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From what I understand plate glass and BK7 have virtually the same expansion coefficients and the difference is virtually nill for our purposes as the reflective covering is on the front of the mirror so colour of the glass does not matter.
Pyrex, however, is another matter. Ever thought about why pyrex can be taken out of an oven a put into water without breaking. Its because it has a very low expansion coefficient so when it is heated is hardly expands or changes shape. As a result when its put into cold water and the outer layer shrinks the movement is not enough to break the glass. Now carry this over into telescopes pyrex virtually does not change shape or size over the temperature we work so pyrex mirrors maintain their shape and, hence, collimation better than plate glass or BK7.
Put this all together and it probably matters most to people who live in Canada or some other really cold country where they take their scopes in and out of heated houses but, over the temperature ranges we experience here average plate glass or BK7 does a great job. Just remember to let the temperatures equilibrate on your scope before using it.
Cheers
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