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

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Dave Rowe wrote:

> 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. 

I believe Sixtests would handle this situation correctly without further data reduction - right Jim?

> 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.  

There can be wider (and fewer, if contiguous) zones, too. This should be calculable, I might try some time.

> 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.

The "motions" of the tester would be greatly magnified, and making an XY dingbat for the Platzeck-Gaviola test would be very much easier, I think.
> 
> Anyway, best of luck.  I've tried figuring optics below f/2.5 and find it nearly impossible.  But, I have only limited patience.

One test that has been discussed a good deal on this list is the pentaprism test, see archives. The setup could use a laser diode at focus, a moving pentaprism (or two flat mirrors, perhaps small diagonals, suitably mounted), and a small aperture reading refractor with a calibrated grating in the eyepiece. 
I haven't heard of anybody trying it, but as far as I can guess, it wouldn't be terribly critical mechanically, and not require very expensive optical components, and still offer sensitivity and accuracy second to none. Also, it would test e.g. Cassegrainian optics that are not so easily tested by the common near-COC tests.

Or if somebody out there has actually tried it, why not tell us?


Nils Olof