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Testing the accuracy of WeDoWo

Summary

The standard star data suggests that
  • full moon or twilight might bias the measurements
  • the measured polarization is slightly lower than the literature value

The test data

High and zero polarization standard stars were observed during five nights in October and November 2012. For the calibrations dome flats were taken with two rotator position angles (ROTPOS=0 and ROTPOS=90) with the same integration time, typically ten+ten frames. The data was reduced in a standard manner, the dome flats were combined after bias subtraction. After this, the standard star data were bias subtracted, trimmed and flatfield corrected. The data from each night data were reduced in a similar way.

In most cases the calculated polarization was less than the literature value, e.g the measured V-band linear polarization of BD+64106 was about 5.4% (nights of vj13,vj15, vk09) using WeDoWo, however the literature value is 5.7%. The zero polarization stars showed low, close to zero (0.1-0.2%), polarization in all but one night. The typical error of degree of polarization of one measurement was about 0.1%. The data from the night of vj24 were different to other nights, the high polarization star measurements show lower polarization ( BD+64106 V(vj24)= 4.5%) than other nights and zero polarization data show higher polarization (HD12021 V(vj24)= 0.8%). We note that during the nights of vj13,vj15, vk09 the moon illumination was between 0 to 20% and 79% in the night of vj24. The B- and i-band data show similar trend to the V-band data.

In addition, in the night of vk13 high polarization stars BD+59389 and HD25443 were observed five and four times, respectively, through i-filter. All measurements show similar values, slightly below the literature value (5.1% vs 5.80%, for BD+59389 and 3.1% vs 4.25%, for HD25443), but the "variability" was within the measurement errors.

Some standard star measurements
Name Filter P literature (%) night P measured (%) P error(%) Comment
BD+64106 B 5.506 ± 0.090 vj13
vj15
5.3
5.3
0.11
0.11
 
BD+64106 V 5.687 ± 0.037 vj13
vj13
vk09
5.3
5.4
5.4
0.14
0.08
0.08


bad seeing
BD+64106 i 4.696 ± 0.052 vj13
vj13
vj15
vj24
vk09
4.7
4.6
4.2
3.8
4.5
0.13
0.09
0.09
0.10
0.10



bad seeing,80%moon
bad seeing
BD+59 389 V 6.701 ± 0.015 vk13
vk13
vk13
vk13
5.8
5.9
5.7
5.8
0.12
0.07
0.07
0.07
 
BD+59 389 i 5.797 ± 0.023 vk13
vk13
vk13
vk13
vk13
5.2
5.1
5.2
5.2
5.1
0.07
0.09
0.09
0.09
0.09
 
HD25443 V 5.127 ± 0.061 vk13
vk13
vk13
vk13
4.1
4.2
4.2
3.1
0.06
0.06
0.06
0.07
twilight
twilight
twilight
twilight
HD25443 R 4.734 ± 0.045 xc11
3.9
0.04
 
HD25443 i 4.249 ± 0.041 vk13
vk13
vk13
vk13
3.1
3.2
3.0
3.1
0.07
0.07
0.07
0.07
twilight
twilight
twilight
twilight
Zero polarization standards
G191B2B B 0.090 ± 0.048 vk13
vk13
0.61
0.77
0.18
0.12
twilight
G191B2B V 0.090 ± 0.048 (B) vk13
0.85
0.09
twilight
G191B2B R 0.090 ± 0.048 (B) xc11
0.10
0.06
twilight
G191B2B i 0.090 ± 0.048 (B) vk13
0.53
0.14
twilight
HD12021 V 0.112 ± 0.025 (B) vj13
vj24
0.23
0.79
0.07
0.08

bad seeing, 80%moon
HD12021 i 0.112 ± 0.025 (B) vj13
vj15
vj24
vk09
0.48
0.30
0.87
0.29
0.11
0.11
0.11
0.11


bad seeing, 80%moon
bad seeing

Back to top Last modified: 27-May-2014