From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 


By using the light bulb test, (and I presume that you are using this test
to check the brightness of a reflected light bulb image when held at an
angle) you will see that the finer grade of grit is doing its job and the
mirror is in contact.  Usually the center gets more action and appears
brighter, then spreads out as you finish that grade. This is for general
surface conditions.  The other check requires a magnifier and bright light
to make sure all pits are taken out by each grade.

The last grade should be done very carefully, short strokes, and an extra
1/2-1 hour of slow, short strokes will ensure that there is no turned edge.
A channeled tool helps here.  If you can be sure of perfect cleanup, use a
steel bar and some coarse grit and carve a couple of grooves across the
tool, just a fraction deep.  This helps grit to flow on the tool and
spreads the action, and may prevent seizure.  The back of a handsaw in a
miter box works good. Clean up!
Ferson (from atm 3) says most polishing problems like severe tde are from
ignoring the final grinding.  I agree.

I have a 6" f/10.5 which is a superb scope.  If you want leave it
spherical, It's 1/8 wave on the wavefront as is (plus any bumps or zones
left on the surface)

Figuring a paraboloid at f/10 is difficult and mine is overcorrected a bit.
My 8" flex scope is adjustable and "perfect".

If you want to do the paraboloid, I'd  recommend that you consider making a
flex cell and use a center bolt pull.  The puller pad is not necessary for
this slight tension. Tolerances are wide for a small slow mirror like this.
Measure the wedge and if it is no more than 1/16" difference, you're
good.

If your glass is 1" thick, it will flex to a 1/25 wave paraboloid with
20# tension.  If .7" thick, it needs 10#. If you make a cell like in S
& T's article November 2000, with the puller pad, you get unimaginable
accuracy (Any zones left on the sphere will be on the paraboloid, but they
are easy to spot with the Foucault.)

Colin

Or



-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of Pat
Bunn
Sent: Wednesday, July 02, 2003 9:06 PM To: atm{at}shore.net
Subject: ATM 6", F10



I started a planetary scope mirror about ten years ago and got it to the
late stages of smoothing before getting discouraged with having to build a
tester and the seemingly complicated polishing and figuring. I am now ready
to build a tester ( I have cut out all the woodwork for the Stellafane
tester) and finish the mirror. The problem that I have is to identify the
place that I stopped at. I remember measuring the focal length with a light
and a wet mirror and testing the reflection of  an incandescent bulb (per
Texereau who I used as a guide to grind the mirror ) as a test for
smoothness. How can I determine that I finished the smoothing and the
mirror is ready for polishing? Any help would be appreciated.

Pat Bunn

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