Peter,
you missed my point a little. Yes the point is to get it to ambient, but if the mirror is say at 39 degrees after a hot day in the sun and the night time temperature drops to 9 degrees it will take 2 hours with 2 peltiers to remove that heat. You must always think of the differential temperature points. You can go below ambient but I usually stop the cooling about 0.3 above ambient and let the scope settle. So two peltiers will work fine in even the most extreme cases. Three I think is overkill.
