Re: Grinding a mirror vs buying complete


"Dan" <mowcat@xxxxxxxxxxxx> wrote in message
news:SUrIf.113267$5D5.476448@xxxxxxxxxxxxxxxxxxxxxx
I am beginning to think about building my own Newtonian. Is there a
fairly
major saving in grinding one's own mirror vs purchasing a completed
mirror
from one of the top makers?

Also, can anyone recommend a good book on making a Newtonian?

Thanks,
Dan
You are very unlikely to make a first mirror, that competes with 'one of
the top makers'. The limits of simple testing, even if you work very
carefully, make this unlikely. Now the 'savings', depend masively on how
you approach things. It is perfectly possible to find pieces of very good
glass, at bargain prices, working from 'second hand', and with care
produce a very good mirror. However for anything larger than perhaps 6",
you also have to be prepared for just how much work is involved.
Cleanliness is vital (a single piece of a coarser grit 'left over' when
switching to a finer grit, can ruin the surface). I have seen people, work
with a piece of scrap glass (often from military suppliers), and with just
a few evenings work, produce an excellent mirror, matching that from a
'good' maker, for a Newtonian. However (conversely), when glass sizes get
larger, and trying to produce faster F ratios, I have seen a guy trying to
get the figure 'right' for weeks without success, till helped.
If you price the time, and buy everything at full price, savings (relative
to off the shelf 'standard' mirrors), are almost non-existent. Relative to
higher quality mirrors, the 'mechanical' costs drop, but the time involved
rises, and whether this is worthwhile is down to how you value your time.
I'd say try a simple design, with a small mirror, and it'll teach you the
problems and show you whether you are likely to succeed. There is a three
volume 'bible' "Amateur telescope making", which goes far further than you
need to, covering just about every design of scope. The first volume
though, concentrates on the Newtonian, and though it is an old book,
getting a copy from a library, is a good way to at least read what is
involved,and decide whether this is for you.
More than thirty years ago, I helped make an 8" Newtonian, and 'on the
way', made a 4" scope as a test. More recently, looking at the prices
being asked, and knowing what was involved, I elected to buy. Major
savings now, only really exist on larger scope.

Best Wishes




.

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