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Hi Chissara.
The BAK7 is a fine glass for a mirror, especially in Australia where we don't get the extremes of cold temperature that they do in parts of America and Europe. For us, you will see absolutely no difference between Bak7 and Pyrex.
However, it's not true to say that you do not need a fan. It's important that the mirror be at ambient temperature for the best possible view. If the daytime and nighttime temperature are considerably different, you'll get use out of the fan to try and bring the mirror to ambient temperature quicker.
If you live in QLD where you get fairly constant temperatures most of the year, a "better" glass such as Pyrex makes even less of a difference.
Myself and others with GSO newtonians with Bak7 glass mirrors have produced fine high-resolution images where good quality, well-collimated optics are very important.
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