RE: ATM An $18.00 Null Test

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

As has already been mentioned, this looks like a variation on the Dall null.
However I think it might have some advantages, in that the lens can probably
be smaller and closer to the knife-edge since the distant light source
reduces the spherical aberration to be countered by the lens.  As well, the
tester is closer to the mirror (again because of the distant light source)
so that it will look larger and details might be easier to see.  Finally,
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.

However, I have often seen the statement that the test must be
monochromatic.  Is there any reason it could not be done with an achromat?
It would still have spherical aberration, but might be colour free enough to
use white light.  I suppose the biggest problem might be getting all the
accurate parameters for the lens.

I am currently figuring a 32cm (12.5") f/6 using the Foucault test.  I
wanted to go to the Ross null but discovered that, with the lens I have, the
lens to mirror separation is almost 7m (23 feet) and I can't measure that
distance to any sort of accuracy.  The reason I wanted to change tests is
that I seem to have got to the point where the central 11.5" of the mirror
is 1/28 wave with a 1/3 wave turned up edge around it.  Aargh!  I have been
trying to plane off that outer rim without ruining the inside (any hints on
this would be appreciated) and want to be able to watch the figure directly
rather than through the murky window of Foucault readings.

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


> ----------
> From:         Aplanatic@aol.com[SMTP:Aplanatic@aol.com]
> Reply To:     Aplanatic@aol.com
> Sent:         Monday, 26 July 1999 20:06
> To:   atm@shore.net
> Subject:      ATM An $18.00 Null Test
> 
> 
        snip
> Since we are using a single, uncorrected lens, the Foucault test should be
> 
> performed in approximately monochromatic light, and the system should be
> ray 
> traced at this wavelength.
> 
        snip