Simulations of instrumental effects on spectropolarimetric solar observations for the VTF

The VTF (Visbile Tuneable Filter) is a two-dimensional high-resolution spectropolarimeter and is currently developed at the Kiepenheuer Institute. As one of the state-of-the-art scientific instruments at the future 4-m-class telescope DKIST on Hawaii it will play a major role in the next decade of solar observations. The setup of the VTF consists of several Fabry-Pérot interferometers (FPI), a wavelength-dependend narrow pre-filter and a polarization modulator. Due to the requested accuracy of the physical measurements, an investigation of the instrumental impacts on the physical data acquisition was required in advance. Based on the inquired simulations, the requirements for manufacturing were inferred and strategies for the data calibration developed.

Simulated maps of a solar region. The panels show a comparison of the full-width-half-maximum values (left) and Doppler velocities (right) for two different instrument setups (1: middle row, three FPIs; 2: bottom row, two FPIs). The panels on the left and top right show the ideal case without errors. The bottom right panels display the resulting maps with induced errors for both instruments.

A measurement of a Quiet Sun region was simulated with and without instrumental defects. For a tele-centric setup of the interferometers in the optical path (like planned for the VTF), the detection of flatness defects of the surfaces is of crucial importance. 

The figure shows the impact of lokal defects of the plate spacing of Fabry-Pérot interferometers on an observation of a granular region on the Sun. The differences for two instrumental configurations become apparent between a Triple-FPI system (instrument 1, middle row) with a high spectral resolution of 200,000 and a Tandem-FPI system (instrument 2, bottom row) with a resolution of 200,000 at a central wavelength of 630.25nm.

To clarify the differences, the panels showing the ideal full-width-half-maximum values (left) are compared with the results including the simulated instrumental impacts of statistically distributed local plate defects with a root mean square of 2.5nm. The obtained Doppler velocities of the ideal case without defects (upper right) and including the flatness errors for both instruments (lower right) emphasize the differences and the relevance of this study.

More information see here.