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VIRCAM Quality Control:
Photometric Standard Stars

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photometric zeropoints | image_quality | ellipticity
 
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photometric zeropoint
image quality
ellipticity
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   Click on HISTORY to see the historical evolution of the trending.


Since most of the VIRCAM science observations contain enough 2MASS standard stars, science images are photometrically self-calibrated by using the 2MASS catalogue. In addition to this calibration plan the observatory maintains a monitoring programme that acquires regularly photometric standar star fields. Several parameters are derived from this observations to monitor several instrument characteristics.

 

top photometric zeropoints

QC1 parameters

parameter QC1 database: table, name procedure
wcs_rms vircam_photstd, wcs_rms - The rms of the world coordinate system derived by comparing the position of the identified photometric standard stars on the detector with the positions of the same sources in the 2MASS catalogue. The pipeline uses a built-in radial polynomial to correct the large scale optical distortion of the FOV induced by the wide field imager. The wcs_rms monitors the residuals between the WCS and the 2MASS star positions

photometric zeropoint

vircam_photstd, qc_magzpt
- Photometric zeropoint for air mass (X) =1. The air mass is taken from the observation and the extinction coefficient is read from the static calibration PRO.CATG=PHOTCAL


The pipeline reads the extinction coefficients and the color terms from a static calibration with PRO.CATG=PHOTCAL_TABLE

From 2009-01-01 to 2010-10-01 the following table was used:

#
# file           phot.fits
# extensions     1
# --------------------------------------------
# XTENSION       1
# Number of columns 5
#
filter    |extinction|   offset|   columns|coleq
Z         |  0.000000| 0.000000|j_m,h_m   |3,-2
Y         |  0.010000| 0.000000|j_m,h_m   |2,-1
J         |  0.080000| 0.000000|j_m       |1
H         |  0.080000| 0.000000|h_m       |1
Ks        |  0.160000| 0.000000|k_m       |1
NB118     |  0.090000| 0.000000|j_m       |1

 

Since 2010-10-01 the following table is used:

#
# file           photcal_2010-01.fits
# extensions     1
# --------------------------------------------
# XTENSION       1
# Number of columns 5
#
filter    |extinction|   offset|   columns|coleq
Z         |  0.050000| 0.000000|j_m,h_m   |1.95,-0.95
Y         |  0.050000| 0.000000|j_m,h_m   |1.55,-0.55
J         |  0.050000| 0.000000|j_m,h_m   |0.93,0.07
H         |  0.050000| 0.000000|j_m,h_m   |0.06,0.94
Ks        |  0.050000| 0.000000|j_m,k_m   |0.02,0.98
NB118     |  0.050000| 0.000000|j_m,h_m   |1.10,-0.10

The following table was used from 2011-06 -01 on:

#
# file           photcal_2010-12.fits
# extensions     1
# --------------------------------------------
# XTENSION       1
# Number of columns 5
#
filter    |extinction|   offset|   columns|coleq
Z         |  0.050000| 0.000000|j_m,h_m   |2.025,-1.025
Y         |  0.050000| 0.000000|j_m,h_m   |1.61,-0.61
J         |  0.050000| 0.000000|j_m,h_m   |0.923,0.077
H         |  0.050000| 0.000000|j_m,h_m   |0.032,0.968
Ks        |  0.050000| 0.000000|j_m,k_m   |0.01,0.99
NB118     |  0.050000| 0.000000|j_m,h_m   |1.10,-0.10
NB980     |  0.050000| 0.000000|j_m,h_m   |1.68,-0.68
      

VISTA M1 recoating interventions:

  • between 2009-08-19 and 2009-09-18
  • between 2011-03-04 and 2011-04-05

VIRCAM data acquired from 2017-03-26 are processes with pipeline version 2.3.0 instead of 2.2. A new PHOTCAL_TABLE table is used from then on:


#
# file           VC_GPCT_091031.fits
# extensions     2
# --------------------------------------------
# XTENSION       1
# Number of columns 9
#
filter_name|atm_extcoef|mag_offset|coleq_columns|   coleq_errcols|         coleq_coefs|gal_extcoef|default_zp|default_zp_err
          Z|   0.050000|  0.000000|    Jmag,Hmag|   e_Jmag,e_Hmag|        2.025,-1.025|   0.370000|  0.000000|      0.000000
          Y|   0.050000|  0.000000|    Jmag,Hmag|   e_Jmag,e_Hmag|          1.61,-0.61|   0.140000|  0.000000|      0.000000
          J|   0.050000|  0.000000|    Jmag,Hmag|   e_Jmag,e_Hmag|         0.923,0.077|   0.010000|  0.000000|      0.000000
          H|   0.050000|  0.000000|    Jmag,Hmag|   e_Jmag,e_Hmag|         0.032,0.968|   0.015000|  0.000000|      0.000000
         Ks|   0.050000|  0.000000|    Jmag,Kmag|   e_Jmag,e_Kmag|           0.01,0.99|   0.005000|  0.000000|      0.000000
      NB118|   0.050000|  0.000000|    Jmag,Hmag|   e_Jmag,e_Hmag|          1.10,-0.10|   0.010000|  0.000000|      0.000000
      NB980|   0.050000|  0.000000|    Jmag,Hmag|   e_Jmag,e_Hmag|          1.68,-0.68|   0.140000|  0.000000|      0.000000
      

top image quality

Independent on the used catalogue, the optical system (the telescope) and the atmosphere (seeing) affect the quality of the image shape of an extraterrestrical 'point source'. In bad seeing nights, the FWHM of the point source images on the detector and the shape of the point spread function are dominated by the turbulence in the atmosphere. In good seeing nights, when the seeing is small enough, the contribution of the optical system, the image quality of the telescope and the instrument become more and more apparent. Two parameters are derived from the standard stars observations:

QC1 parameters

parameter QC1 database: table, name procedure
qc_image_size vircam_photstd, qc_image_size - QC.IMAGE_SIZE is the median of the FWHM of all sources
qc_ellipticity vircam_photstd, qc_ellipticity - QC.ELLIPTICITY is the median of all (1-b/a) values. a being the semi major axis, b being the semi minor axis of the applied elliptical Gaussian fit.

 


 
 
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