Correction of Field Rotator-Induced Flat-Field Systematics - A Case Study Using Archived VLT-FORS Data

TL;DR

This paper investigates and corrects field rotator-induced flat-field systematics in VLT-FORS data, identifying the LADC as the cause and proposing methods to improve photometric accuracy in imaging observations.

Contribution

It presents a detailed analysis of rotating flat-field structures caused by the LADC and offers correction methods applicable to instruments with field rotators.

Findings

Rotating flat-field structures are caused by the LADC.

Uncorrected systematics can cause up to 4% photometric errors.

Correction methods improve photometric accuracy in VLT-FORS data.

Abstract

ESO's two FOcal Reducer and low dispersion Spectrographs (FORS) are the primary optical imaging instruments for the VLT. They are not direct-imaging instruments, as there are several optical elements in the light path. In particular, both instruments are attached to a field rotator. Obtaining truly photometric data with such instruments present a significant challenge. In this paper, we investigate in detail twilight flats taken with the FORS instruments. We find that a large fraction of the structure seen in these flatfields rotates with the field rotator. We discuss in detail the methods we use to determine the cause of this effect. The effect was tracked down to be caused by the Linear Atmospheric Dispersion Corrector (LADC). The results are thus of special interest for designers of instruments with LADCs and developers of calibration plans and pipelines for such instruments. The methods described here to find and correct it, however, are of interest also for other instruments using a field rotator. If not properly corrected, this structure in the flatfield may degrade the photometric accuracy of imaging observations taken with the FORS instruments by adding a systematic error of up to 4% for broad band filters. We discuss several strategies to obtain photometric images in the presence of rotating flatfield pattern.