Re: [ATM] Not tde or tue, important oblate spheroid, ellipsoid

[Bern Kosicki <kosicki@verizon.net>]

The reason I asked for clarification on "what the 1/3 stroke produces- 
spherical or oblate", is that at various times during my (long) polishing 
journey I was in the oblate mode.  But with a recent change I seem to be 
back on the "normal stroke produces sphere" mode.

While in the oblate mode recently, I did a series of four sessions with the 
normal stroke, and got a steadily worse (farther from spherical) figure 
each time.  This was starting from a figure that was pretty close to spherical.

My mirror is 8" f/5.3  I used a mix of about 1/2 G 73 and 1/2 G 65 pitches 
for the lap.

Here's the progression.
1. MOT  Conic constant= 0.029
2. TOT  0.077
3. TOT  0.114
4. TOT  0.166
5  TOT  0.404

In each case, the ROC got steadily smaller as the radius of the zone 
increased (ie, it was just the opposite of a parabola)

The reason for the TOT sessions was to reduce a TUE.

What I found out was that I was not polishing at a high enough room 
temperature.  For each session I soak mirror and tool in 90-95 degree water 
for 20 min, then press with 15 lbs for 1/2 hour.

I thought I was getting good contact, since the drag felt smooth, and I was 
getting significant deformation of the pitch with each session.

But room T was about 65 degrees, and I believe that the lap was probably 
not maintaining its conformation during the session, even though the drag 
continued to feel smooth.

Recently I upped the Room temperature to about 71 degrees, stopped using a 
pre-warming bath, used RT pressing (for a long time), and right away got 
back to a fairly good sphere.

Two recent sessions of TOT show a reducing conic constant from the first to 
second session (0.116 > 0.088).  I'm still trying to wear down (slowly) the 
TUE and improve the sphere.  The TUE does seem to be spreading out from the 
edge with TOT strokes (but the last two sessions showed no improvement in 
the error at the edge- remaining steady at 45 nm)

One problem with increasing the temperature - it also caused rapid drying, 
since in the NE it is cool outside and the relative humidity inside is low- 
about 30-35%.
Therefore I set up in a bathroom, and used a humidifier (the kind that are 
used in bedrooms at night to treat colds), to bring up the RH to about 
50%.  This seems to be a pretty good range for RH, since it leaves about 
the right time between "wets" (the addition of CeO or water).

I'm too embarrassed to say the amount of time that I've been trying 
different things to get a consistent operation of the lap, and also 
operation as predicted by Tex.  I do feel better that now the lap seems to 
be working in the direction predicted by Tex.

I'm about to rebuild my lap (it's the second time I've worn it out) and 
will probably use G 65 degrees.

What is the expert opinion of going to a G 65 degree lap?  What can go 
wrong if I use this pitch instead of the half-half mix of 73 and 65?


Bernie K



At 09:21 AM 2/5/2008, Bob May wrote:
>The subjecgo only gets nastier than this.  How you push the glass
>also makes a difference in what shape you get.  Theory is that
>the 1/3 strokee produces a spherical mirror is, to a fair degree
>true.  However, if you push down on the glass or tool at the
>right time, you can change the shape of the glass surface.
>With that bit of tantalizing, I'm going to back out and let othes
>go about their theories.
>Bob May
>
>rmay at nethere.com
>http: slash /nav.to slash bobmay
>http: slash /bobmay dot astronomy.net
>
>
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