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.