FORS2 Rotating Flat Field Systematics Fixed -- Recent Exchange of FORS LADC Prisms Improves the Long-known Flat-fielding Problem

TL;DR

The paper reports that exchanging the LADC prisms in the FORS2 instrument significantly reduced flat-field artefacts caused by surface inhomogeneities, improving measurement accuracy.

Contribution

This study demonstrates that replacing the FORS2 LADC prisms effectively mitigates long-standing flat-field systematics, enhancing data quality.

Findings

Significant decrease in small-scale artefacts after prism exchange

Improved flat-field uniformity and measurement precision

Validation through analysis of extensive flat-field data

Abstract

For many years the FORS2 instrument has suffered from artefacts in the flat fields caused by surface inhomogeneities on the longitudinal atmospheric dispersion corrector (LADC) that affected high-precision photometric and spectroscopic measurements. Recently, the FORS LADC prisms were exchanged, and our analysis of a large number of flat fields shows that this exchange has resulted in a significant decrease in the level of small-scale artefacts.