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Description
Solar magnetographs are often equipped with Fabry–Pérot etalons to sample spectral lines that are sensitive to the magnetic field. Typically, two etalons with different optical thicknesses and reflectivities are used to reduce unwanted light from secondary orders while improving the spectral resolution. However, using two etalons in spaceborne or balloon-borne applications is usually prohibitive in terms of mass, power consumption and cost. Forcing light to pass through a single etalon twice (double pass) can therefore be preferable in these instruments.
We investigate the effects of a double-pass configuration on the instrument's effective transmission profile and compare the results with those obtained from in-flight calibrations of the double-pass TuMag instrument on board Sunrise.
We assess the spectral performance of double-pass configurations, considering intentional etalon tilts introduced to deal with ghost images in the detector. We evaluate the spectral response and its impact on the effective transmission and the shift in wavelength sampling across the field of view. Our findings show that the predictions are in excellent agreement with the blueshift maps and flat field images calibrated for TuMag during the sunrise flight.