Re: ATM Vacuum Pans. _ Corrector size?

[Aplanatic@aol.com]

I wrote:

> My experience with the Slevogt corrector provides corroborating evidence
> about the flatness required of the vacuum pan...
>
> ...So, I made a two ball spherometer and measured the flatness of the pan 
lip.
> 
> It showed about 400 microinches of waviness as I rotated the spherometer
> around the lip.

Stuart Field wrote:

> I don't understand the geometry of this measurement.  Where were the balls 
> and the tip of the indicator when you did this?

The two balls rested on the pan lip, separated by 180 degrees, i.e., across a 
diameter.  The indicator tip rested in the center of the floor of the pan 
which was machined 150 mils below the lip.  A third arm was fastened to the 
spherometer, orthogonal to the arms containing the balls, and rested on the 
lip during measurements.

The floor of the pan was carefully lapped with 400 grit to provide a smooth, 
flat surface for the indicator tip to rest upon.  This was necessary in order 
to reduce the effect of slight indicator tip misalignments when rotating the 
spherometer.  An aluminum surface ground with 400 grit is very, very smooth, 
at least on the scale of the measurements performed.

After careful lapping of the lip, the spherometer showed no measurable 
difference in lip height, referenced to the center of the floor, during 
rotation.  I used an indicator with 100 microinch graduations and 0.25" total 
travel.

Dave Rowe
Torrance, CA