Pyrex Primary versus BK7 with fan

[NQLD_Newby]

Hello everyone,

I am looking at upgrading to a 10" or 12" Dob next year, and have consequently been reading about the several types available.

Some models like GSO for example, have a BK7 type glass primary and include a fan for cooling equilibrium. Some models have pyrex primary and do not have a fan. Now, I have done some research on the mediums used and realise that pyrex has a better temperature coefficient than glass and doesn't get effected as much. But this brings me to my question.....

Is pyrex that much better, that it doesn't need a fan? Will a pyrex mirror be ready for distortion free viewing in the same amount of time as a BK7 glass with fan?

Is a pyrex mirror better quality, as in more reflective (maybe this has nothing to do with the medium and has more to do with the coatings), or are both equaly as good in this regard.

Thanks for listening and any advice would be greatly appreciated.

[janoskiss (Steve H)]

Regardless of how thermally stable a material the mirror is made of, I'd still use a fully baffled fan (one you have to DIY, because mass produced ones don't come like that) to combat:

* Dew
* Inversion layer above mirror and tube currents

I have a pyrex mirror 10" and it is certainly more tolerant to temperature fluctuations than BK7 or plate glass (which my 8" is) but I still use a fully baffled fan for reasons above. And when temperature diff is great enough pyrex deforms also so can do with help from fan for cooling.

[rmcpb (Rob)]

Pyrex is better but considering where you live and how you are not likely to experience winters nights like Canberra where it just keeps dropping I would say that a BK7 mirror is more than adequate for your needs. I have one in my 8" scope and it works here in the mountains in winter, the fan has helped a lot as well.

Cheers