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
*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.
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).
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)
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:
*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:
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.
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.
*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.
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.
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.