Quality Control and
Data Processing

HC PLOTS
photometric zeropoint
image quality (FWHM of PSF)
ellipticity
error in WCS
pointing RA accuracy
pointing DEC accuracy
QC1 database (advanced users): browse | plot

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.

From 2018 on it can happen that the vircam data reduction pipeline recipe for photometric standard stars cannot process chip #15. The resulting image product is coadded, but the jitter offsets are not taken into account. The failure occurs sporadically in the Z-band and since 2022 also in the Y-band. The reason is the most right channel #16 in chip #15 whith the erratic noise signal. The failure of the recipe can be fixed by modifying the bad pixel map generated by the linearity recipe, in a way that all pixel in channel #16 are flagged in addition. This modified bad pixel map must be used to process the twilight flat to generate modified master twilight flat and confidence map. Both modified twilight flat products and the modified bad pixel map must be used to process the photometric standard star field to ignored the channel #16 area during co-adding and achieve a well combined image and a final photometric zeropoint for chip #15.

An example modified bad pixel map has been ingested in the ESO master calibration archive for this purpose:
filename = VC_MBPM_221210A_X.fits and
dp_id = M.VIRCAM.2022-12-13T13:07:11.166.fits

QC1_parameters

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
QC.MAGZPT	vircam_photstd..qc_magzpt	[mag] photometric zeropoint	HC		[docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter **There might be more than one.

The photometric zeropoints of H, J and K are derived from the 2MASS catalog directly. The zeropoints of Y, Z, NB119 and NB980 are derived via color equations.

Trending

The trending plots show the zeropoints averaged over 16 detectors (upper left box) and the ones resolved per detector (upper right box). Zeropoints might be dependent on the choosen standard star field. For this reason the OBS.ID and the OBS.TARG.NAME is included in the database query (accesible via the [this] link).

Scoring&thresholds Photometric zeropoint

The values are not scored

History

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 catalog 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 re-coating (silver coating with fast digression of reflectivity)
• 2011-03-04 ... 2011-04-05: M1 re-coating (aluminium coating)
• 2018-07-25 ... 2018-08-05: M1 cleaning

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 color 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 data from 2011-10-28:

  #
  # 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
  

The coleq_coefs column of the table should be read as :

Z_VIRCAM     = 2.025 * Jmag_2MASS - 1.025 * Hmag_2MASS
Y_VIRCAM     = 1.610 * Jmag_2MASS - 0.610 * Hmag_2MASS
J_VIRCAM     = 0.923 * Jmag_2MASS + 0.077 * Hmag_2MASS
H_VIRCAM     = 0.032 * Jmag_2MASS + 0.968 * Hmag_2MASS
Ks_VIRCAM    = 0.010 * Jmag_2MASS + 0.990 * Kmag_2MASS
NB118_VIRCAM = 1.100 * Jmag_2MASS - 0.100 * Hmag_2MASS
NB980_VIRCAM = 1.680 * Jmag_2MASS - 0.680 * Hmag_2MASS

• 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

Algorithm Photometric zeropoint

The counts of identified stars are matched with the 2MASS catalog entries.

QC1_parameters

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
QC.IMAGE_SIZE	vircam_photstd..qc_image_size	[pixels] Average FWHM of stellar objects	HC		[docuSys coming]
QC.ELLIPTICITY	vircam_photstd..qc_ellipticity	Average stellar ellipticity (1-b/a)	HC		[docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter **There might be more than one.

Independent on the used catalog, 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:

Trending

The image size is trended here and the ellipticity is trended here.

Scoring&thresholds Image quality

The values are not scored

History

High ellipticity of sources can be due to several reasons, all related to the telescope. E.g.: One or more of the M2 hexapod legs can malfunction, the instrument can continue to observe, while the telescope is already moving, the altitude or azimuth drives might vibrate.

Algorithm Image quality

QC.IMAGE_SIZE is the median of the FWHM of all sources
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.

QC1_parameters

FITS key	QC1 database: table, name	definition	class*	HC_plot**	more docu
QC.WCS_RMS	vircam_photstd..wcs_rms	[arcsec] Average error in WCS fit	HC		[docuSys coming]
QC.WCS_DCRVAL1	vircam_photstd..wcs_dcrval1	[deg] change in crval1	HC		[docuSys coming]
QC.WCS_DCRVAL2	vircam_photstd..wcs_dcrval2	[deg] change in crval2	HC		[docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter **There might be more than one.

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. Each raw fits frame detector extension contains 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 a static optical distortion model to the VIRCAM wide field, a radial polynomial of order=3, where the zero term is a 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 astrometric RMS that can be achieved is the intrinsic RMS of the 2MASS catalog. Note that 1 detector pixel corresponds to 0.339 arcsec.

Trending

The residuals of thew WCS fit are monitored here and the pointing errors in RA and DEC are monitored here and here respectively. The FULL plot of the RMS of the WCS shows a linear long-term increase of the RMS.

Scoring&thresholds Pointing and WCS

The values are not scored

History

No remarks.

Algorithm Pointing and WCS

The rms of the world coordinate system quality ocntrol parameter is 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 catalog. The pipeline uses a built-in static radial polynomial to correct the large scale optical distortion of the FOV induced by the wide field imager.

RMS of the world coordinate system a function of the average number of identified sources. Data cover the range of 2010-2023. All filters are used. The RMS increases rapidely for images with less than 200 sources per detector.

The 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.