Re: [ATM] Polishing with WAO?

["Tom Krajci" <tom_krajci@tularosa.net>]

>From: "Mel Bartels" <mbartels@bbastrodesigns.com>

>...The ultimate tests are 1) what does the coating look
>like and 2) star tests for smoothness and scatter.

Can we quote typical, best, and worst case scatter values for:
- coatings
- dust on the optics
...then can we come up with a good way to evaluate:
- correct grinding (so pits from earlier grades don't spoil your polishing
efforts...is it because amateurs don't have good evaluation methods...or
that many amateurs disregard proven methods?)
- sufficient polishing

What is sufficient polishing?  Depends on how much scatter you get from
coatings, dust, etc.  If you find that a typical mirror coating and dust
scatters 11% (i.e. 11% of all light that comes off the mirror is not
specular reflection)...then is it really so bad to have a polished surface
scatter 0.5% or 1%?  (Assuming scatter from various sources combines in a
random manner, root sum square math shows you that the contribution of 0.5%
or 1% to 11% is pretty small.  But is that a valid assumption for scatter?)

Think of it like an error budget for scope design (flexure, drive
performance, etc.)  Set your goals, set budget limits on the various
contributors (make the goals/limits realistic!)...and don't waste time where
it makes little improvement.  Find the biggest source of error in your
budget (that you can control)...control it, measure it, verify it.

You want a super-polished surface?  Maybe that is only good for bragging
rights around the water cooler, or perhaps marketing.

Here's one handy rule of thumb:  the 80 - 20 rule.  You achieve 80 percent
of your goal with 20 percent of the effort.  It's not too hard to achieve an
80 percent solution, as long as you're honestly measuring your progress.
You want a 90% solution - you're gonna work lots longer for a small gain.
95%?  99%?  Much more work...and can you really measure that level of
performance...or is there more hand waving and smoke blowing going on at
that level?  ;-)

-------------------------------------------
Tom Krajci
Cloudcroft, New Mexico
http://overton2.tamu.edu/aset/krajci/

Center for Backyard Astrophysics (CBA)
http://cba.phys.columbia.edu CBA New Mexico

American Association of Variable Star
Observers (AAVSO): KTC http://www.aavso.org/
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