Re: ATM polishing to evaluate

["AstroJack" <AstroJack@AVATM.com>]

Hi Richard,

There is a test to check to see if you are roughly spherical, and best of
all you can use it from 80 grit on if you like.  Alas it it nothing more
than the sharpie / permanent marker test. The reason is that as long as you
can pass this test, even at 9 micron, then the figure you have should be
close enough to a sphere that polishing correctly should produce a good
result.  As long as the center, middle and edge wear evenly with your rigid
( Tile or Glass) tool you are good to go.  If the center markings persist at
this point your center has a hole and this should be fixed.  Also, if the
edge markings persist then you need to finish working the curve out to the
edge.  If both of those persist, or the middle zones, then perhaps the fire
hydrant test should be used as a follow up.  Also, don't worry about things
like turned down edges and stuff till you polish. Before that just keep it
close to sphereical.  Also remember when you do start to polish that 1/3
center over center (CoC) means your overhang is 1/6 the diameter of your
mirror. 1/6 on one side plus 1/6 on the other gives you the 1/3 total
distance of the stroke desired.  {1/3 on each side =2/3 CoC (during
polishing only, not talking figuring here!) IS a good way to a turned edge.}
Of course so is too little overhang.  But don't worry, we all get the hang
of it sooner or later!  : )

Good Luck!

Jack


----- Original Message -----
From: "Richard Foulk" <richard@skydive1.com>
To: <atm@shore.net>
Sent: Sunday, February 17, 2002 7:01 PM
Subject: ATM polishing to evaluate


>
> Aloha,
>
> Several places I've read that you can't really tell where you're at
> until you polish the mirror -- so that you can Foucalt test.  And if
> there are sufficient flaws you'll need to go back to fine grinding
> to correct them.  Sounds inefficient.
>
> Is there not a more reasonable way?  Isn't there a way to start
> testing a mirror while it's in fine grind?
>
> I've seen some small spherometers on the net that look appropriate
> for zonal testing.  Does anyone use them in that way?
>
> What about a quick wax job of the mirror to make it Foucalt-worthy?
> Would that be worth the trouble?
>
>
> Thanks
>
> Richard
>
>