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Quality Control and
Data Processing
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FORS2: Imaging zeropoints/System efficiency
Photometric zeropoints have been calculated from standard stars observations based on the Landolt catalogue (Landolt 1992, AJ 104, 340, used until 2008-03-31) and the Stetson catalogue (since 2008-04-01). You can find some information about conversion from the FORS2 filters here . There are two kinds of zeropoints - per frame and instrumental. The raw data have been:
The twilight images of FORS2, which are used to create master sky flats, show evidence of a feature, which position depends on the rotator angle (see overview). This limits the achievable photometric accuracy to about 5%. For more information see Moehler et al. (2010, PASP 122, 93). 
Photometric Stability
For the determination of a photometrically stable night we evaluate the variation of the extinction coefficient measured from standard stars frames. Since 2011 the FORS2 calibration plan foresees, for potentially photometric nights the initial acquisition of a low-high-airmass pair of standard fields, followed by further standard fields around midnight and in the morning if SCIENCE data requesting photometric conditions are taken. A night is then defined as stable (S) if
In addition we check for the time range covered by the measurements - if it is less than 1 hour the night is flagged as Sb (for 'stable begin'). Nights with higher extinction variations are marked as not-stable ('N'), while nights with an airmass range of less than 0.4 or only one measurement are flagged as unknown ('U').
Frame Zeropoints
The frame zeropoints are based on a single STD_IMA exposure. They are calculated assuming a fixed extinction coefficient. A second result is calculated by the pipeline from the same input data, this time the extinction coefficient assuming a fixed zeropoint. Disclaimer: Do not use these zeropoints for the reduction of science data. They have been processed assuming standard extinction coefficients and colour terms, which may not be appropriate for an individual night. Instead we recommend to use the instrumetal zeropoints and extinction coefficients (available since 2012-06). These coefficients were hardcoded in the old pipeline (for data observed until 2008-03-31) and are tabulated in configuration files for the new pipeline (for data observed since 2008-04-01). The coefficients are frozen since 2011-11-28. We noticed a problem with some standard star field results, where the pipeline mis-identified standard stars. Usually such mis-identifications are caught during the QC certification process, but a few such cases (≤5%) have slipped through. In such cases typically only few of the identified stars carry high weight. This problem has been fixed with pipeline version 4.9.2, which has been in use since 2011-05-25.
For the trending we select only zeropoints with errors of less than 0.1 mag and with more than 1 standard star found. In the same plot we trend the error of the frame zeropoints to check if outlying zeropoints have larger than usual errors.
Scoring&thresholds Frame Zeropoints
No scoring nor thresholds are currently applied.
The SExtractor software (Bertin & Arnouts, 1996,
A&A 117, 393) is run on the reduced images, to detect standard stars and to extract
their fluxes. A fixed aperture of 10" radius is used and flux from neighbouring
stars within this aperture is corrected. The
fluxes of the unsaturated stars are then converted from ADU per sec into electrons
per sec using the inverse gain stored as FITS keyword "HIERARCH ESO DET OUT1
CONAD". They are then corrected for atmospheric extinction and colour terms
using tabulated
coefficients. For the zero points per frame the optimally weighted average
of the zeropoints from the individual unsaturated stars is determined. The optimal
weights are determined in such a way, that the error on the frame zeropoint is minimised.
Individual zeropoints deviating more than either 0.3 mag or 5 σ from the mean
zeropoint are rejected as outliers, and the mean is re-determined. The sig_zp_frame parameter corresponds to combination of the rms error of the individual zeropoints and the accumulated errors from the frames (raw frame, master bias, master twilight flat).
Extinction Coefficients per Frame
Scoring&thresholds Extinction Coefficients per Frame
No scoring nor thresholds are currently applied.
Algorithm Extinction Coefficients per Frame
Assuming a value for the zeropoint an extinction coefficient is derived. The values assumed for the zeropoints can be found here. If the assumed zeropoints are over/under-estimated, the derived extinction coefficients are correspondingly shifted. The final values per stable night are calculated as below
The error of the extinction coefficient is the rms value.
With a time delay of one month, we calculate in a second step the instrumental zeropoints and extinction coefficients per stable night. For the instrumental zeropoints and extinction coefficients per night, we collect data from at least 7 stable nights around the night of interest. The combinations of high airmass coverage and stable extinction for these nights allows us to determine a common zeropoint and nightly extinction coefficients with the pipeline recipe fors_photometry. The zeropoint and the extinction coefficient derived for the night of interest are stored in the database table fors2_photometry. We take care that data are not combined across mirror coatings.
The extinction coefficient for the night more accurately refers to the times of the night when standard star images were taken. If, for instance, only two standard star images were taken at the beginning of the night, the extinction coefficient describes only the conditions for that short time interval.
For trending we select only data from stable nights, since the fors_photometry recipe is very sensitive to variations of the extinction coefficient during the night, which may affect the determination of the instrumental zeropoint.
Scoring&thresholds Instrumental Zeropoints and Nightly Extinction Coefficients
No scoring nor thresholds are currently applied.
No major events.
Algorithm Instrumental Zeropoints and Nightly Extinction Coefficients
The instrumental zeropoint is zeropoint derived by the procedure described above. Since the recipe can determine the exticntion coefficient from the data this zeropoint should be free from atmospheric effects and reflects only changes in the instrument and/or telescope.
The error is the rms error of the instrumental zeropoint.
The extinction coefficient is derived for the night. It is defined only for stable nights.
The error is the rms error of the extinction coefficient for the night.
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