Stray-light correction in 2D spectroscopy

Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany
e-mail: schliche@kis.uni-freiburg.de

Received: 15 March 2013
Accepted: 22 May 2013

Abstract

Context. In solar physics, spectropolarimeters based on Fabry-Pérot interferometers are commonly used for high spatial resolution observations. In the data pipeline, corrections for scattered light may be performed on each narrow band image.

Aims. We elaborate on the effects of stray-light corrections on Doppler maps.

Methods. First, we demonstrate the basic correction effect in a simplified situation of two profiles that suffer from stray light. Then, we study the correction effects on velocity maps by transforming a Hinode SP map into a two-dimensional spectroscopic data set with i(x,y) at each wavelength point, which mimicks narrow band images. Velocity maps are inferred from line profiles of original and stray-light corrected data.

Results. The correction of scattered light in narrow band images affects the inferred Doppler velocity maps: relative red shifts always become more red, and relative blue shifts become more blue. This trend is independent of whether downflows have dark or bright intensities. As a result, the effects of overcorrection produce both downflows and upflows.

Conclusions. In 2D spectropolarimetry, corrections for scattered light can improve the image intensity and velocity contrast but inherently produce downflow signatures in the penumbra. Hence, such corrections are justified only if the properties of the stray light (seeing, telescope, and instrument) are well known.