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