
FLUX CALIBRATION

- Principles :
The fits table containing the absolute spectrum of the reference star
will be copied into the work directory; this ensures that the original
will remain in a safe place, and will not be inadvertently tempered with.
To be reachable, this table must be stored into directory
{Local XOasis installation path}/oasis-4.3/user/flux_ref/
where a few ones are already provided with the present software
(the ones which had already been of interest for the Lyon team, in fact...).
The following stars are already included, as of August 1998 :
- HR 4963
- HR 5501
- HD 23733
- HD 93521
- BD+332642
If you need another one, you must provide it. You may for instance
download the corresponding ASCII file giving F=F(lambda) for this
particular star from the
ESO spectrophotometric standards library. With your favourite
package (MIDAS, IRAF, ...), create then from that
a FITS table with at least two columns, one for LAMBDA, the other
for F_LAMBDA. But you may create more data columns, with
different flux units, which you may use later into XOasis. The standard name
of such a table is {Name of the star}abs.fits in XOasis, but you may
use any denomination of your own, like Georgette or MonEtoileStandard.
- Use :
- Click on Flux in the main menu, then on Copy reference
table.
- In the new window which pops up, check the table(s) you want to
copy to your work directory, then click on Copy.
- Principles :
A standard star observation is used to calculate the spectral
transfer function of OASIS/TIGER. The total flux of the star is
obtained by summation of all the spectra obtained within a
certain radius, larger than the seeing disc radius.
In the case where it is suspected that the
wings of the star image may be lost due to CCD size limitation,
it is possible to use the AOB theoretical PSF (two gaussians)
to estimate the amount of light lost. The correction curve, or
spectrum, obtained is stored in a Throughput spectrum.
- Use :
Click on Flux in the main menu, then on Compute
flux correction. In the new window :
- Input reference star datacube is the
preprocessed datacube of the standard star. 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. This
must be a FLUX standard, and the absolute spectral
flux of this star must be provided as a table :
- Input flux reference table is the table where
is recorded the outside atmosphere absolute spectrum
of the standard star; same input possibilities as
above. In this table the
Wavelength column and Flux column
are the labels of the two columns of interest, usually Lambda
and F_Lambda (case unsensitive).
- Output flux correction spectrum is the name
the user wish to use for the file holding the flux
correction curve.
- Spatial summation radius is the radius
inside which all the spectra will be summed up to get
the total flux of the reference star .
- Summation center (X,Y) is the center of this
spatial summation. This center must have been obtained before.
Use for instance the Analyse, Integrate spectra
function (see the
Analyse data : spatial or spectral integration section of the
present manual). Integrate the spectra over the whole wavelength
in the cosmics-removed standard star datacube, make an image with
the resulting data, choose View result, and note the
cursor coordinates of the centroid of the star image, as well as
the radius of this image; an eye estimate is good enough for the
present purpose.
This will give you the Summation center, as well as the
Spatial summation radius, which should be large enough
to enclose ALL the star energy.
- Options
- Use PSF estimate :
checking this option means that the user is willing to
use an AOB PSF estimate (made from two gaussians)
to take care of light from
the star possibly falling outside the CCD.
It validates the following sub-options :
- Core sigma is the sigma you adopt for the inner
peak of the AOB PSF, while Halo sigma is the
same parameter, but for the outer PSF halo.
Halo/core ratio is the peak-to-peak ratio of the
two.
- Polynomial smoothing may be used on the
correction curve, with a given Degree.
- Debug : this switches the program
to verbose mode, and more informations are recorded
into the history file (see
Getting started).
- Save values :
All the input values (files names, coordinates)
are saved, and become the new default values for the
user. They can be recalled at will, and are used each
time the Compute throughput 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
Each spectrum in the object datacube to be flux-calibrated
is multiplied by the flux
correction spectrum obtained during the preceding step. Care is taken
of the different integration times and air masses.
- Use
- Input object datacube : the object datacube as
it is after wavelength calibration, flat-fielding,
and cosmic rays removal. 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.
- Input flux correction spectrum : the correction
spectrum just generated during the
Compute throughput phase. Same input possibilities
as above.
- Output flux-calibrated object datacube : the result
will be recorded there. This will be the end product of the
reduction process for this exposure.
- Options
- 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
