Download Chapter 4 Data reduction logical step sequence

FLAMES-UVES Data Reduction Users’ Manual
VLT-MAN-ESO-19500-3016
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The traces are found using the Hough transform technique, internally running the low level HOUGH/ECHELLE
command with appropriate parameters. To improve robustness of the wavelength calibration and the extraction in different settings we have set values of SCAN keyword (see help of hough/echelle) with the MIDAS
keywords DRS_SCAN_MIN (set to 55,73,73) and DRS_SCAN_MAX (set to 1993,1975,1975) to prevent the
case in which it could be detected very small portion of fibres in the two corners in the bottom right hand side
or in the top right hand side of the detector.
While sensible defaults are provided for them, some of the above parameters can be given on the command
line and are passed along unchanged, but it is strongly advised to avoid changing them if their meaning is not
clear to the user (i. e. read the documentation for the HOUGH/ECHELLE MIDAS command before doing
any change of defaults for parameter P8).
These traces are then correlated with the order positions from the first guess order table, and labelled according
to order and fibre number as deduced from the FIBREMASK keyword in the fibre flat field frames.
The orders are then fitted with a polynomial of degrees as defined in the DEFPOL keyword, assuming constant
relative positions of the fibres. The polynomial coefficients are stored in the COEFFD descriptor of the output
order-fibre table, while the relative position of the fibres are stored in the FIBREPOS descriptor of the same
table.
6.4.4 PREBKG/FLAMES
Command Syntax: PREBKG/FLAMES order_table back_table [n_windows]
[x_width_max,y_width_max]
Input:
• order_table: Table with best-guess order positions, created e. g. by OFPOS/FLAMES.
• back_table: Name of the background position table to be created.
• n_windows: Optional number of windows in each inter-order space. The default is set by keyword
BKG_MAX_IO_WIND (500).
• x_width_max,y_width_max: Optional maximum x,y widths of each window. Defaults are set by
keyword BKG_XY_WIN_SZ (6 pixels in x, and 2 pixels in y).
Output:
• back_table: Background position table, for computation of scattered-light background.
Command description:
This command takes as input the order table (e.g. the table raworder.tbl created by PREDICT/FLAMES) to
create a table of positions where the scattered-light background may be computed, in regions not illuminated
by any fibre.
The user is advised to recreate the background table using the order-fibre table from the PRENORM/FLAMES
command before starting optimal (standard) extraction. In general, when a background table and an orderfibre table are required for a DRS command, it is considered good practice to freshly create the background
table using the same order-fibre table to be subsequently used.
Subtracting a poor background may easily make a disaster in subsequent data reduction steps, affecting the
quality of the extracted spectra and the DRS robustness. Therefore, in highly pathological cases, such as
strongly saturated, bright emission lines bleeding in large parts of a frame, it is recommended to run the
SELECT/BACK command on the background table and to make sure that only bona fide background regions
are left selected, before actually using it.
See also the BKFIT/FLAMES command for a standalone background fit on a frame.