The Mirror Blank
Making a 4.25 Inch Dobsonian
Reflector Telescope
Options
There are a few different options of
material to make the mirror. The least
expensive is plate glass. The assumed disadvantage
of a plate glass mirror is that it has the highest index of expansion. When your first take your telescope out
into the code, while the mirror adjusts and its temperature is in flux, the
surface will be more out of whack then with a material with a lower index of
expansion. Alternatives in increasing
price are Pyrex (the trademarked name for
borosilicate glass), quartz, and more exotic blends such as Zero-Dur. There is much debate about this issue. Some would suggest that with an appropriately
thin mirror, the temperature adjustment will go much quicker than a traditional
thickness mirror. Some would suggest
that expansion doesn't matter all that much as the biggest problem is surface
air currents. Sometimes fans are used
to help the mirror adjust more quickly.
I don't have a strong opinion either
way. Fancy glass is fine if you have
the money. It might adjust more
quickly when you are figuring your mirror, but you shouldn't be in a rush
anyway. It only leads to problems. For this project the mirror blank happened
to be Pyrex. The 12.5 inch mirror I
broke was (or at least was supposed to be) plate glass. It turned out to be tempered which is why
it broke.
Grinding
Down the Edge
If your mirror blank doesn't come with
a beveled edge, you will want to grind it down before starting. First Hand Discover sells sharpening stones
for this purpose. If during the rough
grinding this edge wears off, you will want to regrind it. If you fail to do this, there's a good chance
that you will chip the edge of the mirror.