FORS1 Quality Control:
Extinction Coefficients and Colour Terms

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Colour terms for FORS1 and Paranal exctinction coefficients in the UBVRI (and ubvg_HIGH after April 2007) passbands have been determined by QC Garching since P73. This is 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 (Catalogue-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.

top Colour Terms and Extinction Coefficients

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 2007 FORS1 has a blue-sensitive CCD mosaic and new set of high-throughput filters (u_HIGH, b_HIGH, v_HIGH, g_HIGH)
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 band (U_BESS, u_HIGH) zeropoints are not available anymore. At the same time the standard filter set was changed to R_BESS, I_BESS, b_HIGH, v_HIGH, and g_HIGH.

The following tables report the colour and extinction terms together with the zero points for FORS1. Only data taken in photometric nights were included. All estimated uncertainties are one standard deviation.

-0.015 690 0.114 gH gH

P81 May 24 to September 30, 2008

CHIP1

Filter Colour Colour Term Avg. Zero Point Ext. Coeffs N. of Stars RMS deviation Plots (Gif) Plots (Ps)

R (V-R) +0.009+/-0.007 27.957+/-0.011
0.065+/-0.004 462 0.036 R R

I (V-I) -0.067+/-0.006 26.993+/-0.011 0.046+/-0.004 466 0.035 I I

bH (B-V) +0.064+/-0.005 28.513+/-0.011 0.211+/-0.004 453 0.037 bH bH

vH (B-V) +0.048+/-0.003 28.373+/-0.009 0.120+/-0.003 523 0.028 vH vH

gH (B-V) -0.055+/ 0.008
28.813+/-0.016 0.184+/-0.007 363 0.047 gH gH

CHIP2

Filter Colour Colour Term Avg. Zero Point Ext. Coeffs N. of Stars RMS deviation Plots (Gif) Plots (Ps)

R (V-R) -0.032 +/-0.007 27.948+/-0.012 0.081+/-0.004 356 0.033 R R

I (V-I) -0.078 +/-0.006 26.930+/-0.012 0.031+/-0.004 339 0.031 I I

bH (B-V) +0.057+/-0.007 28.412+/-0.015 0.200+/-0.006 339 0.044 bH bH

vH (B-V) +0.037+/-0.003 28.304+/-0.011 0.125+/-0.004 367 0.030 vH vH

gH (B-V) -0.065+/-0.010 28.771+/-0.022 0.198+/-0.010 286 0.059 gH gH

P80 October 2007 to March 2008

CHIP1

Filter Colour Colour Term Avg. Zero Point Ext. Coeffs N. of Stars RMS deviation Plots (Gif) Plots (Ps)

U (U-B) + 0.081+/-0.011 25.976+/-0.035 0.440+/-0.024 239 0.095 U U

B (B-V) -0.102+/-0.007 28.118+/-0.021 0.262+/-0.012 246 0.053 B B

V (B-V) +0.027+/-0.006 28.196+/-0.018 0.131+/-0.010 263 0.045 V V

R (V-R) +0.011+/-0.016 27.979+/-0.025
0.111+/-0.016 269 0.065 R R

I (V-I) -0.041+/-0.010 27.033+/-0.031 0.084+/-0.020 270 0.085 I I

uH (U-B) +0.098+/-0.008 26.496+/-0.027 0.440+/-0.015 183 0.064 uH uH

bH (B-V) +0.073+/-0.007 28.502+/-0.022 0.240+/-0.012 194 0.048 bH bH

vH (B-V) +0.045+/-0.007 28.370+/-0.020 0.144+/-0.011 190 0.043 vH vH

gH (B-V) -0.048+/ 0.008
28.811+/-0.023 0.214+/-0.012 182 0.051 gH gH

CHIP2

Filter Colour Colour Term Avg. Zero Point Ext. Coeffs N. of Stars RMS deviation Plots (Gif) Plots (Ps)

U (U-B) +0.050+/-0.007 25.951+/-0.023 0.465+/-0.014 242 0.055 U U

B (B-V) -0.114+/-0.009 28.052+/-0.026 0.260+/-0.017 245 0.065 B B

V (B-V) +0.009 +/-0.012 28.093+/-0.030 0.099+/-0.020 242 0.078 V V

R (V-R) -0.003 +/-0.025 27.905+/-0.032 0.089+/-0.021 235 0.082 R R

I (V-I) -0.054 +/-0.012 27.016+/-0.029 0.089+/-0.019 232 0.074 I I

uH (U-B) +0.056 +/-0.008 26.439+/-0.024 0.439+/-0.014 185 0.054 uH uH

bH (B-V) +0.048+/-0.016 28.425+/-0.036 0.228+/-0.023 192 0.089 bH bH

vH (B-V) -0.002+/-0.010 28.286+/-0.025 0.120+/-0.015 197 0.059 vH vH

gH (B-V) -0.072+/-0.010 28.757+/-0.024 0.222+/-0.014 196 0.056 gH gH

P79 April 2007 to September 2007

CHIP1

Filter Colour Colour Term Avg. Zero Point Ext. Coeffs N. of Stars RMS deviation Plots (Gif) Plots (Ps)

