FORS Sec Std and Abs Phot Project

FORS Secondary Standard and Absolute Photometry Project: Homepage


FORS1 was one of the first two instruments to be commissioned on the first of the four VLT telescopes, and it has been offered continuously at the VLT since ESO Period 63. In a recent analysis of VLT instrument efficiencies, FORS1 was the outstanding winner in all categories: Number of papers, citations, h-index, m-parameter and citations per paper (Grothkopf et al., 2007, The Messenger, 128, p62 - Table 2). As stable VLT work horses during the past 8 years obviously the two FORSes have in many ways also been instrumental to the definition of the VLT queue observing paradigm which includes both systematic collection of calibration data, online and offline reduction pipelines.

Photometric Calibration of FORS: The original paradigm

The original concept for the photometric calibration and the FORS imaging pipelines involved acquisition of simple twilight images which were used for flatfielding, and observation of Landolt photometric standards. The limitations of this strategy have until now defined the limits of obtainable photometric calibration accuracy with the FORS instruments. Those limitations can basically be broken into two sources of errors:
    Errors introduced by the flatfields
    The lack of photometric standards suitable for 8m class telescopes

The Project

This project started in 2003 and its first aims were to
    Determine the photometric accuracy in the current paradigm
    Test a range of methods to improve on the current paradigm
Those goals have been achived, and a detailed description of the tests, methods, results, and recommended actions has been published in two VLT reports:
    ``FORS: An assessment of obtainable photometric accuracy and outline for strategy for improvement'' (VLT-TRE-ESO-13100-3808), Møller et al., 2005.
    Available here in pdf format and gzipped postscript
    ``Absolute Photometry with FORS: The FORS Absolute Photometry Project'' (VLT-TRE-ESO-13100-4006), Freudling et al., 2006.
    Available here in pdf format
In addition to those milestone reports, several smaller status reports and memos on detailed issues have been presented as conference and Messenger papers. A complete publication list is available.

Following closely the recommendations of the two VLT reports we now aim at a complete redefinition of both pipeline reduction algorithms and calibration plan. The project aim is to develop an integrated method which produces in parallel
    Optimal illumination corrected flat fields (true System Efficiency Frames)
    Nightly atmospheric parameters (extinction)
    Long term tracking of above atmosphere zero-points and colour terms
and at the same time bootstraps itself via definition of a large set of fainter secondary standards appropriate for calibrating current and future large aperture optical telescopes.

    Project team

    W. Freudling
    C. Izzo
    J. Larsen
    S. Moehler
    P. Møller
    F. Patat

Work in progress:
Palle Møller - pmoller[at]
Last updated: 26 July 2007