WFCAM photometric calibration with 2MASS

A modified version of the existing photometric calibration software, fitsio_photom, used for calibrating INT wide field camera (WFC) data against the Landolt catalogue was used for testing the photometric calibration of WFCAM data against the point source catalogue (PSC) from the all-sky data release of 2MASS.

In order to optimise search speed and disk space requirements the 2MASS catalogue was loaded into a series of FITS binary tables, stored on disk at CASU, and containing a subset of the columns available in the full catalogues. The format of these is described in the associated README file. A shared set of C subroutines is then used to access the 2MASS catalogue for positional searches. Due to the differing formats of these catalogues, completely separate sets of routines are used for the point and extended source catalogues.

The 2MASS PSC search code performs a given positional search in three steps:

For the given search window (RA, Dec and a width) the code finds the appropriate set of FITS files containing the desired range in RA.
In each file, the range of rows (sorted by Dec) required is determined by a binary chop.
The rows are then read out one at a time through an interface subroutine.

A standalone program read_psc is provided, which calls the appropriate subroutines to extract all rows in a given window, printing them to the standard output in an ASCII table. The modified version of fitsio_photom calls the subroutines directly.

The program was tested on a sample data file (w_20010607_06523_r.fit) generated from a J-band CIRSI data frame by inserting appropriate WFCAM FITS headers. From a reduced image produced in this manner using a prototype WFCAM pipeline, a catalogue was produced using the existing extraction software, followed by running astrometric refinement and classification routines. The resulting catalogue file was then processed with fitsio_photom. No confidence (weight) map was available so none was used in the extraction, which required turning off the crowded field code in the extraction software.

Results

The calculated J-band zeropoints for each detector are tabulated below.

	Zeropoint
Detector	Median	Mean	Sigma (MAD)	Num stds used
1	21.09	21.09	0.152	28
2	21.07	21.02	0.129	7
3	21.05	21.01	0.142	23
4	21.07	21.04	0.137	30
Overall	21.07	21.04	0.13	88

Conclusions

Given the rapid and efficient 2MASS access code already developed it will be relatively straightfoward to implement routine 2MASS PSC throughput calibration of all WFCAM JHK data frames, at the detector level. This could reasonably be extended to the Z and Y broadbands using extrapolated linear colour equations. Further refinements of the throughput monitoring will require more accurate derived colour equations which can be developed during the WFCAM commissioning and characterisation phases.

Last modified: Mon Jun 30 17:28:14 2003