Re: [ATM] zone one challenge

[Mike Lockwood <melockwo@uiuc.edu>]

Steve,

Great work!  An excellent page and simulation.  Very thought 
provoking, and it really shows the problems with reading zones.  The 
region of similar "grayness" is outside the center line of the zone, 
making an accurate reading difficult.  Of course, similar issues occur 
in the outer zones of fast mirrors, where zones are narrower but the 
shadow gradient is stronger.

Actually, I think your page also demonstrates quite well the problems 
with reading zones that are too wide and the problems that can result. 
  If this were my mask, I would probably have made a new one with two 
zones instead of the one wide one.  Of course this only works on outer 
zones to a certain point of narrowness - after that, readings are very 
difficult indeed.

Other ways I fight Foucault errors:

I like using two different Couder masks for one mirror.  Mask 1 might 
have 7 zones while Mask 2 has 6 zones, or maybe Mask 2 has its zone 
centers where Mask 1 had its zone edges.  If I take readings with both 
masks and they agree, then I am much more confident in the resulting 
error profile.  If they don't agree, the disagreement (over several 
sets of readings) is often consistently in the same portion of the 
mirror.  Then I know that there is measurement uncertainty in that 
area (the height of a raised 70% zone, for example) and can take more 
measurements.

For example, if readings show that the 70% zone is 1/10th wave high, 
that might not sound too bad.  But if another set of readings shows 
1/4 wave high, that is not good, and will alert you to a problem.  The 
truth is likely somewhere in the middle, and you will have to use your 
own judgement in deciding what action to take.  These readings 
actually popped up for me when I was figuring a strongly corrected 
concave test plate (for interference testing a Cassegrain secondary). 
  I worked the high zone down a bit, and with subsequent figuring the 
two sets of readings eventually converged, providing quite similar 
error profiles (given by FigureXP).

I have not, however, noticed high centers in my readings with multiple 
masks.  I do tend to use narrower center zones.

Until I finish my autocollimation flat, I'll be using at least two 
different masks (each with different zone radii) with zones that are 
as narrow as possible (without causing reading difficulty).

	Mike Lockwood


PS: I chose "I", 0.002" inside the proper null.

Koehler, Steve wrote:
>  I have written up my ideas in the following
> report, which includes a challenge to find the null for zone one from a series
> of 15 simulated images.
> 
>     http://www.visi.com/~mkoehler/zoneone/zoneone.html


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