U (U-B) +0.043 +/- 0.008 26.080 +/- 0.031 0.447 +/- 0.018 123 0.054 U U

B (B-V) -0.128 +/- 0.008 28.163 +/- 0.023 0.241 +/- 0.013 140 0.039 B B

V (B-V) -0.015 +/- 0.007 28.227 +/- 0.023 0.112 +/- 0.012 140 0.037 V V

R (V-R) -0.097 +/- 0.015 28.024 +/- 0.025 0.087 +/- 0.014 130 0.041 R R

I (V-I) -0.101 +/- 0.011 27.023 +/- 0.034 0.027 +/- 0.021 123 0.058 I I

uH (U-B) +0.066 +/- 0.012 26.572 +/- 0.049 0.437 +/- 0.033 75 0.055 uH uH

bH (B-V) +0.048 +/- 0.009 28.539 +/- 0.031 0.222 +/- 0.020 86 0.034 bH bH

vH (B-V) +0.019 +/- 0.008 28.410 +/- 0.029 0.139 +/- 0.018 88 0.032 vH vH

gH (B-V) -0.096 +/- 0.008 28.867 +/- 0.032 0.209 +/- 0.022 82 0.030 gH gH

CHIP2

Filter Colour Colour Term Avg. Zero Point Ext. Coeffs N. of Stars RMS deviation Plots (Gif) Plots (Ps)

U (U-B) +0.039 +/- 0.013 26.069 +/- 0.041 0.470 +/- 0.024 95 0.069 U U

B (B-V) -0.124 +/- 0.012 28.221 +/- 0.032 0.316 +/- 0.019 136 0.058 B B

V (B-V) -0.011 +/- 0.012 28.247 +/- 0.033 0.158 +/- 0.018 115 0.058 V V

R (V-R) -0.047 +/- 0.022 28.009 +/- 0.033 0.111 +/- 0.018 111 0.058 R R

I (V-I) -0.100 +/- 0.012 27.038 +/- 0.035 0.037 +/- 0.019 99 0.057 I I

uH (U-B) +0.063 +/- 0.018 26.548 +/- 0.070 0.445 +/- 0.051 62 0.073 uH uH

bH (B-V) +0.059 +/- 0.016 28.563 +/- 0.052 0.282 +/- 0.036 72 0.056 bH bH

vH (B-V) +0.019 +/- 0.014 28.388 +/- 0.046 0.155 +/- 0.030 73 0.051 vH vH

gH (B-V) -0.106 +/- 0.019 28.938 +/- 0.082 0.287 +/- 0.063 69 0.064 gH gH

P78 December 2006 to March 2007

Filter Colour Colour Term Avg. Zero Point Ext. Coeff. N. of Stars RMS deviation Plots (Gif) Plots (PS)

U (U-B) +0.061 +/- 0.007 25.426 +/- 0.025 0.500 +/- 0.016 125 0.033 U U

B (B-V) -0.099 +/- 0.007 27.934 +/- 0.023 0.312 +/- 0.013 108 0.026 B B

V (B-V) +0.010 +/- 0.005 28.194 +/- 0.018 0.175 +/- 0.010 117 0.022 V V

R (V-R) -0.030 +/- 0.014 28.093 +/- 0.026 0.132 +/- 0.015 96 0.029 R R

I (V-I) -0.068 +/- 0.009 27.291 +/- 0.032 0.109 +/- 0.020 104 0.040 I I

P77 April 2006 to September 2006

Filter Colour Colour Term Avg. Zero Point Ext. Coeff. N. of Stars RMS deviation Plots (Gif) Plots (PS)

U (U-B) +0.074 +/- 0.006 25.191 +/- 0.021 0.442 +/- 0.012 176 0.043 U U

B (B-V) -0.088 +/- 0.004 27.699 +/- 0.014 0.200 +/- 0.008 221 0.020 B B

V (B-V) +0.027 +/- 0.003 28.011 +/- 0.012 0.103 +/- 0.005 208 0.020 V V

R (V-R) +0.004 +/- 0.007 27.933 +/- 0.013 0.068 +/- 0.006 191 0.057 R R

I (V-I) -0.061 +/- 0.004 27.112 +/- 0.013 0.024 +/- 0.007 199 0.029 I I

P76 October 2005 to March 2006

Filter Colour Colour Term Avg. Zero Point Ext. Coeff. N. of Stars RMS deviation Plots (Gif) Plots (PS)

