[ATM] Fixing Tom's oblate sphere

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Polk, Tom wrote:
> From: Jerry [mailto:wa4guu@verizon.net]
>> As for the corrective action, just wait until the mirror is fully
>>  polished and then you will (should) go to shorter work sessions
>> for figuring.....
> REPLY: Using a laser pointer, the surface is fully polished except
>  an area ~2.0 diameter at the center. This area follows closely 
> behind.

I don't promote the idea of figuring the mirror before it is
polished out, but I find that the oblate shape usually gets worse
through polishing if one doesn't realize what is going on.

So, I favor the idea of catching it when it is forming and fixing the
problem that is creating it.  Otherwise you have a badly oblate sphere
and a nasty TDE to fix and the mirror maker may not understand why
this has happenned.

When you push the lap forward (working TOT) the front of the lap tends
to dig in naturally because the mirror is concave, even if you are 
pushing straight down.  This is unavoidable.  Also, subconsciously 
applying a little pressure at the leading edge of the lap tends to 
make lap contact FEEL better, but really one is just plowing the edge 
of the lap into what will become a fairly strong up-slope on the 
mirror's surface, relative to the sphere, and this is an oblate 
sphere.  It feels better, so the worker keeps doing it, and the 
condition worsens.  Been there, felt it, done it.

> Last night the polishing sessions were: 15 minutes TOT; hand 
> pressure and stroke as described by Mike Lockwood site (great 
> information, thanks!). 15 min MOM; hand pressure and stroke as 
> described by Mike Lockwood site *Cold Press* 45 minutes Repeat the
>  above set twice (one hour polish time). *Cold Press* 1 hour Repeat
>  first set twice (one hour polish time).
> 
> Looks like progress to me (right? ;-).

The center came down.  I wouldn't be surprised if the polish is much
better there too.  That is probably from the MOT work.

What you really want to get rid of is the rolled edge.  That will just
take time.  Comparing with your old photo, it didn't change.

So, I recommend applying a bit more pressure with the heel of your
hand and your fingers as you move the lap, working TOT.  (In order for 
this to work, the heel of your hand just be on one side of the mirror 
and your fingers on the other, with the center of the mirror somewhere 
in between.)

If that hasn't helped in another hour or two of work, then I will 
prescribe a figuring stroke to speed it up.

How much weight are you pressing with?

> Are my polishing sessions to long? Can I go longer between cold 
> presses?

For right now let's stick with the process you describe above, but do 
two TOT sessions for every one MOT session.  And add a little more 
pressure.

	Mike L.

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