Re: ATM [Fwd: BOUNCE atm@shore.net: Non-member submission from ["Bob Kenne" <bkenne@bigcity.net>]]

[mark cowan <cowanm@open.org>]

> > From: "Bob Kenne" <bkenne@bigcity.net>

> > I am currently working on my first mirror, a 10", F7, 3/8" plate,
> > stabilized with 3/4" ribs and a 1/4" back plate. When I finished hogging it
> > out, I found the focal length to be about 64". <snip>  when
> > I reflect the sun onto a white surface I get a sharp 1" circle at a focal
> > length of 65".

It should be a 0.6" circle, Bob.  The "sharp" part is an illusion (unless you can
see sunspots in there!).

> > Here's the problem.... there
> > remains a 2 1/2"circle on the center of the blank where I'm getting little
> > or no contact. I' not sure if I have a problem here or should just continue
> > with finer grinding.

Yes, it's a problem, specially after 2 hours with 120.  I'd say go back to the
rougher stuff and continue until you get uniform contact.  The sun circle will
decrease, even with plain old water... ;)  Go TOT to keep the FL going longer,
but use long strokes to "push down the edges" while you're doing it (this will
erode the bevel on the mirror so be careful).  You done hogged too hard.

Best of luck,

Mark Cowan
Salem, Oregon