Re: Madden's mirror hole

Bob Madden (madden@netcom.com)
Tue, 28 Mar 1995 17:31:36 -0800 (PST)

Greg, I don' know, but it sounds about like yours. When I have done the Combs test (Ink on glass and 30 sec of rubbing together) I find the center doesn't even polish out. And I have put on about 6 hours of polishing. Mark Van Wettering saw it when I had it spherical up at Chabot. But as usual I'm always trying to make it better. I saw the idea of wax paper in Brown, but felt I didn't need it because I wasn't touching there. I guess I was trying to short cut and should know better. I've spent about 60 hours into this project which was only to be a solar scope, 5.5, f-10. Shoulda been easy.

Say, On another subject, I saw Rich Comb's and Alan Gorski's video on Collumnation today. It is very well done with the esentials and all you need to do a good job. If you are going to RRMC take along an extra $20 for it. Rich has said he will have a few to sell.

):-{])) <---- madden@netcom.com madden@svpal.org Remember amateur astronomers: "keep looking for the next Universe"

On Tue, 28 Mar 1995, Greg Granville wrote:

> In message Tue, 28 Mar 1995 09:09:18 -0800 (PST),
> Bob Madden <madden@netcom.com> writes:
>
> > Phil you have just inspired me to get back to work on my 5.5-inch. I've
> > been so discusted with a hole in the center that I was about to drop it
> > on the concrete floor and start over.
>
> Bob, how bad is your hole in the center? On my last 6" mirror (which was also
> my first mirror) I managed to put a nasty hole in the center TWO times while I
> was getting the feel for parabolizing. Both times, the hole showed up as
> greater than 1/2" deep when focault testing. In both cases, I made a
> wax-paper star and depressed it into the center of the lap for about 15
> minutes. I polished about 15 minutes, pressed the "star" again and after
> repeating this three or four times the huge hole was mostly gone.
> I finally discovered that my lap was to hard, and that I had to apply
> excessive pressure to deepen the center (and then, it went way to far all at
> once)
> I put a few tablespoons of turpentine in my pitch bucket and poured a new
> lap. Once I got it softened up a bit, I was no longer punching massive holes
> in the center of the glass.
>
> > Say, I have a question for you atm'ers: What do you do with used pitch?
> > I'm not planning to, but can it be re used? How many times, if at all?
>
> The pitch I'm using was a hand-me-down from another ATM. It's been used to
> make at least three mirrors - and probably twice that number of laps.
>
> I'm not really sure it the stuff becomes unuseable eventually or not. ???
> ________________________________________________
>
> Greg Granville - N3SPE - (day)814-865-3310
> ARL-PSU gag1@psu.edu (eve)814-339-7244
>