WAVELENGTH CALIBRATION
- Principles :
This function is provided to allow the user to remove
one or several calibration line(s) from the set used to
calibrate the spectra. It is generally
used after the end of the reduction, when it is found that
the calibration failed for a particular lens. This maybe
due, for instance, to a residual cosmic ray misidentified
as (or blended with) a line of the spectral lamp.
By removing this false line (for this particular lens only),
the calibration may be redone,
and give a satisfactory result; remember that this should be done on
the copy of the reference table which has been made at the beginning of
the extraction mask
creation, not on the original one.
As a first step, do not use this function, and jump to the
next paragraph : compute calibration
parameters.
- Use :
- Calibration datacube : the name of the spectral
lamp (or Perot-Fabry) datacube associated with the object
datacube you intend to calibrate later. You can type it in
directly, or use the browse
icon at the end of the field, or drag and drop it from the
Reduction folder.
- Reference wavelength table : it has been choosen
into a data folder provided with the software, and copied into the
work directory at the extraction mask creation step;
You can type the name in directly, or use the browse
icon at the end of the field, or drag and drop it from the
Reduction folder. Choose the
right element (neon, argon, Perot-Fabry, ...).
- Lens number : the lens/spectrum you will to work on.
By clicking on [Accept], a graphics window pops up, showing the Nth
spectrum
just selected (in black), with the theoretical line positions (in red)
overimposed. Ctrl-left-clicking on a line deselect it for the Nth
spectrum (it is now blue, another technological miracle),
and the next wavelength calibration computation for the associated
datacube will not use this particular line for this particular
spectrum.
- Options :
- Save values :
All the input values (files names, coordinates) are saved,
and become the new default values for this user. They can
be recalled at will, and are used each time the
Edit reference table window is opened.
- Recall values :
The values (files names, coordinates) saved by the user,
are loaded to the various input fields.
- Default values :
The input fields are set to the general default values;
for instance, the file names are set to blank.
- Principles :
Upon creation, the spectra contained into the
raw datacube are already wavelength-calibrated to
a good approximation (from a fraction of an Å to 2 Å,
according to the configuration), thanks to the extraction
algorithm. In the present reduction step, the small residuals
are fitted by a second-order polynomial to refine the
calibration.
- Use :
- Calibration data cube : this is the spectral lamp
or Perot-Fabry datacube associated with the object data cube
you are willing to calibrate. You can type the name in
directly, or use the browse
icon at the end of the field, or drag and drop it from the
Reduction folder. In the reduction folder,
it is displayed as a raw calibration datacube.
A new file will be created, with the same name as the
calibration data cube, but with extension .fits; it will
be shown into the reduction folder display as a
Table. In the next reduction step,
this table will be used for the final
wavelength calibration of the raw object datacube.
- Wavelength reference table : it has been choosen
into a data folder provided with the software, and copied into the
work directory at the extraction mask creation step;
You can type the name in directly, or use the browse
icon at the end of the field, or drag and drop it from the
Reduction folder. Choose the
right element (neon, argon, Perot-Fabry, ...).
- If all goes well, something like 1120 spectra should have
been successfully calibrated (watch the message in the output
window). Use the View result button to check the quality
of the calibration. It plots the error as a function of the
spectrum number; all the points should lie near to zero (below
0.05). If a few points show up as a small cloud at the upper
right of the plot, this is the trace of some problem with this
configuration, but they maybe neglected if they lie at the
very edge of the CCD frame; you may loose a few spectra over
the theoretical 1128 ones. Or you may play with the optional
parameters to try to reduce the size of the cloud. For
instance, identify the poor points by using the File,
Plot table, function in the Graph Display menu
of the plot window. Open the table you just write into, and
Plot with label after specifying X Col = a,
Y Col = error, Label = no. Play with the zoom
to get things readable in the cloud. Left mouse button to
define the zoom area, release button to apply.
- Options :
- Polynomial degree for continuum subtraction : this
is for the fit of the underlying continuum of the spectral
lamp or Perot-Fabry, if any. Usually, there is no such
continuum.
- Polynomial degree for calibration : this is for the
fit of the second-order calibration polynomial. Note that
upon creation, the spectra contained into the
raw datacube are already wavelength-calibrated
at (roughly) the Å level, thanks to the extraction
algorithm. This explains that a degree 1 is in fact enough
for the present step, 2 (the default) beeing a conservative
value.
- Debug : this switches the program to verbose mode,
and more informations are recorded into the history
file (see Getting started).
More, checking this option validates the following ones :
- Perform over : this allows to compute the
calibration coefficients over All lenses, or for
One lens. This last possibility is given to allow
the user to play with the optional parameters, in case of
tricky data (never happens, of course...)
to find the right combination. The lens number
must then be provided, and 1 is a default, not
worse than any other value, as this is the central lens.
Change it at will.
- Debug table, Debug spectrum : in debug mode,
a special
debug table will be created, as well as a special
debug spectrum, to hold the temporary results of
the user's tests.
- Save values :
All the input values (files names, coordinates) are saved,
and become the new default values for this user. They can
be recalled at will, and are used each time the
Edit reference table window is opened.
- Recall values :
The values (files names, coordinates) saved by the user,
are loaded into the various input fields.
- Default values :
The input fields are set to the general defaults values;
for instance, the file names are set to blank.
- Principles :
The parameters computed for each spectrum in the preceding
section are used to transform the pre-calibrated spectrum
into a wavelength-calibrated spectrum. A new datacube is
created to hold the spectra set.
- Use :
- Input data cube : this is the data cube created
by the
Extract spectra function. It is displayed as a
Raw object datacube. You can type it in directly,
or use the browse
icon at the end of the field, or drag and drop it from the
Reduction folder.
- Calibration table :
this is the table created during the
Compute calibration
(just preceding) step. It bears the same name as the
Raw calibration datacube associated with the
Raw object datacube you are to calibrate, but
with extension .fits. After the calibration, it will
keep the same name, but will be displayed as Calibrated
table. Same input possibilities as above.
- Output datacube : from the input datacube, a new
datacube file will be created, holding a full set of
wavelength calibrated spectra; the input one was made with
a set of pre-calibrated spectra.. After completion of the
calibration, it will be displayed as Wavelength calib
object datacube. Same input possibilities as
above.
- Wavelength scale :
you may choose to get the wavelength
coordinate of the spectra to be linear (lambda)
or log (log of lambda).
- Options :
- Quality threshold : you may choose here to set it to
None, and all the spectra are calibrated,
regardless of the quality of the calibration fit
(default, and good choice, as least as
a first step), or to Very good, or even to
Excellent if you prefer that; note that there is no option
like Terrible, as OASIS data are supposed to be at least good
;-) ... You may come back to this step later if you discover a lens
exhibiting a strange behaviour.
- Debug : this switches the program to verbose mode,
and more informations are recorded into the history
file (see
Getting started).
- Save values, Recall values,
Default values : as in the previous section.
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Last update: 11/01/1999. Send comments to
martin@cfht.hawaii.edu