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VIRCAM Quality Control:
Photometric Standard Stars

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Trending & QC1
QC links:
photometric zeropoints | image_quality | ellipticity
photometric zeropoint
image quality
error in the WCS
pointing error
QC1 database (advanced users): browse | plot
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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 standard 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

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 photometric zeropoint are derived from the observation of photometric standard star fields and are dependent on several hardware and software components. While the 2MASS catalogue of the same version is used through the live time of the instrument, other components changed and had an impact on the zeropoint values:

A) The following hardware related interventions took place:

  • 2009-08-19 ... 2009-09-18: M1 recoating (silver coating with fast degression of reflectivity)
  • 2011-03-04 ... 2011-04-05: M1 recoating (aluminium coating)

B) The pipeline reads the extinction coefficients and the color terms from a static calibration with PRO.CATG=PHOTCAL_TABLE. Operations started with initial values of extinction and colour terms. These values were improved as more and more standard star observations became available.

  • 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
  • The following table was used for dat from 201-10-28:

# 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

C) The following major pipeline upgrades took place.

  • 2016-12-07: version 2.2.1(UK inkind version, with major changes in the source extraction algorithm)
  • 2017-03-16: version 2.3.0


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 extraterrestrial '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.

top world coordinate system (RA, DEC)

The VISTA telescope control software writes the coordinate system of the sky, the pointing direction, as world coordinates (ZPN projection) into the headers of the fits frames. In each detector extension the same RA and DEC (CRVAL1,2) values and the detector specific reference pixel (CRPIX1,2). The error between the real coordinates and the header coordinates, called pointing error is monitored. Is is a property/quality of the raw frame.
The pipeline, when processing the frames, applies an optical distortion model to the VIRCAM wide field, a radial polynomial of order=3, where the zero term is s shift, the mentioned pointing error.
In the absence of imperfect correction of the wide field optical distortion of VISTA, uncertainty of the barycenter of the telescope and seeing PSF, differential refraction and other effects the minimum RMS that can be achieved is the intrinsic RMS of the 2MASS catalogue.
Note that 1 detector pixel corresponds to 0.339 arcsec.  

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 in the pipeline processed product frame 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.
pointing error vircam_photstd, wcs_dcrval1, wcs_drcval2 - mean shift in arcsec, which was applied by the pipeline to align the raw frame header keywords on the WCS with the points sources in the pipeline product frames. This pointing error comes in two components, one as an offset value for RA (rectaszension) and as an offset value for DEC (declination). The pointing error is a quality measure of the telescope acquisition system and of the raw frame header information. The pipeline reports the pointing error in deg which is written as deg in the QC1DB. In the trending plot the values are scaled by 3600 to show them in arcsec.