There are two effects that should be separately dealt with when reducing WFI data. One is the well known colour transformation equation to the standard system. The second is the variation of zero points as a function of position in the mosaic. In a well designed reduction process we must first correct the instrumental magnitudes using a zero point correction map, and then apply the color transformation equation. The problem is complicated in the case of the WFI due to the large color term for the B filter: the zero point for an O3 star, intrinsic color -0.3, reddened by 1 magnitude to +0.7, differs with respect to the zero point of an unreddened star with intrinsic color of +0.7 by more than 0.1 magnitudes! Most of the Landolt standards have small reddenings, so... caveat emptor.

Photometric ZeroPoints

The following Zeropoints (ZP), Colour Terms (CT) and Extinction coefficients should be used in the following format:

Below are listed key solutions that have been calculated from a reasonable amount of data. You can also view this table for other solutions that do not contain as much data.

March 12, 2003

Perfectly photometric night where a bunch of standard fields were moved around each chip. All parameters fitted simultaneously as free parameters, with good airmass and colour range, and around 300 stars in each fit (yes - even for U). The table below is the average over all chips. I also attempted a solution for each individual chip setting the extinction to that listed below and solving for the zeropoint and colour term in each case. The results of this can be found here.

This is *the* definitive measurement on this page

Janury 16, 2003

Photometric night. Those parameters no fitted simultaneously as free parameters, have no error estimate and were chosen to either minimize the residuals by eye, or taken from previous measurements.

November 08, 2003

Photometric night. The results below are averaged over all chips. Those parameters not fitted simultaneously as free parameters, have no error estimates and their values were either chosen to either minimize the residual by eye, or taken from previous measurements.

July 31, 2003

The data has been obtained during a supposedly photometric night, though upon producing these photometric solutions, it seems clear that the night was not strictly photometric. All parameters were fitted simultaneously as free parameters, except the colour term in the R-band (which was taken as that determined in April).

April 13, 2003

This night was pretty much photometric, but not exactly the nicest night on record. The Extinction and Colour Terms for each filter were determined by hand (hence no error estimates), the ZP was fitted as a free parameter once the other two terms were fixed.