UVES-MOS: calibration files and recipes
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UVES-MOS Calibration files and recipes

BIAS frames (uves_cal_mkmaster)

DPR CATG = CALIB, DPR TYPE = BIAS

Raw bias frame of the two red CCDs

Purpose. Bias frames are measured to monitor the status of the CCD camera. They come in stacks of typically five raw frames. They are routinely measured every night when UVES or UVES-MOS is operational. They are all processed into master_BIAS frames and quality-checked on the mountain and by QC Garching. Since they are very stable, a complete set of master BIAS frames is stored in the calibration archive and delivered only about once per week.

Recipe. The pipeline recipe uves_cal_mkmaster determines median stacks of all input frames and produces a master BIAS frame. The recipe is is called with:
@p uves_cal_mkmaster set_of_input_frames basename_of_output_files ? M M MINIMUM

QC checks. As part of the QC1 checks performed by post-pipeline procedures, median values, read noise and overall structure in the master BIAS are monitored. Each master BIAS is compared to a reference master BIAS.

Click here for the results of these QC checks.

Products.

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

Format check files (flames_cal_predict)

DPR CATG = CALIB, DPR TYPE = LAMP,FMTCHK,SimCal

Raw formatcheck frame (echelle orders from bottom to top)

Purpose. Formatcheck exposures are obtained with a ThAr arc lamp. The simultaneous claibration fibre is used to measure the positions of the calibration lines and to obtain a first-guess dispersion solution. This file is required for automatic pipeline processing only.

The geometric calibration of echelle spectra is a complete definition of the spectral format including the order position and the wavelength associated to each detector pixel. This step was traditionally carried out in echelle spectrographs via visual identification of a few lines. In order to predict the dispersion relation and to efficiently calibrate all possible optical configurations in an automatic way, many new methods had to be developed. The high non-linearity of the dispersion relation has made necessary to develop a physical model of dispersion. The precision with which the geometric calibration is performed determines the accuracy of all successive steps.

The positions of a few hundred well separated ThAr lines contained in a reference table are predicted by the physical model and their central positions are projected onto the format-check frame.The lines are found in the narrow Th-Ar frame by a two-dimensional centering procedure. The initial dispersion relation, usually based on about a hundred initial detections is refined with successive iterations on the spectrum until most lines are found.

Recipe. The pipeline recipe flames_cal_predict does a fit to the physical model to obtain a first guess solution for the dispersion. In principle it is also possible to perform an order tracing here, but this is achieved with a higher precision with order definition flats. The recipe is called with:
@p flames_cal_predict input_frame basename_of_output_files set_of_ref_frames 40,40 0,0 0,0,0 0,0

Guess line table. The guess line table is used downstream in the calibration cascade to achieve the final dispersion solution from ThAr lamp exposures. Although preliminary, this first guess line table is very important for the following steps since it replaces the interactive line selection traditionally done for echelle reduction.

QC checks. The QC checks evaluate and monitor the differences between predicted and measured line positions, the shift against a reference frame, and the number of lines identified by the recipe. The figure below shows examples where differences in X position (dispersion direction) are plotted as a function of the position on the detector. The two left cases show a misalignment: the measured differences show a broad range of values without any structure. A good model prediction is indicated by a thin, well-clustered distirbution of the displayed points (right side).

Match and mismatch of physical model and format check frame. The normal result obtained after successful line matching (right) produces a well concentrated distribution with mean ordinate zero. A misalignment causes a failure of line matching (left). The two plots on top show examples for the upper CCD, the two others for the lower CCD.

The measured differences and shifts in X (dispersion) and Y (cross-dispersion) direction and the number of selected lines are stored in a database to monitor to detect changes of the spectral format.These can be due to shifts of the gratings/cross dispersers of the UVES spectrograph or can be caused by the fibre positioner. A destinction between both is only possible by comparing the results form UVES-MOS format checks with those from the UVES echelle mode (see also the description of UVES FMTCHKs).

Products.

DRS setup table

Apart from the guess line table, a DRS (data reduction system) setup table is produced which contains a FITS header only. This technical table is used by the pipeline to store parameters related to the physical model etc. It is updated through the calibration cascade: the first version is created from format check files, the second version from order definition files, the third and final version from wavelength calibration frames. The first two versions are preliminary only. The DRS table delivered by QC Garching is the final version.

The order and background tables produced by this recipe are guess solutions only and should not be used downstream in the calibration cascade.

Order definition files (flames_cal_orderpos)

DPR CATG = CALIB, DPR TYPE = LAMP,ORDERDEF,SimCal

Raw order definition flat (echelle orders from bottom to top)

Purpose. Order definition flats are taken with the simultaneous calibration fibre and a continuum flat lamp. They are processed to find automatically the position of the SimCal fibre for each echelle order. This file is required for automatic pipeline processing only and is used by the slit flat field and fibre flat field recipes. Additional products are a DRS set-up table and a FIB_ORDERDEF image which is basically the raw orderdefiniton frame splitted into two files (one for each CCD) for further processing.

This kind of exposure provides an accurate position of the spectrum along the cross-dispersion direction. In this step the physical model uses the information on the instrument configuration provided in the FITS header of the raw frame to estimate the number of orders present in the image. Hough Transform detection is then applied to find the central position of each order and an estimate of their slope at the center. Finally, the cross-order profile is centered along the order and a polynomial fit is performed. A description of the Hough Transform and its application can be found in the article of Ballester (1994, Astron.Astrophys. 286, 1011).

