From: "Peter John Smith" 
To: 
Reply-To: "Peter John Smith" 


While appreciating the sentiment that a TDE is bad, and is worth removing,
and always makes for a "poor" image, the simple fact is that
there is SOME point at which the TDE has no impact on the visible image.

Impact on an image should not really be confused with impact on a star test
which is more sensitive.

For SPHERES, either a Foucault or Ronchi will suffice but since it is
almost impossible know what othere are referring to without actually seeing
the test, this is a powerful argument for some attempt at quantification
because when someone says they think they have a TDE, it can range from
terrible to almost unperceptible.  An experienced user can use either
Foucault or Ronchi here to test the edge.  And, a first time user can also
- if he has some guidance on what to look for.

Zonal measurement with a knife is useful but with a narrower TDE is often
difficult or impossible.

I have written two programs which help to interpret and quantify Ronchi patterns.

There are limitations to them, and there are limitations to the whole
method.  But, used sensibly they will give you a reasonable feel for what
is an 'acceptable' edge.

On my web pages (below), go to the section on Ronchi then Software.

The program RonchiZ allows you to simulate Ronchi patterns for defective
mirrors including a turned edge.  It allows you to see what effect it has
on the bands when the shape of the defect is scaled up or down and also the
effect on the strehl.  And it allows the strehl to be estimated when
portions are masked off.


 http://www.users.bigpond.com/pjifl>

for Ronchi testing, special telescope and eyepiece designs, null tests,
Machining, etc.

The Ronchi material contains a downloadable program which simulates
Ronchigrams for defective mirrors with zones, TDE, Calculates Strehl for
defective mirrors etc.

Peter Smith.

--- BBBS/NT v4.00 MP