[ATM] Scattered thoughts

[Scott Milligan]

Tom:

One of the questions/issues you will face will be to decide on the
instantaneous field of view (extent of the solid angle) that your
scatter-sensor sees & collects with each measurement.  As you may already
know, the solid angle into which light scatters after encountering a surface
varies inversely with the size of the scatter site(s), (measured in
comparison to the wavelength, in a direction transverse to the incident
beam), whereas the intensity and polarization state of the scattered waves
often depend strongly on the depth (again relative to wavelength),
distribution (periodic or random), incidence angle, and material (metal or
dielectric).  From the (not extensive) readings that I have digested on the
subject, I recall reaching the conclusion that the strongest interactions
occur with structures having an average transverse extent within the range
1/2 - 200 wavelengths.  

A sensor with a well defined, relatively small field of view might be made
to scan a larger solid angle, in the process giving information regarding
how the scatter varies with increasing angle away from the known, specular
beam path.  Such an instrument is often referred to as an angle resolving
scatterometer.  I can envision a conceptually simple rig consisting of a
pinhole source, mirror test stand, and a single element detector preceded by
a suitable arrangement of lenses and limiting apertures and/or obstructions.
The detector head might be mounted on a swing arm that pivots about the
mirror vertex, such that the distance between the mirror and the detector
remains essentially constant as the angle between the optical axis and
detector is systematically scanned to collect a data set (in one plane, at
least).  An occulting disc sized to block the central maximum of the pinhole
image might be used to filter out the relatively strong intensity associated
with the specular beam path, leaving only the scattered light incident on
the detector, whereas a field stop could precisely define the instantaneous
field of view seen at any moment during the scan by the sensor.  A chopper,
filter wheel, and lock in amp would only make it sexier (an old time ATM
like yourself has all this stuff hanging around in your basement, don't
you?)

If you decide to make it a project, I (and no doubt others on the list) will
be interested to hear what results.  The absolute worst-case scatter I ever
experienced with my own optics occurred about 20 years ago as a casualty of
the New England winter; after an observing session during which the
temperature of my first self-made Newtonian mirror fell well below freezing,
I brought the scope inside to a room that was only kept heated while people
were present.  Condensation formed on the mirror surface, and before it
could evaporate, we all left for the night, turning off the heat.  I
theorize that the dew must have penetrated the (partially) porous quartz
overcoat, froze, and then expanded, created what I'll call a "Callisto"
effect on the coating.  Really bad scatter, had to strip & recoat.


Scott Milligan
 
 

-----Original Message-----
From: atm-bounces at atmlist.net [mailto:atm-bounces at atmlist.net] On Behalf Of
Tom Krajci
Sent: Monday, January 08, 2007 6:01 PM
To: atm at atmlist.net
Subject: [ATM] Measuring scattered light off optical surfaces

What amateur techniques are used to quantitatively measure scattered light
off optical surfaces?  I know of various go/no-go tests for evaluating
quality of polish, but what about quantifying the amount of scatter of a
polished surface, or an aluminized surface?

Thanks in advance.

-------------------------------------------
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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