FORS Sec Std and Abs Phot Project

FORS Secondary Standard and Absolute Photometry Project: Calibration Plan

The new FORS imaging calibration paradigm

We have shown ( (Møller et al., 2005; Freudling et al., 2006) that false illumination of the FORS1 CCD has been the defining limit of its photometric calibration accuracy. The first step to improve the FORS calibration is therefore to obtain data to allow a computation of the illumination correction across the field. This false illumination problem, which is inherent in all calibration employing ``flat field correction'', has been a known problem for several years, but its solution is mostly viewed as too complex and too observing time costly to be addressed in a general way.

In very general terms the solution can be obtained via a set of dithered and rotated exposures of the same stellar field. With the same star observed in several different positions on the CCD, and with a large number of stars, we obtain a number of linear equations which is large enough to strongly over-constrain the determination of relative magnitudes of all objects, of relative extinction of each exposure, and of a set of parameters describing the 2D correction field across the CCD.

Another limitation on the calibration accuracy has been the brightness of the Landolt standards which have forced the use of extremely short exposures introducing small but non-negligible shutter-time errors and also resulting in a very small number of useful standards per frame. The second generation FORS calibration paradigm is designed to solve both of those problems with the same set of calibration data. The new calibration plan does not require more time for calibrations than previously, it merely requires the data to be taken in a different way, and also requires a much more sophisticated pipeline.

The second generation FORS imaging calibration plan

We are in the process of selecting 8 Landolt/Stetson standard fields (see here for current status). Within each of those fields 3 different but largely overlapping pointings will be chosen. Each pointing shall be carefully selected to fulfill the following three conditions:

(1) It must include a large number of Stetson standards suitable for observation with FORS for at least 10 seconds without saturating under typical conditions,

(2) it must include at minimum 2 Landolt stars with published U band photometry,

(3) it must not contain any stars bright enough to heavily bleed during a 10 second exposure in any band.

Finding pointings that fulfill all three criteria is not a trivial task and in a few cases we have had to relax (2) above. Since the project is designed to bootstrap itself across the sky over a two year time span, this relaxation is not critical in any way.

Each of the chosen pointings will be observed at 4 different rotator angles (separated by 90 degrees). Each night one of the FORSes goes on sky the night will start with the selection of a pointing and a rotator angle to observe at. Those will be following a sequential pattern such that 12 nights will ensure that all three pointings are observed at all 4 rotator angles. At the end of night another field, at different airmass, will be observed following the same scheme. At the end of a sequence of 12 observing nights we shall then have a complete set of dithered and rotated frames for a hugely overconstrained solution, and since the selection of pointings and rotations is sequential the same will be true for all following nights.

The on-line pipeline

The new FORS imaging pipeline currently under construction will allow a real time solution of the linear equations. For each night we can therefore use the current nights observations together with those of the previous 11 observing nights to obtain a complete solution for current extinction and illumination correction. The only assumption used for this scheme is that the pixel-to-pixel variations on the CCD, i.e. its underlying physical properties, does not change significantly on a timescale of 12 observing nights.

At each pointing and for each filter we shall initially obtain two exposures, one of length minimum 10 seconds (depending on filter) suitable for the large number of Stetson standards and long enough to ensure that shutter time errors can be ignored, and one short exposure suitable for the Landolt standards which allows us to add the missing bands to the Stetson standards, and also to provide an independent link of Stetson standards to Landolt standards. The short exposures will be terminated when our secondary standard set is deemed complete enough to continue the bootstrapping without the Landolt.

Global properties such as "FORS above atmosphere zero-points", system colour terms, magnitudes of observed standard stars, Paranal extinction curve, can also be obtained the same way on a nightly basis and their variation can be traced. However, better values can be obtained from post-processing of a larger dataset on a less frequent basis.

Palle Møller - pmoller[at]
Last updated: 16 July 2007