Re: ATM better result with slitless tester ?

[Dwight Elvey <elvey@hal.com>]

claudio minutella <minutella@tic.ch> wrote:
> 
> i have a ordinary focault tester (fixed light source) for 
> testing my parabolica, but i am always a bit unsecure about 
> reading the measurements on the several zones. arriving on 
> zone 3 for example, i can move the knive a bit forward and 
> back - without any changes on the zone, both "windows" are 
> the same darkness. "the bit" is about 0,5 mm or 0.2 inches 
> and tehrefore it is a bit difficult to get a correct
> output on the data result of tex.exe

Hi Claudio
 I was looking at your tester but didn't notice if there
was any way to do fine adjustments right and left. The
position of the tester when graying needs to be quite
exact. It needs to be even closer than the adjustment
you are measuring. You can either move the entire tester
or just source or resolving edge but this is a vary
fine adjustment.
 Another thing I'm doing is to have the mask hindged
with a pull string. I can drop the mask at the tester
and adjust until I think things are correct and then
pull the string, bringing the mask up to make the final
light level comparision without the extra light.

> i am asking now, if i can read better the position with
> a slitless tester ... 
Slit-less type
Advantages:
1. self aligning when only one knife edge is used
    for both source and resolving edge
2. Brighter other side light makes it easy to find image and center.
3. Easier to deside which side of focus your at.
Disadvantages:
1. Brightness sometimes masks surface irregularities.

Slit type
Advantages:
1. surface irregulatities have higher contrast.
Disadvantages:
1. Slit must be vary narrow and even. Adjustment is tricky.
2. Light source must be quite bright because most of the light
   is lost.

Both are about as hard to adjust but I think one should be
able to get similar results form either.
 I suspect your troubles are caused by not having enough
lateral adjustment. This is really a fine, fine adjustment.
The other thing that comes to mind is that the surface isn't
fully polished yet.
 Maybe others have more to say about slit verses slit-less.
Dwight