RE: ATM An $18.00 Null Test

["Wagner,Rick [CIS-ISB]" <Rick.Wagner@EC.GC.CA>]

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 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.

Rick Wagner
Ottawa Centre  RASC
45*27'16.6"N  76*15'30.1"W



> ----------
> From:         Ric Rokosz[SMTP:bz737@freenet.toronto.on.ca]
> Sent:         Tuesday, 27 July 1999 22:50
> To:   Wagner, Rick [CIS-ISB]
> Cc:   'atm list'
> Subject:      RE: ATM An $18.00 Null Test
> 
> On Tue, 27 Jul 1999, Wagner,Rick [CIS-ISB] wrote:
> 
> > ing a single pass through the lens means the lens quality is somewhat
> less
> > important.  Given any lens it should be easy enough to use a ray-trace
> > program to minimize spherical aberration by adjusting only the
> mirror-lens
> > and lens-knife edge spacings.
> 
> The lens quality is most important in these tests.The Dall has to be 
> really good to work at all and the Ross is a bit more foregiving if you 
> use only the central area of a large lens.
> 
> I used a surplus lens for a while and then made a custom Ross lens and the
> 
> difference was  amazing - shadows were clearer and easier to see.
> 
> Ric
>