Scattered Light From Telescope Mirrors

1
Scattered Light From Telescope Mirrors
Brian Daugherty, Russell Chipman University of
Arizona College of Optical Sciences

Introduction
Coherent Small-Angle Polarimetric Scatterometer
The Terrestrial Planet Finder Mission seeks to
detect extrasolar planets at a contrast of 1012.
This requires very tight tolerances on all
optical system specifications. Wavefront control
and mirrors substrate surface quality have been
addressed as demonstrated on JPLs high contrast
imaging facility. The effects of near angle
scatter and the slight depolarizing properties of
metal coatings on image formation at these levels
are not well understood. We are developing the
small angle polarimetric scatterometer to
characterize metallic coated mirror samples.
Scattering and Polarization from Mirror
Roughness Scattered light from surface roughness
is typically the primary source of scattered
light in optical systems. This has led to a great
deal of research resulting in several useful
models to characterize this effect. However,
there is no data for near angle depolarization
caused by surface scattering.

• We are developing a system to measure the
  polarimetric and scattered light effects of
  metallic mirror coatings at small scattered light
  angles with these goals
• High dynamic range
• Measure low scattered light levels
• Polarization sensitivity
• Minimize scattered light within test
• The first implementation should measure within
  20 arc-seconds of specular.

Issues
Three scattering and polarization effects can
degrade the performance of the TPF optical system
in the sub arc second region around the Point
Spread Function (PSF) core.
Mirror Coating Anisotropy There has been little
research related to the effects of reflection
coating anisotropy. The columnar microstructure
of metallic coatings leads to an anisotropic
scattered light pattern with polarimetric
dependencies. James Breckenridge of JPL says, In
the case of TPF science, even very small patches
of anisotropy could scatter enough light to make
the background overwhelm objects 1010 times
fainter than a bright star in the field of view.
Polarization Aberrations The differences between
the s- and p- Fresnel reflection coefficients
cause polarization aberrations, the coupling of
small amounts of light between polarization
states. This gives rise to PSF components which
can weakly couple into the first null of the Airy
pattern. These effects can be modeled by CodeV
and ZEMAX polarization ray tracing.
Cassegrain Telescope, Linearly Polarized
Illumination
Our design is a coherent detection polarimetric
scatterometer which scans large, high f,
diffraction limited point spread function with a
frequency shifted reference beam over a pinhole
with several sets of polarizations. Coherent
detection provides high dynamic range and
polarization selectivity dynamic ranges of
140dB and a minimum detectable power of 10-17
watts have been demonstrated elsewhere. Only one
element in the critical path contributes unwanted
scattered light.
Shows the columnar structure found in metal
coatings
Cross Polarized PSF x 100, fills in first dark
ring
Pupil between crossed polarizers
Co-polarized PSF, almost Airy Function
Kassam, S., et all. Applied Optics. Vol. 31, No. 9