Re: ATM Lateral Wire Test: was Caustic Test

["Nils Olof Carlin" <nilsolof.carlin at telia.com>]

John Swensson wrote

> This test has been termed the
> "Lateral Wire Test" by the originators (John Francis and Nils Olaf
> Carlson). 

I suppose I have contributed to the name given by John, but I am not an 
originator - though I have had faith in the test ever since I learned 
about it from John.

I started playing with this and found it to be by far the
> easiest test to perform of any of them. The experts say it gives as good
> an accuracy as the caustic test and much simpler to perform. 

While I believe it has very good potential precision, I don't believe it is 
quite a match for a well-setup Gaviola caustic. I don't know the practical
limits of mirror size and focal ratio, but I suspect it is *particularly* 
suited to small and/or slow mirrors where the Gaviola or Pinstick/Everest 
Foucault - not to mention the "regular" wire test - are totally useless.
> 
> I built a tester just for this test, its really simple, simpler than a
> Foucault tester and much easier to use. I took Peter Smith's idea of
> using a dial indicator as the transverse track and measuring device in
> one, its real simple and works great. Attached to the spindle is a non
> lasing laser diode pointing at the mirror. The advantage of this is that
> the light source moves and the wire stays fixed, thus its easy to put a
> camera or other device behind the wire. Behind the source is a vertical
> wire (or other such device). Thats IT! 

´For a "wire", I have had some success with drawing a line with a X-acto type
pointed knife on clear acrylic sheet - by varying the pressure, you can easily 
make "wires" of different widths and use the one you prefer (if too thin, the
center line is less distinct).



> I just use a regular tape measure to
> measure the distance between mirror and source and wire. It turns out
> you only need something around a 1mm accuracy on that. 

Or even less...
> 
> All these numbers get fed into sixtests using the caustic test setup.

this speaks highly of the versatility of Sixtests
> 
> 
> This test is very sensitive to zones in the mirror. Zones show up as a
> "wiggly" shadow thats really obvious. My current mirror is at the
> spherical stage (for the 4th time!) and shows nice straight ronchi lines
> but gives a scraggly shadow in the lateral wire test, under Foucault I
> can barely see the zones on the surface, when they are not there I get a
> nice straight shadow. 
> 
> This ought to become the standard ATM mirror test, its MUCH easier to do
> than a Foucault, the tester is super easy to build and the results are
> on par with caustic testing.

Not rushing ahead, I would agree that it merits further evaluation, 
determining the practical limits and considerations.

Nils Olof
> 
> John S.
>