From: "James Lerch" 
To: "ATM List" 
Reply-To: "James Lerch" 


---- Original Message -----
From: "Mel Bartels" 
>
> Also matching the detector's spectral response to the eye is important.
> I've seen 10% to 50% errors by not taking this into account.  (I presume
the
> goal of your exercise is to measure coating efficiency for visual use).

Hi Mel,

The purpose of this exercise was three fold

    A) Heck, could it be done? ;)
    B) Just how bad was the 8 month old Silver Coating
    C) Just how good is a 'Reynolds Wrap' Al alloy coating?

The Jury is still out on option A.

Option B has some interesting background.  When I put the Silver coat on
this mirror 8 months ago, it was only a "so-so" coating.  The
mirror is an 11" F/5.8, btw.  With the fresh silver the mirror
performed very well,  as the coating deteriorated the result was increased
"Fire" (Diffuse Glow) around bright objects.

During the previous new moon, we had the opportunity to play with this
11" scope, my 8" binos, and both a 10" & 14" dob. 
All 4 of the scope were in the 60 - 70" Focal Length range.  The
consensus was the 11" performed as one would expect a quality 11"
scope to perform!  (Amazing once you read the next paragraph!)

So, when we took the mirror out for coating a few days ago, we were fairly
amazed at just how POOR the silver looked.  Most significantly, when the
mirror was held between your eye and a 40w fluorescent light, the
transparency of the Silver closely resembled a pair of dark sun glasses!!!
In addition, the coating thickness was inconsistent across the mirror,
looking very much like "Orange peel" textured dry wall!  (the
textured effect would definitely explain the diffuse glow around bright
objects!)

After Coating the mirror with Aluminum (Reynolds Wrap), we anxiously setup
and started observing with the fresh coating.  The first thing we noticed
was the images produced didn't 'Appear' any brighter than the silver
coating (entirely subjective of course!).  What was VERY obvious was the
now non existent diffuse glow around bright objects and several orders of
magnitude increase in contrast!!!   The amount of contrast gain became
obvious when
viewing Saturn at low power, and what at first appeared as a strong lens
flare turned out to be M1!  Keep in mind, were viewing with a 1st quarter
moon up pretty high, we have both Saturn and M1 (With structure directly
viewable, no averted imagination), needless to say we are looking forward
to the next new moon, and the opportunity to play with all 4 telescopes
again!
:)

With regards to Item C of our testing goal, it would be nice to know the
reflectivity of Reynolds Wrap aluminum foil.  I can say that holding a
piece of glass coated with Pure aluminum and one coated with Reynolds Wrap
aluminum alloy, there is no obvious difference in appearance.  (Both look
damn shiny! :)  I choose to stay with the Reynolds wrap alloy after doing
the following experiment.  I washed the two previous examples with Windex
and a paper towel.  The pure Aluminum coating was damaged with sleeks, the
Reynolds wrap was undamaged (Visual Observation!).  At the time I thought
to myself "Hmmm, Reynolds wrap is obviously tougher than pure
aluminum, it looks just as shiny and a roll is a lifetime supply, I think
I'll use that from now on :)"

BTW, I've learned that Reynolds Wrap Aluminum foil is an Alloy of Al, Fe,
& Si.  Percentages unknown, but I'm hoping to have a sample analyzed
soon to find out..



> Another way to take background into account simultaneously is to image the
> background and reflected mirror image onto the same frame.  Or to image
one
> reflection with many reflections onto the same frame.  Perhaps the latter
> might be of some use - if you can get a dozen reflections into the image,
> then that might increase accuracy.  You might be able to get a thin strip
of
> every odd reflection in the image too.  Then you could easily deduce the
> reflectivity of the 2nd mirror being used to make the multiple reflections
> along with the reflectivity of the mirror in question.

Very good idea!  Perhaps a flat piece of glass with two holes in it.  One
hole has an LED in it, a second has the camera.  Place the glass, LED,
Camera combo within a few inches of the mirror being tested, adjust the
angle to get multiple reflections. (Like the 'Artsy Fartsy' Infinity mirror
with lights around the border).  Next measure the change in intensity of
each reflection.  Sounds like an interesting experiment ;)

BTW, does anyone know where I might be able send a coated sample and get a
"Real" reflectivity measurement done across the near IR 
near UV spectrum?

Thanks,
James

--- BBBS/NT v4.00 MP