Download Chapter 4 Data reduction logical step sequence

FLAMES-UVES Data Reduction Users’ Manual
VLT-MAN-ESO-19500-3016
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– NFLATS: A MIDAS descriptor written into the input frame in_frame, containing the size of the
YSHIFT descriptor.
Command description:
The y-axis shift of the orders/fibres in in_frame is found by correlating the frame with a synthetic one composed of Gaussian-shaped pseudo-fibres centred on the positions traced by the order-fibre table, with a variable
offset in the direction perpendicular to dispersion.
The size of the Gaussian shapes is forced to be the same as the one which was used to measure the “zero-point”
shift measured at the PREPFIBRE/FLAMES stage.
The correlation is computed using one pixel every CORRELXSTEP; thus CORRELXSTEP=1 means that
all pixels are used, CORRELXSTEP=2 means that every other pixels is used and so on. The safest bet
(and the default) is CORRELXSTEP=1. Increasing CORRELXSTEP cuts the correlation times significantly,
but CORRELXSTEP values larger than than the size of the resolution element are risky: on a line emission
spectrum it may happen that only the dark parts of the frame are used for the correlation, resulting in numerical
instability and useless results.
The variable y offset is allowed to span the interval between -max_shift and +max_shift, and is referred to
the fibre positions as measured on an all fibres flat-field frame by the PRENORM/FLAMES command, and
stored in the order table.
The maximum of the correlation is determined using a numerical algorithm, with accuracy better than CORRELTOL and the corresponding y offset corrected for the “zero-point” shift measured at the PREPFIBRE/FLAMES
stage.
This final y offset is saved as a descriptor called YSHIFT in the input frame. Since it is referred to a unique
reference position, it is duplicated as many times as the number of normalised fibre flat-field frames.
Note: since this same correlation algorithm is integrated, where needed, in data reduction steps (for speed reasons), the use of this command is mostly reserved for diagnostic purposes, or in cases where the determination
of the y shift is particularly difficult and needs quite a bit of fine tuning of the parameters, as may happen if extremely bright, strongly saturated emission lines are present in the frame. In such a case, it is indeed possible
to perform this step separately, and disable it in the level 2 DRS commands (e. g. EXTRACT/FLAMES) to be
used afterwards on the same frame. However, in such cases, it is more reasonable to switch off the correlation
step completely and to use a standard extraction limited to the cores of the fibres, sacrificing a fraction of the
flux to get reliably extracted spectra (or normalisation factors).
6.5.3 FNDRYSHIFT/FLAMES
Command Syntax: FNDRYSHIFT/FLAMES in_frame fibre_ff_cat slit_ff_cat order_table
[max_shift]
Input:
• in_frame: A single frame (after SPLIT/UVES and PREPPA/FLAMES).
• fibre_ff_cat: A catalog of normalised fibre flat-field frames, as produced by PREPFIBRE/FLAMES.
• slit_ff_cat: A catalog of equalised slit flat-field frames, as produced by PREPSLIT/FLAMES.
• order_table: An order-fibre table, as produced by e. g. OFPOS/FLAMES.
• max_shift: Maximum y shift (world coordinates) to be searched between in_frame and flat-field
frames. Default value is keyword MAXYSHIFT.
Output: