efficient (TCE) of 3.25 as opposed to BK7 which has a TCE of 7.1. That is 2.18 times not 3. However, it gets more complicated than just looking at the TCE, as you know. BK7 has a somewhat higher specific heat than pyrex (18% or 858 as opposed to 726). While it takes longer to cool from a given temperature, it also takes longer and requires more heat to heat up to that temperature. The practical effect of this, is that the temperature differential between the mirror temperature and ambient is usually slightly less for a BK7 mirror than it is for pyrex. This depends on ambient thermal conditions. However, given consistent ambient temperatures for both mirrors throughout the day, the BK7 mirror is usually cooling from a slightly lower temperature than the pyrex mirror. Thermal conductivity of both is almost the same.
True, pyrex is an inherently better substrate than BK7 for large, thick front surface optics, no one is saying it's not. IMO it's getting way too technical to worry about when you're talking about mass produced Chinese and Taiwanese mirrors in 10" Aperture. If I was buying a premium 10" mirror, of course I would want it out of pyrex. However, when your talking a 38mm thick 10 " diameter "mass produced" mirror with a fan blowing on it, it doesn't really matter squat, what it's made out of. Base the decision on which scope to buy, around factors other than the mirror substrate. Who cares if it takes an extra 5 or 10 minutes for the BK7 mirror to deliver diffraction limited images, as opposed to the pyrex mirror ? I can guarantee that most "mass produced" 10" pyrex mirrors won't deliver lunar and planetary views as good as my 10" mass produced BK7 mirror, regardless of how cool they get or how many fans they have blowing on them. I got lucky and got a good one, but clearly optical quality out of the box is a lot more important an issue than what it's made out of.
Something else to consider, a lot of people, not me, observe planets by letting them drift across the FOV from edge to edge. The drift method. As you are well aware a large part of the FOV in an F5 newt is not diffraction limited anyway, because of off axis comatic blur. Yet people still observe with that part of the mirror. Other factors which IMO are more important. How well is the scope collimated ? Is it really a diffraction limited mirror, or close to it ? What is the skill of the observer? How accute is his eyesight? What are the quality of eyepieces being used? How good is the seeing?
There are a lot more things to affect optical performance in such a scope, than worrying about what substrate a 10" mass produced mirror is made out of.
CS-John B