GIRAFFE: Calibration recipes
BIAS frames: gimasterbias
DPR.CATG = CALIB, DPR.TYPE = BIAS
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Raw BIAS frame
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Purpose
Bias frames are measured to monitor the status of the CCD. They come
in stacks of 5 raw frames. They are routinely measured every night when
GIRAFFE is operational, in the read modes "Normal" and "Fast". They are
processed by the pipeline into a MASTER_BIAS. They are scored and
quality-checked by QC Garching.
Recipe
The pipeline recipe gimasterbias processes GIRAFFE BIAS frames. These
are combined and median-stacked.
QC checks
The pipeline measures median values, read noise and global
structure in the master BIAS and in the first raw frame.
Products
product category (HIERARCH ESO PRO CATG) | product code | format |
MASTER_BIAS | MBIA | 2D like the raw frame, with pre and overscan truncated |
DARK frames: gimasterdark
DPR.CATG = CALIB, DPR.TYPE = DARK
Purpose
Dark frames are measured to monitor the status of the CCD. They come in
stacks of 3 raw frames, each with 3600 sec integration time. They are taken
about monthly. They are processed into MASTER_DARK frames and are quality-checked
by QC Garching.
For data taken with the old CCD (until 2008-03), the master_dark is also
useful for subtracting the amplifier glow in the upper right edge of the CCD,
if this becomes an issue for the data reduction. There is no amplifier glow
with the current CCD (used after 2008-03).
Recipe
The pipeline recipe gimasterdark processes GIRAFFE DARK frames. These are
combined and median-stacked.
QC checks
The pipeline measures median values and dark current in the master DARK
frame.
Products
product category (HIERARCH ESO PRO CATG) | product code | format | comments |
MASTER_DARK | MDRK | 2D like the raw frame, with pre and overscan truncated | useful for background subtraction in science files until 2008-03 (old CCD ). The current CCD does not require master dark subtraction, but these files are still measured for monitoring. |
FLAT frames: gimasterflat
DPR.CATG = CALIB, DPR.TYPE = LAMP,FLAT or LAMP,FLAT,NASMYTH
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Raw FLAT frame
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Purpose
Flat frames are normally illuminated by the calibration unit on OzPoz, with
the lamp moved across the fibres which are arranged in a spiral pattern
('robotic flats'). While this is a good approach for the Medusa and IFU
fibres, the Argus fibres are not well illuminated by this procedure.
Therefore, there are also Argus flats taken on the illuminated Nasmyth screen
('nasmyth flats').
The flats measure fixed-pattern noise (pixel-to-pixel sensitivity variations),
fibre signal localization and width, relative transmission (fibre-to-fibre),
fringing (in the red), and the approximate response function. Normally 3 flats
are measured and stacked by the pipeline to suppress cosmic ray hits.
Recipe
The pipeline recipe gimasterflat processes GIRAFFE FLAT frames. The input raw
frames (typically three) are debiased, combined, median-stacked and normalized
to 1 sec. Next all fibres are localized by a profile fit in the course of the
extraction procedure and extracted. Finally they are normalized such that the
signal from the fibre with the highest transmission is set to one, and all
other fibres are normalized relative to that fibre. In the normalization
procedure, all SIMCAL fibres are neglected, as well as the SKY fibres for the
IFU and Argus slits. The product files have one column per extracted fibre,
plus a binary table as extension, the fibre_setup table which is described
here.
Only localized and extracted fibres get a column in the products, not
any "missing" fibres (both non-allocatable and non-allocated ones).
QC checks
The pipeline measures median/mean and rms of the following parameters:
relative fibre transmission; fibre signal curvature; signal width, and the
flat-field lamp efficiency.
Products
product category (HIERARCH ESO PRO CATG) | product code | format | comments |
FF_EXTSPECTRA | PFEX | 2D, pixels vs fibre index | extracted and normalized spectra |
FF_EXTERRORS | PFEE | same as PFEX | corresponding extraction errors |
FF_LOCCENTROID | PLOC | same as PFEX | centre (X px) of extracted signal per fibre |
FF_LOCWIDTH | PLOW | same as PFEX | width (px) of extracted signal per fibre |
The error file (PRO.CATG = FF_EXTERRORS) contains the calculated standard
deviation per pixel of the FF_EXTSPECTRA file (containing: photon noise, read
noise).
