Quick links: calibrations | QC
parameters | QC report | scoring & certification
Quality Control and documentation
The reprocessed UVES data set has a monitored and controlled quality. Data quality
has been assured by
- using certified calibration products
- measurement of quality control parameters
- certification based on quality control parameters (scoring)
The data quality has been monitored by quality plots which also serve
as spectral previews.
top Certified calibration
All calibration products in
the archive have been certified earlier. They have been processed
by earlier versions of the pipeline, analyzed and certified by the QC group, and
stored in the archive, as part of the quality control process routinely implemented
for all VLT and VLTI instruments.
The optimal association of calibrations and science data is another
aspect of QC. It is guaranteed that always the closest-in-time certified calibration
products are selected for processing. Thereby the instrument (and atmosphere) conditions
during science exposures are generally matched best with those at the time when the
calibrations were taken.
Find more about: UVES
calibrations | calibration
top QC parameters
The pipeline measures properties of the product files. These numbers
are called QC parameters. They form the backbone of the QC process for the UVES
science data. The QC parameters are written into the product headers and into
the QC1 database where they are available for statistical studies, plotting etc.
The QC parameters of the UVES science data are available here: plotter | browser.
The most important QC parameters are:
top Reports and previews
- qc_s2n (pipeline-derived S/N, averaged across the spectrum)
- mean_reduced (mean signal of extracted spectrum)
- qc_resolmed (resolving power, derived from the arclamp spectrum
used for calibration)
Graphical reports are available to serve as previews and to indicate
the quality of the products. There is one such report per CCD (i.e. one for a blue
exposure, two for a red exposure, three in total for a dichroic twin exposure). The
reports are stored in the archive and are part of the data set delivered in the downloads.
They have names like
belongs to the data set with ID UVES.2005-10-18T00:18:06.165).
Example of a QC report and preview
for a UVES product file
This product spectrum is always the extracted spectrum, not the
flux-calibrated spectrum. The reason for this is that this type of spectrum
always exists in a data set.
Top panel: date, setup information,
"product": this is the internal name (header key "PIPEFILE") of
the extracted product spectrum. This name can be easily used to match all poducts derived from
the same raw file (same data set).
"sciname": this is the final name of the
product, as it is archived; it includes information about the type, OB, time,
"OB, target": OB ID and target name
sketch of the slit (length and width to scale), the average object position, and the seeing
|Box 1: scans
through the raw file; left: middle CCD row; right: middle CCD column
(ADU vs. X pixels)
|Box 2: cross-dispersion
profile in raw file, closeup view, on a logarithmic scale to show
the full dynamic range (log ADU vs. X pixels)
Box 3: Three spectral windows
(6 A wide) of the product spectrum at full resolution. They
are marked in plot 4 (below) by the red broken lines and labelled as a-c.Horizontal
broken line indicates zero flux.
|Box 4: Preview
of the product spectrum and its S/N. The full spectrum is shown
here, with a 5 A rebinned version shown in red. The Y scale is in extracted ADU.
Nominal positions of some prominent spectral lines are marked blue for orientation.
The S/N plot is the ratio of the extracted spectrum and the
errorbar file. It gives a crude indication of the spectral quality. It is
not intended to replace precise measurements.
|Box 5: Raw
data histograms. One histogram is shown around the background level (100...400
ADU; log frequency). The approximate BIAS level is marked by the red line.
Any shift of the maximum away from the BIAS level indicates contributions from
sky background. The second histogram focuses on the saturation limit and
indicates any pixels in that range (the particular example here indeed shows
|Bottom panel: overview of
exposure and quality control parameters. From left to right: Exposure time, airmass,
seeing, slit width; wavelength start, end, step size (all in angstrom); number
of orders, mean S/N, mean position, mean profile width.
top Scoring and certification
All UVES products have undergone a certification procedure. This was either automatic
(assisted by scoring), or interactive. The scoring process has been tuned to apply
for more than 90% of all UVES products created (after all, this is mass production).
[If you do not want to know more about the certification, you
can skip this section.]
Scoring. The QC parameters have been used for automatic scoring of
the product quality. This is a novel QC concept to assess the quality of pipeline
products in an automatic way. The set of QC parameters, as measured by the pipeline,
is tested against configured thresholds. For each parameter, compliance results in
a score 0, while failure gives a score 1. In the process of UVES reprocessing,
data products with a non-zero total score are flagged for further inspection, while
products with score 0 are automatically certified.
The score results have been archived
and are available in the requested data sets under the names <dateset_id>.html
as HTML file, e.g. UVES.2005-07-03T04:36:52.519.html.
There is also a more technical score file, <dataset_id>.tlog.
The scored parameters are:
- wlen_start (start of wavelength scale) and qc_nord (number of extracted orders):
outliers may indicate a failure of the dispersion solution
- qc_fwhm: if this number is found to be below 0.5 arcs or above 3 arcs, this is
most likely indicating a problem which needs inspection
- qc_resolmed: if this number deviates from average values expected for the slit
width used, this may indicate a problem with the grating
- num_sat: the number of pixels in the raw file above 60,000 ADU
- diff_temp: temperature difference between science exposure and arclamp calibration;
should be below 2 degrees, otherwise thermal grating drifts may result in a shift
of the wavelength scale
- mean_airm: should be below 2, otherwise differential atmospheric dispersion may
corrupt the extraction
- qc_pos: should be sufficiently off the slit edges, otherwise the extraction might
Example of a UVES product score
|Parameter score report
time range: 2005-01-21 ... 2005-04-21
Scores are collected per QC1 parameter.
||score result: 2/16 best: 0/16
This example contains scores for
8 QC1 parameters and their evaluation for the 2 red CCDs and product files. The
little squares can be activated per mouse click to connect directly to the QC1
database and display a plot of that parameter and all other similar parameters
in a time window spanning 3 months.
The scoring scheme of UVES reprocessing is set up to display:
- green OK if total score = 0
- yellow OK if score 1 or 2
- red NOK is larger than 2
Certification. All products with non-zero
scores have been reviewed. As a result of this review, there might have been comments
about unusual features (e.g. "spectrum close to saturation but still looks ok").
These comments are meant to provide help or further information, but in no case imply
any guarantee. In particular, there is also no guarantee for completeness. There
might also always be products scored OK but having "hidden" problems.
optional comment files always refer to a data set and are identfied by the name <dataset_id>.cmt,