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.

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 remaining 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 of the same night.

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.

The photometric standard star fields are observed in standard modes: with the standard UBVRI filters and standard CCD mode. (post-detector-upgrade: low gain, 2x2 bin; pre-detector-upgrade: high gain 1x1 bin). The raw data have been

• de-biassed,
• flattened (with a normalized twilight sky flat).

The fluxes are then converted from ADU per sec intoelectrons per sec using the conversion factor from ADU to electrons of the FITS keyword "HIERARCH ESO DET OUT1 CONAD". For the pre-upgrade time, when the CCD had 4 ports, the CONAD used was the average between the 4 ports keyword values "HIERARCH ESO DET OUTi CONAD" with i = 1...4. The final zeropoints have been obtained applying standard extinction coefficients and colour coefficients as selected, per period, from here.

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.

 
 

QC home  QC and trending  Paranal autrep db

 

---

Send comments to <smoehler@eso.org> Last update: April 24, 2007