RE: ATM An $18.00 Null Test

[Ric Rokosz <bz737@freenet.toronto.on.ca>]

On Wed, 28 Jul 1999, Wagner,Rick [CIS-ISB] wrote:
 
> In the Dall null the lens has to be small in order to avoid having it
> interfere with the light cone returning to the knife edge.  In this
> variation of the Dall however, the lens needn't be small since the light
> cone is diverted out the side by a diagonal.  Therefore you can use the
> central portions of a large lens just like in the Ross null.

I'd really be concerned about the quality of the diagonal in this 
setup.At what wave rating does the quality of the null fall off - is a 
1/10 wave surface useable -a 1/4 wave - 2 waves - 10 waves.From my 
experience in refiguring the quality of the secondary is most important.
In the back of Texerau's book he mentions he improved the quailty of the 
82 inch Mcdonald scope by refiguring the coude secondaries(!).

> 
> I am interested in the improvement you saw with the home made lens.  To
> wwhat do you credit the improvement?  Figure, surface quality, coatings?
> That might be an interesting project for the future.  I have a surplus Ross
> lens I bought off Peter Ceravolo.  He says it is quite good but I have no
> way to test it.  I have a 10" perforated sphere that I was going to try as a
> Waineo null but was having problems figuring it to a satisfactory sphere.

The surplus lens I used is probably the same one you have - 6.3 inches 
bk7 r=9.065 or so.I got mine from Bob Sandness which is where peter got 
his - a large patch from a surplus house in Toronto.I understand that 
Peter tested them all out on a interferometer and some of the quality 
couldn't be beleived.Perhaps he sold you one of the better ones.
The improvement in my homemade lens was mainly the accuracy of the convex 
side.It came out to better than 1/8wave and the plano ( tested with the 
Ritchey-Common setup)  at 1/4 wave.

The lens from Peter is most useable.Search the archives for a long thread 
on the Ross.Also read the TM articles very carefully.One critical point 
not mentioned on the list is that the RC changes as the curve is roughed 
in and you have to re-measure it when your're close to the ideal shape 
and then recalculate the Ross spacing.Subtle this but crtical.

Ric
Toronto