Re: [ATM] Making the pitch tool

[Joe <nss@mwt.net>]

John Stephen wrote:
(snip)
I made one polishing pass on the mirror and stuck it on my focault tester

> only to find that the focal length is about two inches too short, making it
> approx. F3.75.  That wouldn't be a big deal except that I'm replacing an
> existing mirror (F4) and need to match it better than that.  In hindsight, I
> should have used the spherometer to measure the original mirror as a sanity
> check, but I foolishly assumed that the place I bought the spherometer from
> had verified its operation.  Stupid stupid stupid.

John ,, Hi there.

I know in these days of everything high tech and all that,, what I'm about to
say may seem really old school, or cave man tech,,  But when it comes to Mirrors
(concave) I have NEVER used a spherometer.

I got burned on my very first mirror I made by a bad one also.

During rough grinding when I want to check the focal length I simply smear some
corn oil or a dish washing soap onto the face of the mirror, and if it's sunny
out simply use the sun and directly measure the focal length.

If it's cloudy (or night time)  I set the mirror against a wall (preferably
white ) and with a flashlight shine a light at it and keep backing up till the
reflection is in best focus.  Of course with this set up divide the measurement
by two!

This works better and better as you get into the finer and finer grits, the
image focused gets crisper as the mirror gets smoother,, like Duh?

This I do all the time.

Joe

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