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Mark,
I appreciate the point you are trying to make regarding the issues involved in cooling large thick mirrors. As you correctly point out, the issues increase logarithmically as the aperture and thickness of the mirror inceases. I agree 100%, that in these situations the selection of the mirror substrate is very important. You also correctly point this out on the thread concerning the 16" GSO mirrors.
Reading between the lines the question the original poster is asking is this. Should I buy a 10"/F5 GSO dob with a BK7 mirror and supplied with a cooling fan. Or, should I buy a 10"/F4.7 Synta dob with a pyrex mirror and no cooling fan. The answer is, for all intents and purposes it doesn't matter and he should base his decision around factors other than the mirror substrate.
CS-John B
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