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SCIENCE DATA: GENERAL

On Paranal, the quick-look pipelines make an effort to automatically reduce all science data. The reduction is performed using standard calibration solutions from a local calibration database which is refreshed every few months. Generally any pipeline processing on the site is done on a best-effort basis. Its purpose is to offer a quick look to assess data quality etc.

Until the end of P87 (September 2011) science data have been processed by QC Garching with the best possible (certified) calibrations solutions. The products were ingested into the Science Archive and delivered to the PIs.

This service has been terminated with the begin of October 2011. The documentation of science pipelines and science processing is kept here for its heritage value.

UVES PIPELINE: SCIENCE REDUCTION

The UVES pipeline is designed to reduce raw SCIENCE spectra taken in Echelle mode to a level almost free from instrumental signature. Formally, there are two pipelines: the UVES-Echelle pipeline, and the FLAMES/UVES (or UVES MOS) pipeline. The UVES-Echelle pipeline is handled here. The UVES MOS pipeline is handled here.

On Paranal, the quick-look pipeline makes an attempt to automatically reduce all science data taken in Echelle and MOS modes. The reduction is performed using standard calibration solutions from a local calibration database which is refreshed every now and then. All settings are supported. Furthermore, for non-standard wavelength settings used in Visitor Mode, a complete set of calibration files is acquired and processed in order to obtain solutions. This often works fine, but since the number of possible non-standard UVES configurations is very high, there is poor experience with the pipeline behaviour in some of these cases. Generally any pipeline processing on the site is done on a best-effort basis.

SCIENCE RAW DATA


These types of raw science files exist:

frame DPR TYPE Purpose
SCIENCE OBJECT science exposure (observed before 2002-06-29)
SCIENCE OBJECT,POINT science exposure (point-like source)*
SCIENCE OBJECT,EXTENDED science exposure (extended source)*
TEST SLIT slit acquisition image (observed before 2002-11-23)
ACQUISITION SLIT slit acquisition image

*The destinction between point-like and extended sources is made by the user during the Phase 2 proposal.

Raw science data are usually accompanied by slit viewing images.

 

REDUCTION STEPS


Recipe. A science raw file is pipeline-processed by the pipeline recipe uves_obs_scired. Data are corrected for bias, interorder background, sky background, sky emission lines and cosmic ray hits. They are flattened, optimally extracted and finally merged. A response correction is also performed if an appropriate response curve exists for the setting. Find here a detailed record of the reduction steps.

Extraction. The UVES pipeline knows three extraction modes:

  • optimum (OPT)
  • average (AVG)
  • two-dimensional (2D)

Optimum extraction is the default extraction mode for "ECHELLE" and "ECHELLE,ABSORPTION-CELL" observations; average extraction for "ECHELLE,SLICER" and "ECHELLE,ABSORPTION-CELL,SLICER" observations. A two-dimensional extraction is performed if the DPR TYPE keyword is "OBJECT,EXTENDED".

Optimum extraction provides an optimized signal-to-noise ratio and is suitable for point sources. Average extraction is an alternative mode which simply averages any signal along the slit and above the sky background level. It may be a good choice in case of failure of the optimum extraction, e.g. at high exposure level. Two-dimensional extraction actually does not extract the signal at all, but provides a flattened and wavelength-calibrated solution with the flux redistributed into wavelength-slit coordinate space. This means that any spatial information along the slit is preserved and can be manually extracted later. Find more information here.

Optimum extraction. In optimal extraction mode (OPT) the UVES pipeline assumes a Gaussian profile for the cross-dispersion flux distribution. This procedure

  • yields optimum S/N,
  • automatically discriminates against any non-Gaussian components in the cross-dispersion profile.

This means specifically

  • automatic sky subtraction (since the sky background is treated as a pedestal),
  • automatic cosmic ray rejection (whenever the hit has a non-Gaussian distribution),
  • automatic sky emission line removal (since these fill up the whole slit and have a rectangular profile).

The spectra are then flattened in order-bin space, i.e. with extracted flats. Flat extraction is done with the weight factors derived from the science spectrum.

The UVES pipeline also offers flattening in pixel space (the science spectrum is divided by the flat field before extraction). Checks done on high-S/N, featureless spectra show that the pixel-space flattening may yield slightly (less than 10%) higher S/N than extracted flattening. However, telluric lines in the flats are found to propagate much stronger into the extracted science spectrum in the case of pixel-space flattening. Since telluric lines in the flats fill the whole slit, they are effectively removed by the optimum extraction.

Flattening corrects for the blaze function within the orders. In the red settings (860 nm), flattening also efficiently removes fringing.

A response correction (flux calibration) can be applied to the extracted spectra. This additional step can be performed for all settings with master response curves (i.e. the standard settings). Check out for more information and downloads here.

In principal any instrument response curve derived from a recent STD star spectrum can also be used for flux calibration. The difference between those "individual" response curves and the master response curves is the critical compilation of the latter ones, and the averaging of many individual ones into the master. In any case it should be clear that UVES cannot provide anything better than a relative flux calibration, mainly due to the slit losses and due to the observations taken under uncontrolled photometric conditions.

