Special enhanced silver mirror coatings were designed and fabricated to minimize the polarization introduced by a three-mirror
off-axis high-accuracy telescope. A system diattenuation of approximately 1% in the VIS-NIR was achieved by
both reducing the diattenuation from each mirror individually and by balancing the diattenuations introduced by the
three mirrors over the spectral range. This process of low-polarization engineering involves minimizing system
polarization introduced by surface geometry, thin film coatings and birefringent elements, and measuring the system. In
this report we will outline a methodology to minimize instrumental polarization aberrations, with an emphasis on
achieving low diattenuation in the MSPI camera, given its off-axis geometry and coating design constraints imposed by
the space-based application. This polarization balancing technique for mirror coatings can be applied to astrophysics
applications.
When the optical elements of a system are not collinear, there are advantages to aligning all elements simultaneously.
This paper presents the steps taken to prepare for system alignment and the alignment plan for such a system. A
tolerance analysis of the system defines the compensators necessary for system alignment and allows an estimate of the
expected magnitude of initial aberrations present in the system. Polarization and pupil aberrations are characterized in
order to further understand expected system aberrations before alignment. A two step alignment plan is outlined. First,
a CCD array placed at the focal plane indicates spot size and shape as elements are aligned. Once spot size is
minimized, the CCD array is replaced by a ball bearing for retroreflection. Useful interferograms can be obtained with
which remaining aberrations can be minimized. This technique is presented as the alignment plan for an off-axis
telescope system consisting of one spherical and two ellipsoidal mirrors.
Polarization-sensitive optical systems include those requiring very accurate irradiance measurements and those where
polarization is the intended measurement. Low-polarization optical system design is the process of minimizing system
polarization introduced by surface geometry, thin film coatings and birefringent elements, and measuring system
components to verify polarization performance. The complicated, multi-step, iterative low polarization optical system
design process requires initial system design, witness sample fabrication and measurement, reverse engineering of
fabricated coatings and coating redesign, end-to-end system polarization aberration analysis, and system measurement
and calibration. Most of this process will be spent iterating between design and measurement phases until a final design
is reached that can be fabricated and calibrated to perform within the desired system tolerances. This work discusses
low polarization optical system design using a three-mirror off-axis camera as an example.
Dust is known to aggravate respiratory diseases. This is an issue in the desert southwestern United States, where windblown dust events are common. The Public Health Applications in Remote Sensing (PHAiRS) project aims to address this problem by using remote-sensing products to assist in public health decision support. As part of PHAiRS, a model for simulating desert dust cycles, the Dust Regional Atmospheric Modeling (DREAM) system is employed to forecast dust events in the southwestern US. Thus far, DREAM has been validated in the southwestern US only in the lower part of the atmosphere by comparison with measurement and analysis products from surface synoptic, surface Meteorological Aerodrome Report (METAR), and upper-air radiosonde. This study examines the validity of the DREAM algorithm dust load prediction in the desert southwestern United States by comparison with satellite-based MODIS level 2 and MODIS Deep Blue aerosol products, and ground-based observations from the AERONET network of sunphotometers. Results indicate that there are difficulties obtaining MODIS L2 aerosol optical thickness (AOT) data in the desert southwest due to low AOT algorithm performance over areas with high surface reflectances. MODIS Deep Blue aerosol products show improvement, but the temporal and vertical resolution of MODIS data limit its utility for DREAM evaluation. AERONET AOT data show low correlation to DREAM dust load predictions. The potential contribution of space- or ground-based lidar to the PHAiRS project is also examined.
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