Re: ATM Artificial Star for Testing (Long)

[Aplanatic@aol.com]

Hi Mark:

The problem with the artificial star test is the distance required to the 
point source.  In order not to introduce spherical aberration into the system 
at a meaningful level, the point source for an 8" f/6 scope must be at least 
160 feet from the scope.  At this distance the normalized transverse 
aberration at best focus is about 0.5.  At 80 feet the normalized transverse 
aberration is about unity, which is clearly too large for a good artificial 
star test.  So, aside from making a small point source (which must have an 
effective angular diameter of less than about 3e-6 radians), the problem is 
finding a good site to do the test.  During the day or in the early evening 
the thermal effects due to ground heating and cooling are a big problem.  The 
image is likely to be useless.  I once tried the same method on the street in 
front of our house (for the null KE test) and found that the no useful 
information could be gained.  It might work in the early hours of the morning 
or perhaps over water.  If the source is at 160 feet an illuminated pinhole 
that is 3 mils in diameter is small enough.  A small ball bearing illuminated 
by a diode laser would also work, I believe.  I would place the laser as far 
from the bearing as possible while still being able to hit it.

Dave Rowe
Torrance, CA
Medium Format Astrophotography:
http://members.aol.com/aplanatic/photos/astro.html