FORS1:
photometric zeropoints

QC1 PLOTS ZEROPOINTS: per frame per night

QC1 ASCII DATA:

Year:
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*Chip:**
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QC1 database (expert mode): zp_frame

zp_night

*: use "any" for data until 2002-10-01, as there is no chip id in the database for old data.

Warning

Do not use these zeropoints for science data. They are reduced assuming standard extinction coefficients which may not be appropriate for an individual night!

The night zero points are determined applying extinction coefficients and colour terms which are read from the QC1 Database. The coefficients are updated after every period and may cause jumps of up to 0.1 mag in the night zeropoints, if large changes occur.

General

Photometric zeropoints have been calculated starting from exposures of standard stars taken from the Landolt catalogue (Landolt 1992, AJ 104, 340). There are two kinds of zeropoints - per frame and per night. For the zero points per frame the average of the zeropoints from the individual stars is determined. Zeropoints fainter than this average by more than 0.3 mag are excluded and the average is re-determined. For the zeropoints per night a pre-selection is done: If there are more than 3 stars in a given field, a linear regression of zeropoint versus colour is obtained. Stars deviating by more than 1 sigma (uncertainty of fit) from this relation are discarded. Then all stars reamining for that night are used to determine the zero point for the night. Average zeropoints for the previous periods can be found here; older ones are given here The raw data have been:

de-biassed,
flat-fielded with a normalized twilight sky flat.

The SExtractor (Source Extractor) software see, 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.

Before 1999-12-11, a different method has been used, the adaptive aperture method using "first moments". This gives magnitudes which are systematically lower by about 0.06 mags or more. The fluxes are then converted from ADU per sec intoelectrons per sec. The conversion factors are averaged over the four CCD ports and are read from the FITS keywords "HIERARCH ESO DET OUTi CONAD" with i = 1...4 .

All steps above are performed automatically using the FORS1+2 pipeline.

The final zeropoints are obtained applying standard extinction coefficients and colour coefficients.

Zeropoints trending

Zeropoints tables and plots can be accessed from the above interface. Currently, only zeropoint values obtained from observations performed with the Standard Resolution (SR) collimator are trended.
We warn the users that the zeropoints have been computed using constant extinction coefficients and color terms, with the purpose of instrument and site performance monitoring only. Though they are reasonably precise, they should not be used for accurate photometric calibrations.

The extinction coefficients and colour terms used for the determination of frame zeropoints can be found here.

Colour terms and extinction coefficients

Detailed colour terms and extinction coefficients have been derived. Click here for a description and results.

Paranal sky transparency

Starting with March 2001, a prototype of the ASM LINE OF SIGHT SKY ABSORPTION MONITOR (LOSSAM) has been implemented and made available. After clicking the button, select the month and the date of interest.

Photometric stability

In this plot we show the behavior of the U_BESSEL zeropoints computed for ANTU+FORS1 in the period Feb-Dec 2000, which starts just after an ANTU-M1 re-aluminization. The dotted line is a least squares fit to the full data set, while the solid line is the result of the fitting after a 1 sigma clipping. The rejected points are plotted as empty circles. Here we implicitely assume that the remaining points were obtained in photometric conditions. A clear trending is visible, which can be explained as an eficiency loss in the main mirror reflectivity. Similar behaviors are visible also in BVR and I, with smaller slopes. In the case of U passband, the zeropoint decrease rate is as high as 0.13 mag/year, which implies an efficiency loss of more than 10% in one year.

This loss has been significantly reduced by cleaning the mirror in between re-coatings.

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FORS1: photometric zeropoints

Warning

FORS1:
photometric zeropoints