Narrow-band Spectropolarimeter

The mission of this instrument is to spectrally isolate narrow-bandpass images of the Sun at the highest possible spatial and temporal resolution. Observations with this instrument should allow rapid imaging spectrometry, Stokes imaging polarimetry, accurate surface photometry and spectroheliograms that will result in Doppler velocity maps, transverse flows and imaging magnetograms that track evolutionary changes of solar activity.

The technical feasibility of a narrow-band tunable-filter imaging system has been studied. From the analysis, it turns out that only a Fabry-Perot interferometer-based design with air gaps can meet most of the science requirements. There are two different philosophies in mounting the etalons within the optical light beam. In the collimated version, the etalons are mounted in the parallel light beam near an image of the telescope entrance pupil; in the telecentric configuration the etalons are mounted in the convergent light beam near an image plane. Both configurations have their own advantages and disadvantages and put similar demands on the quality and size of the required etalons.

Fabry Perot interferometers have a very small free spectral range. So, an ordinary interference filter cannot be used as an order sorter. A solution to this problem is the use of multiple etalons in series.