RE: ATM An $18.00 Null Test

["Bertapelle, Ken" <Bertapelle.Ken@tci.com>]

Dave, et. al.,

I have a question about null tests that's been nagging at me for
a while.  I have a slitless focault tester that works as follows:
An intense light source is delivered via fiber optic cable to a
little gadget that I'll refer to as the "viewing node".  The node
has a small opaque window in it that points toward the mirror and
allows the light to shine on the mirror.  Also mounted on the node
and right next to the light window is a sliver of razor that acts 
as the knife edge for the focault test.  The whole node, not 
counting mounting hardware, is about the size of a dime, and maybe
3/8" thick.  (BTW, neither the design nor the construction are my
own - a friend has loaned me the tester so I can make a copy.)

I've found that this focault tester works very well because it 
delivers a *lot* of light, and the shadows are much easier to 
see than those with the first tester I made patterned after the
design in Sam Brown's book.

However, the light source and knife edge move together on the 
viewing node, and they are only separated by about 1/8".  Thus, in
any null test the light would have to go through the lens twice -
once on its way to the mirror, and once on return.  My question is
whether a null test can be made to work with such a tester.  If
so, what kind of lens does one use?  

Thanks in advance.

- KenB


> -----Original Message-----
> From: Aplanatic@aol.com [mailto:Aplanatic@aol.com]
> Sent: Monday, July 26, 1999 6:06 PM
> To: atm@shore.net
> Subject: ATM An $18.00 Null Test
> 
> 
> 
> I have discovered that one can null test almost any 
> paraboloidal (or other 
> conic section) mirror using a single lens in front of the 
> knife edge and a 
> good quality diagonal mirror.  
> <snip>
> Dave Rowe
> Torrance, CA
> Medium Format Astrophotography:
> http://members.aol.com/aplanatic/photos/astro.html
>