QC PLOTS
	CURRENT	HISTORY
nightly extinction
QC1 database (advanced users): browse | plot
colour terms and long-term extinction
QC1 database (advanced users): browse | plot

Since P84 we use the pipeline recipe fors_photometry (see the pipeline manual for details) to determine nightly extinction coefficients. For that purpose we download for each night, which has certified standard star observations, the certified standard star data from within +/- 14 nights of that night.Then we use these data to fit a common zeropoint and extinction coefficients per night, while keeping the colour terms fixed. We do not select the data according to the quality of the nights, i.e. data from both photometric and non-photoemtric (in general clear) nights enter into the fit. For reasons currently not understood, but probably related to underestimated errors in the input catalogues, the errors provided by fors_photometry are severely underestimated. Due to the need to have +/- 14 nights with certified data the nightly extinction values are determined with a 4 week delay.

Colour terms for FORS2 and Paranal exctinction coefficients in the UBVRI (and bv_HIGH after April 2009, P83) passbands have been determined by QC Garching since P73. Until P83 this was done with the method described below (see History). Since P83 we use the pipeline recipe fors_photometry, which allows to use the identified standard stars from many nights to determine zeropoints, colour terms, and extinction coefficients (see the pipeline manual for details). We now use a two step approach. First we determine from all observed standard star fields (i.e. not only photometric nights) a common zeropoint and colour term for each filter and detector, together with nightly extinction coefficients.Using this zeropoint and colour term we then derive a common extinction coefficient for the whole period, again for each filter and detector. These extinction coefficients are intended solely for health check monitoring and should not be used for any scientific analysis.

For reasons currently not understood, but probably related to underestimated errors in the input catalogues, the errors provided by fors_photometry are severely underestimated. The number of stars used includes stars with very low weight (see the pipeline manual for details)

P85

April - September 2010

P84

October 2009 - March 2010

P83

April to September 2009

History

• Washing or re-coating of mirrors affects the zeropoints, as does the move from one telescope to another. The dates of such interventions can be found here.
• Until March 2006, the instrumental magnitudes were estimated with the MIDAS intergrate/aperture command instead of using the SExtractor magnitudes from the pipeline, which were unrealiable due to an incorrect setting of the SExtractor MASK parameter.
• Since April 2008 the new imaging pipeline has been in use. It uses the Stetson catalogue, which does not contain U band data. Therefore U_SPEC zeropoints are not available anymore.
• Since April 2009 the standard filter set contains R_BESS, I_BESS, b_HIGH, and v_HIGH.

Until P83 the determination of the photometric coefficients was done via a one-step multilinear least squares fit to the data produced by the pipeline, using a simplified version of the method described by Harris et al. 1981. The details on the implemented procedure are decribed here. Basically the method finds the best fit solution, solving simultaneously for the zero-point, colour term and extinction coefficient. This has the advantage that different standard star fields can be used, without the need of airmass pairs. As a consequence of this, data from many nights can be combined together to give a more reliable solution. This is usually done with data for a period. If, however, during the period a mirror has been re-coated the data are split in samples before and after the coating. Coating dates can be found here.

Data for this procedure are collected every time a standard star field is processed. Before April 2008, the instrumental magnitudes were plotted versus the colour for the given filter. Then a least square fit with 1-sigma clipping was performed and all stars within+/-1 sigma were then stored in an ASCII file. Since April 2008 a new pipeline is in use, which relies on the Stetson catalog and therefore has many more stars. It assigns weights to the stars (see the pipeline manual for details) and all stars with absolute normalized weights > 0.5/n_stars are stored in the ASCII file. This way one avoids to collect data for stars that hardly influence the final zero point.

Plot Description: In the upper left box the original magnitude differences (Landolt-Instrumental) are plotted as a function of the standard star colour, e.g. (B-V) for the V band. Triangles represent observations performed at an airmass > 1.2, while circles refer to data taken at an airmass <= 1.2.
After the solution is found, the corrected differences are again plotted (lower plot in the upper left box). Empty symbols represent the data which have been rejected, i.e. those points which deviate more than 2 times the RMS deviation (sigma), which gives an estimate of the measurement error. The distribution of the computed zeropoint deviations from the average value are shown in the upper right box. The dotted line represent the data which have been rejected. The behaviour of the observed magnitude differences (Landolt-Instrumental) after correcting for the colour term (left panel) and the extinction coefficient (right panel) are shown in the lower boxes.

The following tables report the colour and extinction terms together with the zero points for FORS2. Only data taken in photometric nights were included. All estimated uncertainties are one standard deviation.

P82

Due to the re-coating of the main mirror in November 2008, there were not sufficiently many data to determine coefficients for the time October 1 to November 4, 2008. For this time please use the coefficients from P81.

November 2008 to March 2009

P81

P80

P79

P78

P77

Due to the washing of the secondary mirror in May 2006, there were not sufficiently many data to determine coefficients for the time April 1 to May 10, 2006. For this time please use the coefficients from P76.

May to September 2006

CHIP1
Filter	Colour	Colour Term	Avg. Zero Point	Ext. Coeff.	N. of Stars	RMS deviation	Plots (Gif)	Plots (PS)
U	(U-B)	+0.090+/- 0.013	24.196+/- 0.048	0.447+/- 0.036	82	0.061	U	U
B	(B-V)	-0.032+/- 0.013	27.612+/- 0.033	0.184+/- 0.021	133	0.029	B	B
V	(B-V)	+0.023+/- 0.007	28.134+/- 0.021	0.124+/- 0.011	192	0.020	V	V
R	(V-R)	+0.037+/- 0.019	28.323+/- 0.030	0.063+/- 0.016	160	0.067	R	R
I	(V-I)	-0.000+/- 0.011	27.647+/- 0.039	0.001+/- 0.023	175	0.100	I	I

CHIP2
Filter	Colour	Colour Term	Avg. Zero Point	Ext. Coeff.	N. of Stars	RMS deviation	Plots (Gif)	Plots (PS)
U	(U-B)	+0.097+/- 0.016	24.258+/- 0.054	0.414+/- 0.040	75	0.067	U	U
B	(B-V)	-0.033+/- 0.012	27.673+/- 0.036	0.200+/- 0.024	97	0.047	B	B
V	(B-V)	+0.001+/- 0.009	28.145+/- 0.026	0.119+/- 0.015	121	0.040	V	V
R	(V-R)	-0.024+/- 0.029	28.363+/- 0.040	0.082+/- 0.022	101	0.068	R	R
I	(V-I)	-0.058+/- 0.014	27.635+/- 0.037	0.000+/- 0.022	117	0.068	I	I

P76

Due to the re-coating of the main mirror end of December 2005 there are two tables for P76.

December 25, 2005 - March 2006

CHIP1
Filter	Colour	Colour Term	Avg. Zero Point	Ext. Coeff.	N. of Stars	RMS deviation	Plots (Gif)	Plots (PS)
U	(U-B)	+0.113+/- 0.011	24.039+/- 0.032	0.462+/- 0.020	176	0.074	U	U
B	(B-V)	-0.026+/- 0.005	27.575+/- 0.019	0.224+/- 0.010	206	0.041	B	B
V	(B-V)	+0.027+/- 0.003	28.075+/- 0.013	0.122+/- 0.006	218	0.026	V	V
R	(V-R)	+0.026+/- 0.005	28.337+/- 0.023	0.090+/- 0.013	163	0.040	R	R
I	(V-I)	-0.029+/- 0.006	27.678+/- 0.024	0.044+/- 0.014	185	0.044	I	I

CHIP2
Filter	Colour	Colour Term	Avg. Zero Point	Ext. Coeff.	N. of Stars	RMS deviation	Plots (Gif)	Plots (PS)
U	(U-B)	+0.113+/- 0.012	24.062+/- 0.034	0.426+/- 0.021	185	0.084	U	U
B	(B-V)	-0.042+/- 0.005	27.603+/- 0.019	0.216+/- 0.010	194	0.039	B	B
V	(B-V)	+0.016+/- 0.004	28.074+/- 0.016	0.110+/- 0.007	179	0.029	V	V
R	(V-R)	-0.049+/- 0.007	28.348+/- 0.028	0.090+/- 0.016	150	0.055	R	R
I	(V-I)	+0.027+/- 0.007	27.683+/- 0.032	0.064+/- 0.020	160	0.064	I	I

