Subject: Size distribution of contaminates on optical surfaces

From: Walter Siegmund

Submitted: Fri, 3 Oct 1997 15:42:45 -0600

Message number: 183 (previous: 182, next: 184 up: Index)

Most of the work on particles and optics contamination was done by John
Varsik while he was at NSO, Sacramento Peak. (He is now at Big Bear.)
He found that the particles between 30 and 40 microns in diameter are
the dominant scattering contaminates on dirty mirrors. See the URL at
the bottom for the details.


The number density of airborne particles falls very roughly as d^-4
where d is the particle diameter. Consequently, the density of 1 micron
particles is a million times larger than 30 micron particles and it is
correspondingly much easier to measure the density of 1 micron
particles than 30 micron particles.  John Varsik has looked for
commercial instrumentation to measure 30 micron particle density
without success. It would be possible to design and build such an
instrument, but my group does not have funding or resources to pursue
this at present. 


In the meantime, we must rely on surrogate measures of 30 micron
particle density, e.g., 1 micron counts or visibility of a flashlight
beam. Neither is particularly satisfactory since both sense particle
smaller than the dominant size for contamination and the power law
exponent is quite variable. Flashlight beam visibility is most
sensitive to ~3 micron particles, depends on the observer, flashlight
and battery charge, is qualitative and requires active participation by
the observer.  To some extent, the limitations of these techniques are
complimentary. I believe that both should be used until something
better is available.


               Walter Siegmund


http://www.sunspot.noao.edu/~varsik/dust/absindex.html


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