[ATM] Gimbaled Secondary mirror (report)

[<atm@hudler.org>]

I ran all of your concerns about the secondary mirror travel and tip/tilt
questions past the optical designer for this scope (see below). He does this
as a professional and uses software that I can only dream of using.

Again guys thanks so much for your input on this it was and is very
valuable.

Jack

Report:

Here is the scoop.  For your nominal system
over a 1.2 degree full FOV (best focus set
for on-axis), the RMS wavefront error for your
system is:


      FIELD               RMS        STREHL
      FRACT    DEG      (WAVES)      

  X   0.00    0.00                  
  Y   0.00    0.00     0.006483     0.998342

  X   0.00    0.00                  
  Y   0.67    0.40     0.719598     0.000000

  X   0.00    0.00                 
  Y   1.00    0.60     1.600025     0.000000

On-axis, your system is great.  Anything less
than 0.07 waves is indistinguishable from
"perfect", according to Lord Rayleigh, and 
you beat this by a factor of 10x.

The Strehl Ratio is a measure of the peak intensity
of an image of a star to the maximum peak intensity
possible.  Rayleigh's criterion corresponds to
a Strehl of about 0.8.  Off-axis, your performance is 
degraded primarily due to field curvature and
a little astigmatism. Not too much of a problem 
visually, more of an issue for flat detectors 
like CCDs or Film.

When you shift the Secondary 50 mm closer to the
the primary to increase the BFL, you introduce more
spherical aberration (think Hubble, although that
was a different cause and effect!).

The results are:

      FIELD               RMS        STREHL
      FRACT    DEG      (WAVES)      

  X   0.00    0.00                  
  Y   0.00    0.00     0.080060     0.776436

  X   0.00    0.00                 
  Y   0.67    0.40     0.694836     0.000000

  X   0.00    0.00                 
  Y   1.00    0.60     1.554631     0.000000

You can see that off-axis, not much has changed.
On-axis, you went from 10x better than imperceptible
from perfect, to about 1x imperceptible from
perfect.  Frankly, it will take a very discriminating
observer to notice the difference.

Based on this, my conclusion is that 50 mm is OK.
Less is better, and I would not shift the secondary
by much more than 50 mm.  For example, 0.15 waves
RMS (about 0.5 waves P-V which is very noticeable)
would take about an 85 mm of shift.  To stay below
0.07 waves, you should keep the shift less than
45 mm.  Again, this would essentially be 
imperceptible from perfection.


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