fwiw) Some numbers with respect to the heat capacity of a pyrex mirror blank:
The density of pyrex is given as 2.23g/cubic cm
source:http://physics.nist.gov/cgi-bin/Star...s.pl?matno=169
The mass of a 25"x2" blank works out to 35kg. (at this density of material)
The specific heat of pyrex is quoted as being between 0.75 & 0.85 kJ/Kg/K (depending on the source) Let us pick the middle ground and work with a value of 0.8 kJ/Kg/K
source:http://www.engineeringtoolbox.com/sp...ids-d_154.html
It follows then that for every degree celsius (or kelvin) you wish to cool a 25" telescope mirror, you need to remove 28 kilo Joules of energy.
To convert Joules to Watts (and thereby add a time component) divide the number of Joules by 3600. (coz there is that many seconds in an hour)
ie) To cool a 25" mirror by one degree celsius in one hour, you need to continuously extract 7.8 Watts of heat over that hour.
Whilst this may seem like an abstract exercise, it may be useful information if you have a clear idea of the ambient temperature gradient over a given night and you also have some idea of the extent to which your telescope mirror will lag the air temperature. Under those conditions you should 'theoretically' be able to tune the quantity of heat removed by an active cooling system to keep the substrate at a temperature close to ambient.
For example, it should be reasonably straight forward to set up a PC based PID controller with a feed forward component that monitors the air temperature and can in essence accurately predict how much extra cooling the mirror needs, converts it to a chopped DC power signal to your peltiers, and Bob's your uncle.
Going by the above figures for the 25", let's say we have a night with a fairly steep cooling profile and the primary mirror temperature is lagging ambient by 4 degrees celsius, you would need to pull out 31 Watts of heat to get that critter under control. Peltiers are typically only 30% efficient so you are talking nearly 100Watts of electrical power, and also the requirement of dumping all that heat somewhere (other than in the telescope structure).
This is perhaps a little complex for your average amateur telescope maker, but for a commercial telescope manufacturer it is entirely possible.
~c