Photometric zero-points of EFOSC

Average values

Good average values for the zeropoints (in adu), extinction and colour terms are given in the follwoing table. These are averages of good nights between the re-aluminsation of the NTT M1 in June 2008 and the end of November 2008. They are based on interactive fits allowing each of these parameters to vary, with hundreds of stars per night, even (on some nights) in the U. Note that these fits were done with raw data.

Filter Zeropoint Extinction Colour term
U 24.101 +/- 0.076 0.448 +/- 0.061 -0.110 +/- 0.045 (U-B)
B 26.023 +/- 0.073 0.251 +/- 0.059 -0.051 +/- 0.040 (B-V)
V 26.089 +/- 0.061 0.132 +/- 0.049 -0.036 +/- 0.036 (B-V)
R 26.188 +/- 0.063 0.096 +/- 0.052 -0.037 +/- 0.043 (V-R)
I 25.353 +/- 0.065 0.069 +/- 0.052 -0.007 +/- 0.047 (R-I)

Automatic ZP monitoring:

Starting April 2008, EFOSC2 is permanently mounted on the NTT, and photometric standards are observed on any suitable calibration night (and often by visitors). Photometric standard star fields of different colours are observed at different airmasses to get a full photometric solution. The photometry is now performed by an automatic IRAF routine described below.

The results are stored on the web as postscript-figure and data-table:

and are summarised on this ZP evolution page.

Old results, for EFOSC2 on the ESO 3.6m (August 2003 - March 2008):

Since August 2003, the zeropoints of EFOSC2 have been measured for the standard filters UBVRi on each photometric night that EFOSC was mounted on the 3.6m. For the photometry, the TMAG-package by Olivier Hainaut was used.

The results are stored on the web as postscript-figure and data-table:

and are summarised on this ZP evolution page.

Old ZP monitoring pages:

The following pages contain ZP results before 2003.

The WCSstandards package

The WCSstandards package is an automatic routine written by Colin Snodgrass using IRAF to find standard stars based on the WCS solution for the frames, which all EFOSC2 frames now have. The page above has more details, here we describe it from an EFOSC2 user point of view.

The routine performs the following steps:

  • For every frame in the current directory:
    • Read the image headers, skip if it is not an image.
    • If required, improve the default wcs solution based on the USNO B1 catalog stars in the frame.
    • Identify the standard stars in the field and their x,y pixel co-ordinates. Skip to the next frame if there are no standards in the field.
    • Measure the photometry for each standard star using APPHOT.
    • Write a catalog of matched instrumental and catalog magnitudes.
  • Combine all catalogs for the night, matching observations of the same stars in different filters at each epoch
  • Solve for the zeropoints etc, either interactively using PHOTCAL or automatically by a least-squares fit with assumed extinction and colour terms.
  • Display the solutions found and write these results to the zeropoints database, to be included in the next update of the ZP evolution page.

Instructions for use

The routine is already installed on the offline machine at the NTT, and can be used directly by visiting astronomers to test the quality of the night after standard stars are observed. Follow these basic steps:

  • Start IRAF - make sure you are in an xgterm for the interactive fit gui to be displayed properly, and that you have a display window (eg ds9) open if you want to see the stars matched in each frame:
    ds9 &
    cd ~/iraf
  • Change directory to the directory where you have your standard star data: e.g. the raw data directory for the current night cd /data/raw/2008-11-23
  • Run WCSstandards routine (see below for the different options for each argument): WCSstandards EFOSC2 display+ inter- refine+ fix- tidy-
  • Sit back and wait
  • If you chose the interactive fit (inter+) then you will be asked to select which stars to use, using the IRAF PHOTCAL fitparams task. When you are happy with the fit for each filter, hit 'q' and save the parameters before moving to the next filter (say yes when prompted twice). Good instructions on the use of fitparams (and on IRAF photometry generally) can be found here.
  • If you set inter- then the median ZP will be measured automatically, and an estimate of whether the night is photometric or not will be displayed.
  • If the night appears to be photometric, please ask your support astronomer to update the ZP evolution page with the new values (which will have been archived on the offline machine automatically, but have to be manually updated on the www site so they can be checked first).

The arguments supplied to WCSstandards can be changed according to your preference:

  • instrument - the instrument parameter must be EFOSC2
  • display - set to true (display+) to display each frame and mark the coordinates of the stars found on the display
  • interactive - set to true (inter+) to use IRAF PHOTCAL task 'fitparams' to solve for zeropoints etc interactively. If it is false (inter-) then the zeropoints will be found from the median ZP for all stars using assumed extinction and colour term values, and the user will not have to take any further action.
  • refine - set to true (refine+) to do two passes at easuring the x,y pixel coordinates of the stars. The 1st pass will find the shift between the WCS and the actual positions for the brighter stars, and the second will then find fainter stars better.
  • fix - if the offset between the WCS and the actual positions is larger, the stars will not be found. Set to true (fix+) to refine the image WCS before looking for the standards.
  • tidy - set to true (tidy+) to delete temporary files at the end of the routine. Be careful, as this uses wildcards and could possibly delete other files if you have named them similarly.