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WFS Photometric Calibration


Nightly Zero-Points

These nightly zero-points are predicated on the fact that the data has been processed through the WFS pipeline (see pipeline processing for more details). For ease of cross-calibration we are providing photometry based in the Landolt (1992, AJ 104, 340) and SDSS systems, see the section on colour conversions for more details. There is evidence for colour term non-linearity in the SDSS-Landolt cross-calibration which has been ignored for the moment.

This table presents our best estimate of the nightly averaged zero-points for each filter at airmass=1.0, assuming the extinction for the night is nominal (see extinction). The steps involved in going from the processed observations of standard fields to the zero-points are:

  1. For a given pointing in a given filter, we derived the k-sigma clipped average zero-point. This makes of the median of the zeropoints and the mdeian of the absolute deviations to derive a robust estimate of sigma.
  2. Then for all the standard fields measured on this night, we derive the average value, after rejecting outliers.
  3. We also measure the zero-point averaged over the entire run. This value is used when insufficient standards were measured on a particular night.
  4. We also give an 'error' in the table. Given that relatively few standards fields are measured in a typical WFS night (around 4 or 5 is usual), it's hard to derive guassian errors. We have found that the spread in the data can be related to gaussian sigma in a reasonably robust way. The value we give approximates to the one-sigma error on the zeropoint, and is generally less than 1-2%. Nights with large values for the error (say 5% or larger) must be treated with caution, either because the night was non-photometric, or because relatively few standard stars were measured.



The zero-point assumes default extinction and unit airmass, ie. for other situations

ZP --> ZP - [sec(z)-1]*extinct + extinct_default - extinct

Average extinction coefficients for La Palma of U = 0.46, B = 0.22, V = 0.12, R = 0.08, I = 0.04, Z = 0.05, g' = 0.19, r' = 0.09, i' = 0.05 have been assumed for the photometric nights used to derive the zero-points and colour terms. For details of nightly individual extinction measurements and more information regarding the La Palma extinction curve see the Carlsberg Meridian Telescope extinction measurements or the Mercator Telescope extinction measurements.


Gain Variations

The effective gain (e-/ADU) of the system after pipeline processing was 3.12 pre-August 1999 and appeared to be 2.6: September 1999 through December 1999 post-mscred taking over the flatfield gain renormalisation. However, the gain and non-linearity of CCD#1 was changed sometime between the December 1999 and March 2000 runs, such that the average gain (ie. effectively what mscred uses) is now currently 2.75: , so to convert the zero-points to photons/s add 1.24, 1.05:, 1.09: magnitudes respectively.


Photometry & Catalogue FITS Headers

The following keyword information can now be used to generate more accurate magnitudes:

APCORPK = '          2.812'    / Stellar aperture correction - peak height
APCOR1  = '          0.635'    / Stellar aperture correction - core1 flux
APCOR   = '          0.114'    / Stellar aperture correction - core flux
APCOR2  = '          0.028'    / Stellar aperture correction - core2 flux
APCOR3  = '          0.000'    / Stellar aperture correction - core3 flux
APCOR4  = '          0.000'    / Stellar aperture correction - core4 flux

Aperture corrections in magnitudes needed to correct the assorted aperture-
like measures produced in the catalogues onto the equivalent of a total flux
stellar system.  These constitute the components of a curve-of-growth analysis
contained within the catalogues with pixel "radii":

peak pixel, 1/2 x rcore, rcore, sqrt(2) x rcore, 2 x rcore, 2 sqrt(2) x rcore

to be used in the sense that corrected_photometry = 2.5*log10(flux) + apcor
They also work well as a first order seeing correction for faint galaxies.

The remaining information necessary for photometric calibration is included
in the following keywords:-

PERCORR =              -0.006  / Sky calibration correction (mags)

this is a correction based on the median dark sky recorded in science frames
compared to the median for all the CCDs and as such is an ancillary correction
to the gain correction derived from the flatfield (usually twilight flats)
data.  This correction is to be used in the same sense as before in that

      corrected_photometry = 2.5*log10(flux) + apcor + percorr

MAGZPT  =               22.10  / Photometric ZP (mags) for default extinction

Nightly-derived CCD zero-point in the sense of what magnitude object gives a
total (corrected) flux of 1 count/s.  These ZPs are appropriate for generating
magnitudes in the natural CCD+filter system based on Vega, see
for more details on colour equations etc..  The
nightly ZPs have been derived from a robust average of all photometric
standards observed on any particular night, corrected for airmass but assuming
the default extinction values listed at the preceding web address.
For other airmass or other values of the extinction use

       ZP --> ZP - [sec(z)-1]*extinct + extinct_default-extinct

You can then make use of any of the assorted flux estimators to produce
magnitudes via

       Mag = ZP - 2.5*log10(flux/exptime) - apcor - percorr

Note that for the so-called total and isophotal flux options it is not
possible to have a single-valued aperture correction

MAGZRR  =                0.15  / Photometric ZP error (mags)

error in the zero point.  If good photometric night this error will be at
the level of a few percent.  Values of 0.05 and above indicate correspondingly
non-photometric night and worse.

This photometric information supercedes earlier release information.

Photometry Summary Plots (by run)