[ballaratdragons (Ken)]

Well,

Not quite right El. Sorry.

Dave, Pyrex is far better than BK7.

Cheaper mirrors are made of Plate Glass which is easy to recognise by it's green tinge (in the glass, not in the reflective aluminium coating).

BK7 is a higher grade 'optical glass' used worlwide as a very good mirror material and a lower thermal co-efficient to plate glass (quicker cooling) and does not have the green tinge to it, it is clear. It is a very good quality Glass.

Pyrex is recognisable usually by it's price!!! Although unrecognisable from BK7, Pyrex is hardly affected by thermal changes which gives it a very short cool-down time and is a far better quality material.

As far as weather in Australia, we do not have the extremes as do other colder countries. So, BK7 is perfect for the Australian climate.

As for Crayford, they are far, far better than Rack & Pinion focusers as they are much smoother, easier to focus, have adjustable friction, a focus lock, have nil backlash and are better made.

They are most definately worth the extra!!!!

Hope that helps.