QC PLOTS
	CURRENT	HISTORY
zeropoints U
zeropoints B
zeropoints V
zeropoints R
zeropoints I
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Photometric zeropoints are calculated from exposures of selected Stetson photometric standard star fields that contain also some Landolt stars (Landolt 1992, AJ 104, 340). Stetson fields are often very rich of stars and permit an accurate estimate of zeropoints. However the Stetson catalog is not complete in all bands and in particular U photometry is missing. For U we compute zeropoints using the Landolt stars present in the fields, if any.

WARNING: Zeropoints have been computed using constant extinction coefficients and color terms, with the main purpose of instrument and site performance monitoring. Though they are reasonably precise, they may be not suitable for accurate photometric calibrations.

Standard star fields are de-biased and flat-fielded using the most recent bias frames and normalized twilight sky flats. 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 instrumental magnitude ("MAG_BEST" Sextractor parameter). The "MAG_BEST" estimate is in agreement with the values obtained with fixed aperture of 10" diameter.

The instrumental magnitude Minstr = -2.5Log(Flux[ADU/sec]), is then corrected for:

• Airmass
• Colour term: this is based on the Stetson/Landolt standard colours, not the instrumental colours
• Flux is transformed into [e/sec].

Minstr_corrected = -2.5*Log(Flux[ADU/sec] -2.5*Log(CONAD) + col_term*Colour - cext*Airmass

CONAD is the conversion factor ADU to electrons written in the FITS keyword "HIERARCH ESO DET OUT CONAD". The typical value for the CONAD, in the low gain mode used for Imaging is 1.8 e/ADU. The difference between the corrected instrumental magnitude and the magnitude of the Stetson/Landolt standard gives a measure of the zeropoint (at zero airmass and zero colour). For each night we take the median of all the calculated star zeropoints

Zp = Mcatalog - Minstr_corrected.

magnitude = Minstr_corrected + Zp = -2.5*Log(Flux[ADU/sec] + col_term*Colour - cext*Airmass + Zp .

If Zp is in (ADU/sec) like in the reduced images header, Minstr_corrected should not be corrected for the CONAD.

WARNING: Zeropoints have been computed using constant extinction coefficients and color terms, with the main purpose of instrument and site performance monitoring. Though they are reasonably precise, they may be not suitable for accurate photometric calibrations.

QC1 parameters

Trending

Zeropoints are used to monitor the total throughput of the telescope and the instrument. For trending, averages of all fields observed during each night are used. Zeropoints from individual frames can be obtained via the general QC1 database interface.

For zeropoint computations, average extinction coefficients extimated by FORS are used and the colour terms are set to zero.

Colour terms and exctinction coefficients for VIMOS in the UBVRI passbands have been determined applying the method described by Harris et al. 1981, PASP 93, 507. This method finds the best fit solution, solving simultaneously for the zero-point, colour term and extinction coefficient in the usual photometric equation:

Mcatalog = 2.5*Log(Flux[e/sec]) - col_term*Colour + cext*Airmass +Zp .

To obtain a reliable solution the data from many nights have to be combined.

The following tables report the computed colour terms for VIMOS obtained with data taken between April 2003 and August 2003. Only data taken in photometric nights were included. All estimated uncertainties are one standard deviation.

Quadrant 1

Filter	Colour	Colour Term	Avg. Zero Point	N. of Stars	RMS deviation	Colour Range
U	(U-B)	-0.028 +/- 0.022	26.52 +/- 0.22	61	0.12	0 ... +2.1
B	(B-V)	-0.074 +/- 0.005	28.23 +/- 0.02	1043	0.049	0 ... +2.5
V	(B-V)	-0.021 +/- 0.005	27.88 +/- 0.014	1133	0.047	0 ... +2.5
R	(V-R)	-0.110 +/- 0.007	27.83 +/- 0.016	933	0.039	0 ... +1.3
I	(V-I)	+0.026 +/- 0.004	26.99 +/- 0.015	960	0.047	0 ... +2.6

Quadrant 2

Filter	Colour	Colour Term	Avg. Zero Point	N. of Stars	RMS deviation	Colour Range
U	(U-B)	-0.029 +/- 0.018	26.81 +/- 0.14	113	0.126	0 ... +2.1
B	(B-V)	-0.076 +/- 0.005	27.95 +/- 0.02	909	0.050	0 ... +2.2
V	(B-V)	-0.029 +/- 0.006	27.81 +/- 0.023	1094	0.067	0 ... +2.2
R	(V-R)	-0.112 +/- 0.007	27.68 +/- 0.014	898	0.040	0 ... +1.3
I	(V-I)	+0.030 +/- 0.006	26.97 +/- 0.023	906	0.067	0 ... +2.6

Quadrant 3

Filter	Colour	Colour Term	Avg. Zero Point	N. of Stars	RMS deviation	Colour Range
U	(U-B)	+0.009 +/- 0.028	26.74 +/- 0.28	81	0.17	0 ... +2.1
B	(B-V)	-0.076 +/- 0.006	28.0 +/- 0.02	900	0.048	0 ... +2.2
V	(B-V)	-0.020 +/- 0.005	27.64 +/- 0.017	1025	0.042	0 ... +2.2
R	(V-R)	-0.068 +/- 0.008	27.63 +/- 0.016	888	0.041	0 ... +1.3
I	(V-I)	+0.031 +/- 0.005	26.86 +/- 0.022	1024	0.047	0 ... +2.6

Quadrant 4

Filter	Colour	Colour Term	Avg. Zero Point	N. of Stars	RMS deviation	Colour Range
U	(U-B)	-0.036 +/- 0.025	26.91 +/- 0.27	71	0.13	0 ... +2.1
B	(B-V)	-0.081 +/- 0.006	28.31 +/- 0.02	938	0.060	0 ... +2.2
V	(B-V)	-0.035 +/- 0.004	27.75 +/- 0.016	1204	0.044	0 ... +2.2
R	(V-R)	-0.087 +/- 0.010	27.83 +/- 0.021	904	0.051	0 ... +1.2
I	(V-I)	+0.035 +/- 0.006	26.99 +/- 0.027	1066	0.075	0 ... +2.6