Recipe. The flames_cal_orderpos recipe processes the order definition flats. The positions are found through a Hough transform algorithm. The recipe script call is:
@p flames_cal_orderpos set_of_input_frames basename_of_output_files

Products.

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

Slit flat field calibration files (flames_cal_mkmaster)
DPR CATG = CALIB, DPR TYPE = LAMP,SFLAT

Slit flat field calibration frame (echelle orders from bottm to top)

Purpose. Slit flat field calibration frames are taken with a long slit and the continuum flat lamp. They measure fringing, fixed-pattern noise and the approximate blaze function. They are measured in three different slit positions with a stack of (typically) three frames at each position. The master slit flats are required for processing the fibre flats.

Recipe. The flames_cal_mkmaster recipe uses the same algorithm as the uves_cal_mkmaster recipe. It creates, after bias subtraction, a median stack from all raw input frames of each slit position in order to reject statistical outliers due to e.g. cosmic ray events. The recipe script call is:
@p flames_cal_mkmaster set_of_input_frames basename_of_output_files + M M MEDIAN

QC checks. The flat field lamp efficiency and stability are checked.

Products.

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

Fibre flat field calibration files (flames_cal_prep_sff_ofpos)

DPR CATG = CALIB, DPR TYPE = LAMP,FLAT,ODD,OzPoz / LAMP,FLAT,EVEN,OzPoz / LAMP,FLAT,ALL,OzPoz
DPR CATG = CALIB, DPR TYPE = LAMP,FLAT,ODD,SimCal / LAMP,FLAT,EVEN,SimCal / LAMP,FLAT,ALL,SimCal

All fibre flat field calibration frame (echelle orders from bottom to top, each order contains eight fibres)

Purpose. Fibre flat fields are taken with the calibration unit of the FLAMES fibre positioner and a flat field lamp. They come in three different varieties: with only ODD numbered fibres illuminated, with only EVEN numbered fibres illuminted, and with ALL fibres illuminated. Only science fibres are used; the SimCal fibre is dark. Fibre flats with DPR.TYPE=LAMP,FLAT,ODD,SimCal, etc. are intended to be used for science observations using simultaneous calibration. For other science observations, the OzPoz fibre flats are taken.

Three different types of fibre flat fields are needed because the images of the fibres on the detector partly overlap in the wings of the cross-dispersion profile in all-fibre flats. The recipe uses odd and even flats to determine the positions of all fibres and orders by making use of a Hough transform and stores the polynomial fit in the order definition table. A description of the Hough Transform and its application can be found in the article of Ballester (1994, Astron.Astrophys. 286, 1011).

The slit flat fields are equalised in the overlapping regions. A miminum set that covers the complete slit length used by the science observations is chosen as output. The odd/even fibre flats are normalised and cleaned for bad pixels. The fibre traces in the all-fibre flat are extracted and stored for relative normalisation between extracted science spectra from different fibres.

Recipe. The flames_cal_prep_sff_ofpos creates the final order defintion table with the positions of all fibres and orders, determines normalisation factors between the fibres and creates a minimum set of cleaned and normalised slit and fibre flats as input for the science reduction. The recipe script is called with:
@p flames_cal_prep_sff_ofpos set_of_input_frames basename_of_output_files set_of_ref_frames OPTIMAL M none 50000 Y

QC checks. The pipeline recipe logs the absolute and relative transmission of every fibre. This is used to monitor the performance of the fibres (changes in transparency) and the stability of the flat field lamp .

Products.

The above products scheme is used for products in Service Mode packages delivered for Period 74 or later. Some of the products are data cubes consisting of up to three individual products of the same type. The original files can be retrieved by using the command DECUBI/FLA of the MIDAS FLAMES context. The data cubes can be used by the science recipe if this is specified by the relevant recipe parameter.

Earlier Service Mode packages do not contain data cubes. The following scheme is an example; the actual scheme may be different because of varying numbers of products:

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

In the example shown in the table, the SLIT_FF_DT2, etc. frames have only be produced for the red lower chip. If these files have also been created for the red upper chip and/or the SLIT_FF_DT3, etc. frames have been produced in addition then the total number of products and the numbering will be different.

Wavelength calibration frame (echelle orders from bottom to top)

Purpose. Wavelength calibration frames are taken with the calibration unit of the FLAMES fibre positioner and a ThAr arc lamp. They are used to find the final dispersion relation for each fibre. Arc lamp frames with DPR.TYPE=LAMP,WAVE,SimCal are intended to be used for science observations using simultaneous calibration. For other science observations, the OzPoz WAVEs are taken.

Recipe. The flames_cal_wavecal recipe has been build around the uves_cal_wavecal algorithm. It uses the first-guess linetable, the DRS setup table, and the final order table (see FFLAT recipe) to find the strongest lines in the arc frame for every fibre. Then the reference linetable is used to match the fainter lines from which the dispersion relation is determined. The recipe script call is:
@p flames_cal_wavecal input_frame basename_of_output_files set_of_ref_frames AUTO Y N 4,0.6

QC checks. The QC check evaluates the distribution of resolving power R over the detector for one fibre, the mean resolution per fibre, the distribution of the fit residuals and the distribution of the selected lines across the detector. Median and mean values of R and the line FWHM, the standard deviation of the dispersion solution, and the numbers of identified and selected lines are stored in a database.

Products.

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

Send comments to <John.Pritchard@eso.org>  Last update: Nov 5, 2004