Subject: Thoughput of 3.5-m mirrors

From: Walter Siegmund

Submitted: Mon, 26 Aug 1996 17:51:47 -0700

Message number: 78 (previous: 77, next: 79 up: Index)

Our estimates of the throughput of the 3.5-m mirrors have been based on
measurements using a Microscan scatterometer (SMS, Portland OR). It may be
that these measurements include a component of the scattered light, in
addition to reflected light. The result is an overestimate of the
reflectivity of these optics by perhaps 10% per surface.

John Varsik of the National Solar Observatory has made preliminary
measurements of the particle size distribution on a mirror surface that has
been exposed to contaminates in a manner similar to the 3.5-m primary
mirror. He uses a microscope to image the mirror surface. His results
suggest that the particles between 30 and 40 microns in diameter are the
dominant scattering contaminates.

These particles remove light in two ways. As tiny bits of (mostly)
transparent silicate crystals, they refract and reflect light over a fairly
large angles. By disturbing the wavefront, they defract light over a solid
half angle of lambda/diameter or about 1 degree.

The Microscan has an aperture of about 1 mm about 30 mm from the surface.
Thus, it appears to include light over a solid half-angle of 1 degree or
more as reflected. Consequently, it may count diffracted light as reflected
(plus a bit of the refracted light). The diffracted light will not enter a
spectrograph slit because of its very small solid angle.

How big is the effect? If the Microscan measures 80% reflectivity, i.e.,
10% loss from the coating and 10% detected scattering, the actual
scattering may be 20% and the actual reflectivity 70%. For two surfaces,
the reflectivity may be 49% rather than 64%. This is a significant effect.

A couple of final comments: (1) Jim Gunn has designed a multispectral
reflectometer. We may wish to think a bit more about the reflectance solid
angle for this instrument. (2) The particle counter that monitors air
quality at APO does not sample enough air to acquire good statistics for
particles larger than about 1 microns. If particles between 30 and 40
microns in diameter are the dominant scattering contaminates, we should be
counting these particles instead.

--Walter Siegmund

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