Products of the Nasmyth flats have similar pro.catg values (with FF replaced
by NF), and somewhat different product codes which are listed here.
Telluric lines
The red nasmyth flats occasionally show telluric line
absorption, due to the lightpath between the calibration lamp, the nasmyth
screen and the detector. Note that these features propagate further in the
course of the science file reduction and partially cancel out (overlap with) stellar telluric
features. See the following plot:
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Comparison of the spectrum of an early-type star as acquired with UVES (blue, bottom), and the signal of an IFU1 L887.1 nasmyth flat
(black; collapsed across all fibres, thereby removing the residual fringing
and enhancing the telluric absorption). In this example the flat has been
rebinned to prepare it for collapsing and comparison with a UVES science
spectrum (normally flats come in pixel space) but this does not affect the
statement about the telluric lines.
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The telluric lines in the extreme red part of the nasmyth
flats are also visible in 2D view e.g. on the FF_EXTSPECTRA products, this
time with the original (unbinned) extracted fibres in pixel space. Here the
telluric features show up as U-shaped darker features.
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CCD signature
On the extracted fibre signal (FF_EXTSPECTRA) of the current
CCD you can sometimes identify regular, low-amplitude features aligned with
CCD rows. These are due to the manufacturing process, have a vertical period
of 512 pixels and sometimes extend across the whole frame, sometimes across
the half. They are caused by small gain variations and correctly recorded in
the flats. They are much better visible on well-exposed image flats taken for
detector monitoring (see below). In the example below, you can also identify
vertical patterns at even lower amplitude, due to the same effect.
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CCD horizontal gain pattern visible on an extracted fibre flat (FF_EXTSPECTRA).
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CCD gain pattern on a well-exposed image flat (taken for detector monitoring).
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Relative fibre transmission
An important property of the GIRAFFE
fibres is the stability of their relative transmission, since the SKY signal
can only be recorded in dedicated (IFU and Argus) or user-selected (Medusa)
fibres. The level of precision of sky subtraction is therefore limited by the
accuracy of the relative fibre transmission as recorded by the flats. The
following figure demonstrates that the sky signal can be measured with a
precision of much better than 1% for most of the fibres.
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Stability of relative fibre transmission: transmission values
(normalized to a mean value of 1) from 94 days of Medusa1 flats are plotted on
top. The residual scatter is plotted below, with the two broken lines marking
1% and 3% rms, respectively. Most fibres have a transmission stable at the 0.3
% level over about 100 days.
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Illumination
While Medusa and IFU flats are sufficiently uniform when
obtained with the robotic lamp, the Argus fibres are not completely
illuminated by the robotic calibration lamp. The differences between a
'robotic' Argus flat and a Nasmyth flat are shown below.
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An Argus Nasmyth flat (below) and a robotic flat (top). The
extracted spectrum has been collapsed in dispersion direction, to display the
fibre-to-fibre efficiency variation and the illumination differences. The
spikes are due to SKY fibres (15 in total), and SIMCAL fibres (5 in total).
The robotic flat has strong ilumination artefacts (the deep vertical
triangles) which would propagate into the science data if used for
flat-fielding. The Nasmyth flat has a much more uniform illumination which
makes it a much better choice for science flat-fielding.
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The next figure compares a Nasmyth flat and a sky flat. The sky flat has been
taken in twilight conditions on the sky and avoids all residual illumination
artefacts which could degrade the Nasmyth flat.
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Comparison of a sky flat (red) and a Nasmyth flat (black). No residual
illumination effects are visible. In particular, the spikes of width 2-3
fibres, and the degraded transmission at the end, are a real effect and not
caused by the Nasmyth illumination.
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This figure demonstrates that Argus data can be reliably flattened with the
Nasmyth flats. This is also evident from the following figure which compares
the non-flattened skyflat (below) and the skyflat after flattening with the
nasmyth flat (top). The bumps and wiggles have disappeared, and the residual
transmission differences are about 2.4% rms.
