The NASA Interface Region Imaging Spectrograph (IRIS) mission is a Small Explorer (SMEX) satellite mission
designed to study plasma dynamics in the “interface region” between the Sun’s chromosphere and corona with high
spatial, spectral, and temporal resolution. The primary instrument is a dual Czerny-Turner spectrograph fed by a 20-cm
Cassegrain telescope measuring near- and far-ultraviolet (NUV, FUV) spectral lines in the ranges 133-141 nm and 278-
283 nm. To determine the position of the slit on the solar disk, a slit-jaw imaging system is used. The NUV slit-jaw
imaging system produces high spatial resolution images at two positions in the Mg II 280 nm spectral line complex using
a birefringent Solc filter with two wide-band interference pre-filters for spectral order selection. The Solc filter produces
a 0.36 nm full-width at half-maximum (FWHM) filter profile with low sidelobes and a peak transmission of 15% at
279.6 nm. The filter consists of two “wire grid’’ polarizers surrounding 8 quartz waveplates configured in a modified
Solc “fan” rotational pattern. The elements are optically coupled using DC200 silicon-based grease. The NUV Solc filter
is sealed in a windowed cell to prevent silicon contamination of the FUV channel. The design of the sealed cell and
assembly of the filter into the cell were among the most challenging optomechanical aspects of the IRIS spectrograph
Meadowlark Optics has successfully built and demonstrated a liquid crystal based tunable filter with novel FWHM
tunability. This allows separate control over both the location of a narrow spectral bandpass and the width of the
bandpass function. This non-mechanical, imaging filter thus enables random access of the visible to near IR spectrum
and also controlling the specificity of the transmitted light. We will discuss both the relative trade-offs in this filter
design space and present data from functional units.
Since the human eye is insensitive to polarization, there is a large amount of information in many situations which is not readily utilized. Measuring the polarization state of light is useful in many research fields including biology, chemistry, astronomy and remote sensing. The first portion of the paper discusses the simple application of accurately measuring the retardance value and fast axis position of an unknown waveplate. We will mention some of the many other polarimetry applications especially in the context of non-mechanical, liquid crystal based polarimeter experimental technique. Some of these examples are from biology showing tissue birefringence changes, astronomy for solar imaging, polarimetric visualization and landmine detection.
A new technology for performing high-precision Stokes polarimetry is presented. One traditional Stokes polarimetry configuration relies on mechanical devices such as rapidly rotating waveplates that are undesirable in vibration-sensitive optics experiments. Another traditional technique requires division of a light signal into four components that are measured individually; this technique is limited to applications in which signal levels are sufficient that intensity reduction does not diminish the signal-to-noise ratio. A new technology presented here is similar to the rotating waveplate approach, but two liquid crystal variable retarders (LCVR’s) are used instead of waveplates. A Stokes polarimeter instrument based on this technology has been made commercially-available. The theory of operation is detailed, and an accuracy assessment was conducted. Measurement reproducibility was verified and used to produce empirical estimates of uncertainty in measured components of a Stokes vector. Uncertainty propagation was applied to polarization parameters calculated from Stokes vector components to further the accuracy assessment. A calibrated polarimeter measures four Stokes components with 10-3 precision and average predicted uncertainties less than ±2x10-3. An experiment was conducted in which the linear polarization angles were measured with a LC polarimeter and with a photodiode for comparison. Observed discrepancies between polarization angle measurements made with a polarimeter and those made with a photodetector were nominally within ±0.3°.