U (U-B) +0.052 +/- 0.013 25.222 +/- 0.052 0.451 +/- 0.036 140 0.085 U U

B (B-V) -0.099 +/- 0.007 27.832 +/- 0.026 0.286 +/- 0.018 274 0.054 B B

V (B-V) +0.033 +/- 0.010 28.056 +/- 0.030 0.125 +/- 0.020 238 0.057 V V

R (V-R) +0.066 +/- 0.010 27.923 +/- 0.030 0.062 +/- 0.021 241 0.057 R R

I (V-I) -0.031 +/- 0.005 27.119 +/- 0.031 0.036 +/- 0.021 223 0.058 I I

P75 June 22 to September 30, 2005

Filter Colour Colour Term Avg. Zero Point Ext. Coeff. N. of Stars RMS deviation Plots (Gif) Plots (PS)

U (U-B) +0.067 +/- 0.008 25.290 +/- 0.034 0.425 +/- 0.023 111 0.047 U U

B (B-V) -0.088 +/- 0.007 27.870 +/- 0.024 0.268 +/- 0.014 184 0.049 B B

V (B-V) +0.030 +/- 0.006 28.149 +/- 0.021 0.147 +/- 0.011 164 0.037 V V

R (V-R) +0.005 +/- 0.013 28.069 +/- 0.026 0.105 +/- 0.012 164 0.037 R R

I (V-I) -0.063 +/- 0.010 27.256 +/- 0.036 0.067 +/- 0.023 170 0.073 I I

April 1 to June 12, 2005

Filter Colour Colour Term Avg. Zero Point Ext. Coeff. N. of Stars RMS deviation Plots (Gif) Plots (PS)

U (U-B) +0.077 +/- 0.014 25.201 +/- 0.039 0.442 +/- 0.023 64 0.049 U U

B (B-V) -0.058 +/- 0.007 27.714 +/- 0.025 0.232 +/- 0.011 83 0.030 B B

V (B-V) +0.053 +/- 0.007 28.040 +/- 0.025 0.132 +/- 0.012 80 0.029 V V

R (V-R) +0.056 +/- 0.017 27.936 +/- 0.030 0.079 +/- 0.016 86 0.047 R R

I (V-I) -0.018 +/- 0.010 27.115 +/- 0.034 0.046 +/- 0.019 70 0.041 I I

P74

Filter Colour Colour Term Avg. Zero Point Ext. Coeff. N. of Stars RMS deviation Plots (Gif) Plots (PS)

U (U-B) +0.072 +/- 0.013 25.037 +/- 0.058 0.297 +/- 0.047 96 0.054 U U

B (B-V) -0.083 +/- 0.010 27.632 +/- 0.046 0.145 +/- 0.035 108 0.044 B B

V (B-V) +0.028 +/- 0.006 28.004 +/- 0.028 0.083 +/- 0.021 116 0.027 V V

R (V-R) +0.012 +/- 0.013 27.972 +/- 0.029 0.087 +/- 0.021 117 0.032 R R

I (V-I) -0.041 +/- 0.008 27.072 +/- 0.030 0.003 +/- 0.020 114 0.037 I I

P73 June 6 to September 30, 2004

Filter Colour Colour Term Avg. Zero Point Ext. Coeff. N. of Stars RMS deviation Plots (Gif) Plots (PS)

U (U-B) +0.076 +/- 0.014 25.192 +/- 0.058 0.278 +/- 0.035 46 0.058 U U

B (B-V) -0.092 +/- 0.017 27.759 +/- 0.074 0.208 +/- 0.054 49 0.052 B B

V (B-V) +0.020 +/- 0.008 28.066 +/- 0.032 0.094 +/- 0.016 51 0.027 V V

R (V-R) -0.003 +/- 0.020 27.951 +/- 0.041 0.037 +/- 0.024 59 0.040 R R

I (V-I) -0.051 +/- 0.011 27.155 +/- 0.043 0.025 +/- 0.024 41 0.033 I I

April 1 to May 31, 2004

Filter Colour Colour Term Avg. Zero Point Ext. Coeff. N. of Stars RMS deviation Plots (Gif) Plots (PS)

U (U-B) +0.123 +/- 0.021 25.433 +/- 0.111 0.650 +/- 0.087 15 0.038 U U

B (B-V) -0.067 +/- 0.015 27.854 +/- 0.072 0.346 +/- 0.050 15 0.024 B B

V (B-V) +0.015 +/- 0.011 28.188 +/- 0.061 0.223 +/- 0.042 16 0.019 V V

R (V-R) +0.014 +/- 0.014 28.067 +/- 0.014 0.150 +/- 0.029 16 0.013 R R

I (V-I) -0.053 +/- 0.009 27.312 +/- 0.055 0.168 +/- 0.038 16 0.017 I I