From: "Tom Krajci" 
To: 
Reply-To: "Tom Krajci" 


From: "John Sherman" 

>..I have a couple questions about getting a 22" mirror coated.

>For deep sky viewing, it seems to me that an enhanced coating is
superior.
>It seems that the image is not only brighter, but that there is also
better
>contrast. Is that correct?

Enhanced coatings do reflect more light...but it's only about 5-10 percent
more than aluminum coatings.  Are you sure you can see a 5-10% difference?

Try this simple test.  Take a piece of clean window glass (no
anti-reflection coatings allowed)...look at a typical night sky scene
through it (naked eye, or through a scope, and make sure you don't have
problems with streetlights or other distracting reflections on the window
glass).  Now look at the scene without the glass.  See how much better it
is?  (I can't tell the difference, but your mileage may vary.) This roughly
approximates the *brightness* difference between enhanced and non-enhanced
coatings.  Are you sure you want to spend the extra bucks for this
improvement in image brightness?

But what about *scattering* of light by the more complex/multi-layer
coatings?  That may be more damaging to contrast...especially for lower
quality/higher-scatter coatings.

The $64 dollar question.  How does the typical ATM measure scatter from an
optical surface or coating?  One test was described by Suiter (in
ATMJ?)...that tested the entire telescope system contrast (baffling, dirt,
coating scatter, etc.).  It required a CCD, computer, and an inexpensive
light trap.

Who has conducted this (or similar) tests?

I'll make a bold, daring prediction:  fewer than ten, repeat ten, folks on
this list will reply that they have conducted this sort of test. Sigh, this
is the ATM list...which explicitly states "making" telescopes,
but testing them is optional.  ;-)  ...no!...  :-(

>...Is there a way I can test mirror coatings for quality? I'm thinking
the
>amount (%) of reflectivity can be estimated by shining a laser into a
>photometer, and then reflecting the laser off of the mirror into the
>photometer. Has anyone actually done such things? On an amateur level?

In my opinion the more important test should be for scatter/contrast.  A
five percent gain in brightness (at some financial expense) can be totally
ruined by a larger amount of scattering.  The less-complex coating scheme
may be better, and less expensive too.

Good luck finding reliable, consistent, verifiable information on scatter
values for various coatings (and various coating techniques...oy,
vey...this can be quite a can or worms).

Tom Krajci
Tashkent, Uzbekistan


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