October 2005 - December 10, 2005

CHIP1
Filter	Colour	Colour Term	Avg. Zero Point	Ext. Coeff.	N. of Stars	RMS deviation	Plots (Gif)	Plots (PS)
U	(U-B)	+0.059+/- 0.006	24.013+/- 0.027	0.459+/- 0.012	73	0.035	U	U
B	(B-V)	-0.011+/- 0.007	27.517+/- 0.024	0.237+/- 0.009	76	0.029	B	B
V	(B-V)	+0.033+/- 0.005	28.032+/- 0.018	0.133+/- 0.006	78	0.020	V	V
R	(V-R)	-0.015+/- 0.015	28.289+/- 0.026	0.089+/- 0.010	69	0.033	R	R
I	(V-I)	-0.028+/- 0.009	27.629+/- 0.030	0.032+/- 0.013	83	0.044	I	I

CHIP2
Filter	Colour	Colour Term	Avg. Zero Point	Ext. Coeff.	N. of Stars	RMS deviation	Plots (Gif)	Plots (PS)
U	(U-B)	+0.059+/- 0.010	24.060+/- 0.034	0.442+/- 0.016	69	0.050	U	U
B	(B-V)	+0.006+/- 0.011	27.503+/- 0.024	0.221+/- 0.012	73	0.042	B	U
V	(B-V)	+0.036+/- 0.007	28.003+/- 0.021	0.121+/- 0.007	77	0.026	V	V
R	(V-R)	+0.055+/- 0.020	28.243+/- 0.029	0.083+/- 0.012	73	0.040	R	R
I	(V-I)	-0.013+/- 0.007	27.627+/- 0.024	0.044+/- 0.010	79	0.032	I	I

P75

CHIP1
Filter	Colour	Colour Term	Avg. Zero Point	Ext. Coeff.	N. of Stars	RMS deviation	Plots (Gif)	Plots (PS)
U	(U-B)	+0.075+/- 0.006	23.994+/- 0.022	0.435+/- 0.013	268	0.061	U	U
B	(B-V)	-0.028+/- 0.005	27.510+/- 0.015	0.206+/- 0.007	368	0.043	B	B
V	(B-V)	+0.025+/- 0.003	28.046+/- 0.011	0.121+/- 0.005	375	0.029	V	V
R	(V-R)	+0.011+/- 0.008	28.270+/- 0.016	0.073+/- 0.008	309	0.042	R	R
I	(V-I)	-0.023+/- 0.004	27.642+/- 0.017	0.035+/- 0.009	316	0.046	I	I

CHIP2
Filter	Colour	Colour Term	Avg. Zero Point	Ext. Coeff.	N. of Stars	RMS deviation	Plots (Gif)	Plots (PS)
U	(U-B)	+0.066+/- 0.004	24.108+/- 0.016	0.445+/- 0.008	230	0.035	U	U
B	(B-V)	-0.036+/- 0.003	27.554+/- 0.012	0.218+/- 0.005	289	0.027	B	U
V	(B-V)	+0.023+/- 0.004	28.025+/- 0.015	0.108+/- 0.007	285	0.037	V	V
R	(V-R)	+0.013+/- 0.005	28.264+/- 0.011	0.073+/- 0.005	252	0.023	R	R
I	(V-I)	-0.018+/- 0.004	27.627+/- 0.014	0.024+/- 0.007	253	0.032	I	I

P74

CHIP1
Filter	Colour	Colour Term	Avg. Zero Point	Ext. Coeff.	N. of Stars	RMS deviation	Plots (Gif)	Plots (PS)
U	(U-B)	+0.055+/- 0.010	24.048+/- 0.054	0.406+/- 0.045	148	0.067	U	U
B	(B-V)	-0.031+/- 0.007	27.595+/- 0.033	0.217+/- 0.025	188	0.044	B	B
V	(B-V)	+0.019+/- 0.005	28.184+/- 0.021	0.203+/- 0.014	203	0.032	V	V
R	(V-R)	-0.060+/- 0.008	28.452+/- 0.022	0.175+/- 0.015	181	0.036	R	R
I	(V-I)	-0.030+/- 0.006	27.817+/- 0.031	0.159+/- 0.022	186	0.048	I	I

