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


Greetings All,

Last night was interesting. A friend and I attempted to measure the
reflectivity difference between an eight month old silver coating, the
freshly stripped mirror surface, and the surface after aluminizing with
Reynolds wrap.

Anyway, here's what we did, with the results below

Starting with the eight month old Silver coating: (Note, this 8mo old
SILVER coating was "OH MY GOD" terrible, being very thin,
blotchy, and rather disgusting in general!)

#1 Setup for a Foucault test using a slitless tester and a blue LED
#2 Set the longitudinal distance to null zone 1 (radius = 1.9")
#3 Laterally moved knife out of return beam 0.05" (fully illuminated mirror)
#4 Used a Sony digital camera {at} F/2.3, Full Optical Zoom, and exposure of
1/80 sec
#5 Recorded image of mirror as a TIFF file (only format that doesn't use
compression)
#6 Adjusted LED power to get a best guess of 66% pixel saturation

Next we stripped the mirror coating, washed it, and put it back on the test
stand.  Snapped another image using the same camera settings as above,
however this time we DID NOT adjust the Pot for the LED!

Next we coated the mirror, then snapped another image.

At this point, I had three images of the mirror, and 'Theoretically' the
only thing that changed was the coating (or lack there of) on the mirror.
Next I found the average Red, Blue, and Green pixel values for a 150x150
square at the center of the mirror.  Here's the results

Uncoated Total = 15
Red = 3
Green = 2
Blue = 10

Silver Coat Total = 171
Red = 0
Green = 8
Blue = 163

Aluminum Coat Total = 174
Red = 0
Green = 27
Blue = 147

So, Now what to do with this Data? Can I convert this into a % reflectivity
knowing that the uncoated mirror surface should have a reflectivity of 4%??

Another interesting item is the Green pixel values for the three tests, any
thoughts here? (They appear more inline with what I'd anticipated given the
condition of the original silver coating)

Finally, if interested, you can download the images for the three tests here:

Cropped Bitmaps:
http://lerch.no-ip.com/atm/reflect_bmp.zip (3.4MB)

Original TIFF Files
http://lerch.no-ip.com/atm/reflect_tiff.zip (11MB)

Take Care,
James Lerch
http://lerch.no-ip.com/atm (My Telescope Construction, Testing, and Coating site)

--- BBBS/NT v4.00 MP