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Comparison of extracted but not flat-fielded sky_flat (below), and the
sky_flat flattened with the Nasmyth flat (top).
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WAVE frames: giwavecalibration
DPR.CATG = CALIB, DPR.TYPE = LAMP,WAVE
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Raw WAVE frame
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Closeup to display single emission lines from single fibres.
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Purpose
WAVE frames are illuminated by the arclamp calibration unit on OzPoz, with the
lamp moved across the fibres which are arranged in a spiral pattern.
The arclamp frames are used to derive the global dispersion solution for
science (and STD) frames.
Recipe
The pipeline recipe giwavecalibration processes GIRAFFE WAVE frames. The input
raw frame is debiased. The corresponding localization (PLOC) and width (PWID)
solutions from the gimasterflat recipe are used to localize and extract the
fibre signals. The line catalog is searched for matching emission lines which
are then identified in the fibre signals. Blended, saturated and too faint
lines are rejected, as well as lines with a non-Gaussian PSF. The Giraffe
optical model is globally fitted. The residuals between that solution and the
positions of the identified emission lines are fitted by a 2D Chebychev
polynomial to obtain the complete dispersion solution. Small fibre-to-fibre
differences are taken into account by the static slit geometry setup table and
the grating table. The extracted arclamp frame is finally wavelength
calibrated and rebinned, to deliver a product suitable for quality control
checks.
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Extracted arclamp frame (left) and rebinned arclamp frame
(right).
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Each extracted fibre contributes to the global pattern which should
give a straight line for each emission line. The straightness of the emission
lines is a sensitive check on the precision of the dispersion solution.
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QC checks
The pipeline measures the mean flux level of the raw input frame, the number
of saturated pixels, the calibration lamp efficiency, the rms of the
dispersion solution, the number of lines found, accepted, and used for the
fit.
Products
product category (HIERARCH ESO PRO CATG) | product code | format | comments |
ARC_EXTSPECTRA | PAES | 2D, pixels vs fibre index | extracted spectra |
ARC_RBNSPECTRA | PRBS | 2D, nm vs. fibre index | wavelength calibrated and extracted arclamp spectrum |
ARC_RBNERRORS | PRBE | same as PRBS | corresponding errors |
DISPERSION_SOLUTION | PDIS | table | dispersion solution coefficients |
LINE_DATA | PLIN | table | detailed list of line parameters |
The error files (PRO.CATG = ARC_EXTERRORS and ARC_RBNERRORS) contain the
calculated standard deviation per pixel of the ARC_EXTSPECTRA file
(containing: photon noise, read noise).
STD frames: gistandard
DPR.CATG = CALIB, DPR.TYPE = STD,OzPoz (or SimCal), DPR.TECH=IFU
Purpose
Flux STD frames are regularly taken only for ARGUS and IFU observations. They
can be used for response correction and flux calibration. They are usually
followed by three Nasmyth flat exposures. For efficiency monitoring, there is
also a dedicated set of Argus settings taken about once a month.
Recipe
The recipe processes the flux STD data just like the science data,
with two important differences: no fibre-to-fibre transmission correction is
applied, and (for Argus data only) a response curve and an efficiency curve
are calculated as the final step.
QC checks
Checks are done for over and underexposure.
Products
product category (HIERARCH ESO PRO CATG) | product code | format | comments |
STD_RBNSPECTRA | PSRS | 2D, wavelength bins vs. fibre index | rebinned extracted spectra, one column per fibre |
STD_RBNERRORS | PSRE | same as PSRS | corresponding extraction error |
STD_RCSPECTRA | PIMG | 2D, spatial coordinates | reconstructed image (IFU and ARGUS) |
STD_RCERRORS | PIME | same as PIMG | corresponding error (IFU and Argus) |
EFFICIENCY_CURVE | PEFF | table | |
RESPONSE_CURVE | PRSP | table | |
The error file (PRO.CATG = ARC_RBNERRORS) contains the calculated standard
deviation per pixel of the ARC_RBNSPECTRA file (containing: photon noise, read
noise). This error is propagated to the error file of the recombined
spectro-image, STD_RCERRORS.
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