SPINOR Optics
Polarization Optics
All the optical mounts built for the Advanced
Stokes Polarimeter are used for SPINOR as well. Every ASP
optical element is replaced by a new achromatic one.
Entrance Window Calibration Polarizer
The entrance window calibration polarizer is an array of 13 VersaLight
wafers. The wafer diameter is approximately 150mm (with a flat on
one side aligned to the polarization direction). The mechanical
mount developed for DST calibration that fits over the entrance window
of the telescope has been modified to accept these achromatic Versalight polarizers.
Calibration Linear Polarizer
The calibration linear polarizer is VersaLight selected for good
performance over a 50mm aperture and AR coated for 450nm to
1600nm. The location of the calibration Linear polarizer is the
stage closest to the exit port of the telescope in the Calibration
Modulation Unit.
Calibration Retarder
The calibration retarder is a 50mm diameter bi-crystalline
achromat of the design shown in the plot, but with thicknesses
scaled for quarter wave retardance between 450nm to 1600nm and AR
coated for the same range. The
location of
the calibration retarder is the stage just above the calibration linear
polarizer in the Calibration Modulation Unit where the light beam is
f/72.
Polarization Modulator
The polarization modulator is a 50mm diameter bi-crystalline
achromat with thicknesses scaled for 0.35 wave retardance from 450nm
to 1600nm and AR coated for the same range. The
location of
the polarization modulator is just after the fast mirror near the
exit of the Calibration Modulation Unit where the beam is f/36.
This retarder design includes positive and negative
birefringent crystals has a low variation of retardance
vs. angle. It is likely
that the temperature will need to be monitored as there is a small
variation of retardance
vs temperature
that diminishes
with wavelength.
Polarizing Beam Splitter
The polarizing beam splitter uses a VersiLight beam splitting cube, is
16.5mm per side and is the same design as that developed for the
ASP. As with the ASP, the image of the slit is split into two
beams separated vertically. Each beam is polarized at 45º to the orientation of the
rulings on the grating. The polarizing beam splitter is just behind the
entrance slit of the
Horizontal spectrograph.
Polarization
Optics Specifications
Polarimeter Response
The polarimeter response matrix maps the input Stokes vector from the
DST to the matrix output from SPINOR. The modulator retardance
varies as a function of wavelength. Between the modulator and
polarizing beam splitter there are some number of mirrors. These
affect the polarimeter response especially when, as in this case, the
beam splitter operates at 45º
to the orientation of the mirror reflections. The polarimeter
response in the product of the Mueller matrices for the modulator,
mirrors, and polarizing beam splitter, and a demodulation matrix
determined by how successive camera reads at different rotation
positions of the modulator are summed . The resulting SPINOR polarimeter response matrix looks rather
gruesome, but is just information on what to do with the detected
Stokes vector in order to recover the vector arriving at the
polarimeter.
Gratings
Since SPINOR operates over a much wider wavelength range compared to
ASP, different grating choices are appropriate. Depending upon
the spectral line of interest and spectral resolution desired different
gratings might be selected. Higher blaze angle gives higher
spectral resolution for the same spatial sample size. Since
SPINOR will operate with AO, its design should tend towards higher
spatial resolution than the ASP, but since it is a research instrument
and cannot be optimized for a single line, not as high as DLSP.
These spectrograph issues
lead to a spectrograph using a 40 mm
slit and 1000mm camera lens.
The Infrared Triplet is of special interest for SPINOR as the three
lines are inaccessible to ASP both due to the calibration optics and
the performance of the 316 line/mm grating at these wavelengths.
Four possible SPINOR gratings
have good performance for near infrared lines. Additional grating
selections from the NSO
grating list
might be appropriate for your needs. For your own needs try
this spread sheet.