Re: ATM An $18.00 Null Test

[Aplanatic@aol.com]

Hi Dwight, Ken:

Dwight wrote:

>  These test that allow multiple elements would be even
>  better if someone could think of a self aligning setup
>  that could be used to do the initial placement of the
>  elements. This type of setup would make it most valued
>  by me.

This depends on what you mean be self-aligned.  In the "Modified Dall Null 
Test" where the point source is displaced outward from the RoC, the correct 
strategy for lens placement is:

Measure the RoC of the mirror.
Place the point source and measure its distance to the mirror vertex
Obtain the lens parameters from the manufacturer or by measurement.
Ray trace the system, finding the nulling location of the lens.
Mount the lens onto the KE holder so that it moves with the KE, at the 
specified distance from the KE.
Move the KE plus lens arrangement in and out to find the focal location.
Test the mirror in the "usual" way to determine the figure, given that it is 
a null test.

One must be careful to center the lens in the optical axis.  I am working on 
a fixture for my tester that will allow x-y motion of the lens to easily 
center it up.  I will soon have a testing system together to give the idea a 
try.  I have a 10" f/6 mirror with a figure known to be good to about 1/10 
wave, so it should produce a reasonable null with this method.  Will let you 
know.

The "real" reason that I am interested in this test method is to test a new 
Cassegrain telescope, 13" in aperture.  In lieu of making a large 
autocollimation flat, this test looks like a very nice way of finishing the 
figuring of the primary.  I won't go into the details of using it to test a 
Cassegrain, suffice it to say that the point source will be located on my 
neighbor's garage about 100 feet from the scope.  This is far enough so that 
all of the primary that is interesting to test will be illuminated even when 
the secondary is in place.

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