Products. The following reduction products per CCD (i.e. 1 BLUE for the blue arm, 1 REDLower and 1 REDUpper for the red arm) are created by the pipeline, if flux calibration is provided:

product category (PRO CATG) product number* format comments
RED_SCI_POINT or RED_SCI_EXTENDED 0000 (+0013) 1D (wavelength space) final science spectrum
(RV corrections to heliocentric and barycentric scale are provided in FITS header)
ERRORBAR_SCI_POINT or ERRORBAR_SCI_EXTENDED 0005 (+0018) 1D (wavelength space) errorbar file** corresponding to RED_SCI
(contains the standard deviation of the extraction as error estimate for the extraction procedure)
ORDER_TRACE 0008 (+0021) tfits table position and FWHM information per order
CRMASK 0009 (+0022) tfits table cosmic ray mask table
FLUXCAL_SCIENCE 0011 (+0024) 1D (wavelength space) flux-calibrated science spectrum (in 10E-16 erg/s/cm**2/A)
FLUXCAL_ERRORBAR 0012 (+0025) 1D (wavelength space) errorbar file corresponding to FLUXCAL_SCIENCE
* The numbers in brackets refer to REDU CCD, others to BLUE or REDL.
** defined by the variance of the chi-square fit to the signal obtained during optimum extraction. The error is determined by the boundary of the chi ... chi+1 domain. The errorbar file then gives the size of the bar which is symmetrical around the most likely value.

If flux calibration is not provided:

product category (PRO CATG) product number* format comments
RED_SCI_POINT or RED_SCI_EXTENDED 0000 (+0011) 1D (wavelength space) final science spectrum
(RV corrections to heliocentric and barycentric scale are provided in FITS header)
ERRORBAR_SCI_POINT or ERRORBAR_SCI_EXTENDED 0005 (+0016) 1D (wavelength space) errorbar file** corresponding to RED_SCI
(contains the standard deviation of the extraction as error estimate for the extraction procedure)
ORDER_TRACE 0008 (+0019) tfits table position and FWHM information per order
CRMASK 0009 (+0020) tfits table cosmic ray mask table
* The numbers in brackets refer to REDU CCD, others to BLUE or REDL.
** defined by the variance of the chi-square fit to the signal obtained during optimum extraction. The error is determined by the boundary of the chi ... chi+1 domain. The errorbar file then gives the size of the bar which is symmetrical around the most likely value.

For slicer observations reduced with average extraction:

product category (PRO CATG) product number* format comments
RED_SCIENCE_SLICER 0000 (+0010) 1D (wavelength space) final science spectrum
(RV corrections to heliocentric and barycentric scale are provided in FITS header)
ERRORBAR_SCIENCE_SLICER 0005 (+0015) 1D (wavelength space) errorbar file corresponding to RED_SCIENCE
(contains the standard deviation of the extraction as error estimate for the extraction procedure)
FLUXCAL_SCIENCE 0008 (+0018) 1D (wavelength space) flux-calibrated science spectrum (in 10E-16 erg/s/cm**2/A)
FLUXCAL_ERRORBAR 0009 (+0019) 1D (wavelength space) errorbar file corresponding to FLUXCAL_SCIENCE
* The numbers in brackets refer to REDU CCD, others to BLUE or REDL.

For slicer observations without flux-calibration:

product category (PRO CATG) product number* format comments
RED_SCIENCE_SLICER 0000 (+0008) 1D (wavelength space) final science spectrum
(RV corrections to heliocentric and barycentric scale are provided in FITS header)
ERRORBAR_SCIENCE_SLICER 0005 (+0013) 1D (wavelength space) errorbar file corresponding to RED_SCIENCE
(contains the standard deviation of the extraction as error estimate for the extraction procedure)

* The numbers in brackets refer to REDU CCD, others to BLUE or REDL.

ASSUMPTIONS MADE


The UVES pipeline implicitly makes the following assumptions on the raw spectra:

  • the object is a point source,
  • the object is centred on the slit,
  • the object has some continuum, i.e. is not a pure emission line object,
  • the signal-to-noise ratio is not too high ( < 50 approximately).

Only if these assumptions are readily met by the nature of the observations, useful results can be expected.

 

CALIBRATION PRODUCTS USED


The successful execution of science reduction requires a complete set of valid calibration products available.

All calibration data are processed by QC Garching and are quality-checked. Certified calibration products are stored in the archive.

Precision


Wavelength calibration.
The precision of the wavelength calibration in the course of science reduction is determined by

  • the precision of the dispersion solution used for resampling,
  • the difference of temperature between the science observation and the calibration solution used.

Typical values for the standard deviation (in milli-Angstroem) of the dispersion solution are given here. They are of the order of 2-8 milli-Angstroem.

The overall UVES spectrum shifts on the detector mainly depend on the air temperature in the UVES enclosure. The shift in the red arm is, in first approximation, 0.35 pixels/Degree C in the dispersion direction. One pixel is ~1.2 km/sec. The corresponding value in the blue arm is about 0.05 pixel/Degree C. Since 2002-01, an active compensation for such drifts has been implemented.

The temperature information is recorded in the file headers. By comparing the temperatures at the times of the science exposure and of the wavelength calibration applied to the data, the magnitude of the possible shift can be estimated from the gradients given above.

CCD defects. The UVES pipeline does not apply any correction for cosmetic defects of the CCDs. These are minimal because of the good cosmetic quality of the devices. However, there are a few defects which show up, notably in long integrations on faint objects on the two CCDs of the red arm.

There are four trails of hot pixels which appear in long exposures in the blue-side quadrant of the EEV chip (bluer side of the red arm mosaic, X coordinates 3896, 3963, 4052 and 4140 in an unbinned fits file, middle of the chip in y). They occupy a single column and are almost parallel to the orders. They appear as broadish emission in the bluer part of the extracted spectrum of a faint object.

In the MIT-LL chip (red side of the CCD mosaic of the red arm) there is a trap in the column X1609 which might show up as a slight depression over ~130 pixels in the extracted spectrum of one order. In long, binned exposures this chip shows also an emission band starting on the red side and extending over the rows 2790-2850 with decreasing intensity toward the blue side of the echelle format. Since this band is perpendicular to the spectrum, it is usually well subtracted in the sky subtraction step of the optimal extraction.


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