Re: ATM 11.2" F/2 Primary testing questions :)

[Aplanatic@aol.com]

Hi James:

There's nothing like a null test for fast mirrors, IMHO.  So, I would use one or more of the following tests:

1) Ross Null Test.  This requires an on-axis light source, a lens, a ray tracing program and accurate placement of the nulling lens with respect to the pinhole or slit and the KE.  Most likely use can find a suitable lens from Edmunds Optical or Mels Griot for this application.  To ray trace the situation start with a point source, pass it through the lens, reflect from the mirror and pass the light back through the lens to the focal point.  The light source can be moved longitudinally so as to bring the focal point back to the light source, and the lens can be moved longitudinally to get the best overall null across the aperture of the mirror.  Residual spherical aberration in this test (if significant) can be calculated and removed during data reduction.  One should use a monochromatic light source for this test, but I have found that a filtered red LED does a passable job.

2) Asymmetric Dall Null Test.  As far as I know, I'm the only one that has ever used this one.  Place a point source as far away as practical from the mirror.  In my case, I'm able to get the point source about 30 meters from the mirror by opening my garage door and placing the light source across our street on a neighbor's fence.  By ray tracing, find a lens that when placed in front of the KE causes a null for your paraboloid.  You will need a secondary diagonal mirror in front of the primary in order to inspect the reflected light, and this diagonal will prevent you from measuring the central area of the mirror, usually not much of a problem.

3) Not Quite a Null Test (NQNT).  This is identical to 2) above, but without the nulling lens.  By calculation or ray tracing, you can determine the residual longitudinal aberration for a point source not quite at infinity, and then do some data reduction to find the surface profile of the mirror under this test condition.  Because the light source is much farther away than in the conventional Foucault test, the zones are considerably easier to read and the longitudinal aberration is quite a bit smaller.  I've used this test on a 10" f/3.3 paraboloid at 30 meters with good results.  Once again, you will need a diagonal to inspect the reflected light.  The inverse of this test is also interesting, where the light source is placed close to the mirror and the reflected light is inspected as far away as possible.  I've never tried this, but it should work in principle.

4) Point Source at Infinity.  The obvious one.  In your case, the point source must be very far away for high accuracy, probably not an option.

5) Secondary Collimation Telescope.  Use a known good paraboloidal mirror to form a collimated beam that is equivalent to a point source at infinity (after correctly focusing it).  This is my favorite test of all.  I have several carefully figured mirrors in the f/6 to f/8 range that I use for this purpose.  The only significant problem with this test is the care required in collimating the setup.  Doable, but not trivial.

Anyway, best of luck.  I've tried figuring optics below f/2.5 and find it nearly impossible.  But, I have only limited patience.

Dave Rowe