CHIP2
Filter	Colour	Colour Term	Avg. Zero Point	Ext. Coeff.	N. of Stars	RMS deviation	Plots (Gif)	Plots (PS)
U	(U-B)	+0.059+/- 0.009	24.087+/- 0.042	0.374+/- 0.033	142	0.062	U	U
B	(B-V)	-0.039+/- 0.007	27.635+/- 0.029	0.237+/- 0.021	175	0.040	B	B
V	(B-V)	+0.019+/- 0.005	28.135+/- 0.025	0.169+/- 0.019	176	0.031	V	V
R	(V-R)	-0.025+/- 0.010	28.415+/- 0.027	0.171+/- 0.020	162	0.035	R	R
I	(V-I)	-0.021+/- 0.006	27.802+/- 0.030	0.155+/- 0.022	154	0.038	I	I

P73 June 24 to September 30, 2004

CHIP1
Filter	Colour	Colour Term	Avg. Zero Point	Ext. Coeff.	N. of Stars	RMS deviation	Plots (Gif)	Plots (PS)
U	(U-B)	+0.070+/- 0.015	24.147+/- 0.104	0.447+/- 0.093	49	0.048	U	U
B	(B-V)	-0.028+/- 0.009	27.793+/- 0.042	0.350+/- 0.034	60	0.028	B	B
V	(B-V)	+0.015+/- 0.011	28.278+/- 0.032	0.250+/- 0.019	64	0.034	V	V
R	(V-R)	-0.032+/- 0.011	28.522+/- 0.022	0.210+/- 0.008	42	0.015	R	R
I	(V-I)	-0.025+/- 0.006	27.859+/- 0.024	0.162+/- 0.011	53	0.048	I	I

CHIP2
Filter	Colour	Colour Term	Avg. Zero Point	Ext. Coeff.	N. of Stars	RMS deviation	Plots (Gif)	Plots (PS)
U	(U-B)	+0.100+/- 0.016	24.332+/- 0.157	0.537+/- 0.138	25	0.046	U	U
B	(B-V)	-0.036+/- 0.013	27.749+/- 0.061	0.285+/- 0.046	33	0.040	B	B
V	(B-V)	+0.015+/- 0.013	28.247+/- 0.065	0.222+/- 0.044	25	0.027	V	V
R	(V-R)	+0.018+/- 0.019	28.469+/- 0.048	0.190+/- 0.029	21	0.019	R	R
I	(V-I)	-0.009+/- 0.009	27.778+/- 0.050	0.100+/- 0.031	25	0.023	I	I

April 1 to June 12, 2004

CHIP1
Filter	Colour	Colour Term	Avg. Zero Point	Ext. Coeff.	N. of Stars	RMS deviation	Plots (Gif)	Plots (PS)
U	(U-B)	+0.086+/- 0.012	24.147+/- 0.044	0.436+/- 0.023	44	0.044	U	U
B	(B-V)	-0.025+/- 0.008	27.562+/- 0.030	0.161+/- 0.014	52	0.029	B	B
V	(B-V)	+0.021+/- 0.006	28.075+/- 0.026	0.074+/- 0.012	51	0.022	V	V
R	(V-R)	+0.040+/- 0.015	28.262+/- 0.032	0.011+/- 0.015	43	0.028	R	R
I	(V-I)	-0.003+/- 0.009	27.575+/- 0.038	0.000+/- 0.020	43	0.034	I	I

CHIP2
Filter	Colour	Colour Term	Avg. Zero Point	Ext. Coeff.	N. of Stars	RMS deviation	Plots (Gif)	Plots (PS)
U	(U-B)	+0.079+/- 0.009	24.227+/- 0.039	0.421+/- 0.019	37	0.034	U	U
B	(B-V)	-0.027+/- 0.009	27.616+/- 0.037	0.180+/- 0.017	34	0.028	B	B
V	(B-V)	+0.025+/- 0.007	28.056+/- 0.030	0.065+/- 0.013	38	0.023	V	V
R	(V-R)	+0.049+/- 0.016	28.261+/- 0.037	0.018+/- 0.016	31	0.026	R	R
I	(V-I)	+0.002+/- 0.012	27.617+/- 0.045	0.000+/- 0.021	33	0.037	I	I