From: Mark Holm 
To: atm{at}shore.net
Reply-To: Mark Holm 


> First question: My center zonal mask (1) is cut so that the two open
> slots on the mask have an inside radius of 1" and outside radius of
> 2.236".  Am I correct in using the null point of this zone as 0.000"
> during the analysis?
>
>



Unless you can measure all the way from the mirror vertex to the knife edge with
0.001" accuracy, you will have to arbitrarily set some zone position
to zero. It is customary, though by no means required, that the central
zone, or the zone
closest to the center of the mirror is set to zero.  Sometimes, you will
find, if you develop a central hill, that setting the central zone to zero
will cause zones a little farther out to have negative values.  This is
probably not the figure you are after, but it is mathematically OK, and
really does indicate the actual shape of the mirror.  Just run the numbers
in your analysis algorithm or software of choice.  In the Texerau
algorithm, and in Sixtests.exe, the negative
values won't cause any problem.  Can't say for sure in Figure 4.5, but
probably the same.

In Sixtests, and probably in Figure 4.5, you enter a mirror to knife edge
distance.  This is a rough (+- 1 mm) value that you measure with a tape
measure.
  The appropriate time to measure it is when your knife edge is at the zone
null
position corresponding to the zone you set to zero.  This nicety is probably not
  realized by many atm's and doesn't cause much error on typical mirrors
because
a millimeter or two change in R doesn't really change the relationships of
the zonal null values that much.

The setting to zero does not need to be a physical thing.  Your micrometer
can read 0.314 or whatever, so long as you have enough travel to get your
measurements in its range.  Simply subtract 0.314, or whatever, from all of
the zonal values before plugging them into the analysis.

If, like my most recent tester, yours goes the "wrong way", just
invert the subtraction to get values that increase as the knife edge moves
away from the mirror.

Mark Holm
mdhom{at}telerama.com

--- BBBS/NT v4.00 MP