Re: Newbie seeking advice on mirror grinding

From: kbpayh (kbpayh_at_gmail.com)
Date: 03/29/05 

Date: 29 Mar 2005 10:59:36 -0800

David Dawson wrote:
> Hi,
> I am a newbie to telescope making.

Me too!

> The focal length is unfortunately now only 49 inches This is an
improvement
> over what is was since I started grinding (with 120 carbo) and the 8
inch
> tool on the bottom.
>
The first time through I ground my mirror down too much as well. This
must be a common mistake among newbies. My problem was using feeler
gauges to measure sagitta. Somehow the focal length ended up 6" too
short. I switched to a drill bit combined with the spit test (using a
candle for the source), and I'm now back within 2" of desired ROC.

> What advice do you have for me?

To fix my mistake I went TOT and used chordal strokes until the drill
bit just slightly rolled around under the straight edge. Then I
continued with TOT, but swtiched to the normal stroke. This seemed to
work for me.

Good luck with your mirror.