Off-disk straylight measurements for the Swedish 1-meter Solar Telescope

TL;DR

This study measures and models the straylight in the Swedish 1-meter Solar Telescope, finding that far-wing scattered light is minimal and other factors mainly reduce image contrast.

Contribution

It provides a method to quantify off-disk straylight using standard science cameras and models the PSF to assess its impact on solar observations.

Findings

Straylight adds a few percent of local granulation intensity.

Far-wing straylight extends to several solar radii but is minor.

Most straylight originates within approximately 1 arcminute of the disk.

Abstract

Context. Accurate photometry with ground based solar telescopes requires characterization of straylight. Scattering in Earth's atmosphere and in the telescope optics are potentially significant sources of straylight, for which the point spread function (PSF) has wings that reach very far. This kind of straylight produces an aureola, extending several solar radii off the solar disk. Aims. We want to measure such straylight using the ordinary science instrumentation. Methods. We scanned the intensity on and far off the solar disk by use of the science cameras in several different wavelength bands on a day with low-dust conditions. We characterized the far wing straylight by fitting a model to the recorded intensities involving a multi-component straylight PSF and the limb darkening of the disk. Results. The measured scattered light adds an approximately constant fraction of the local granulation intensity to science images at any position on the disk. The fraction varied over the day but never exceeded a few percent. The PSFs have weak tails that extend to several solar radii but most of the scattered light originates within ~1'. Conclusions. Far-wing scattered light contributes only a small amount of straylight in SST data. Other sources of straylight are primarily responsible for the reduced contrast in SST images.