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Jay... the BK7 substrate has lower thermal mass though a higher coefficient of thermal expansion, meaning it will maintain its correction slightly (imperceptibly) worse than pyrex but will reach thermal equilibrium slightly (imperceptibly) faster than pyrex and show less interference from tube currents whilst doing so.
The quality of the optics in GSO's up to 12" are unlikely to win any awards but are absolutely fine for imaging.
The issues that prevent these scopes from routinely challenging high ticket price OTA's are mostly mechanical in nature.
~c
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