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Now you have in total three possibilities to perform your wavelength calibration:
The first fit is used to identify as many lines as possible in the corresponding
CCD row. For the identified lines a polynomial fit of chosen order is
performed (using Legendre or Chebyshev polynomials - the type being selected
with the keyword ).
The automatic line identification is repeated with this fit in order to
identify additional lines
to further improve the dispersion curve. This iteration is repeated until
a stable solution is obtained or the maximum number of cycles
(
) is reached.
According to the fit mode you choose
if lines with residual greater than the tolerance () are thrown
out or not. The fit modes available are:
The calibration itself is performed in the same way as in the LONG context: noch einfügen
The resulting dispersion coefficients are stored in table
.tbl, together with the r.m.s. error of the fit, the slitlet
and the y-coordinates (world and pixel coordinates).
Also a plot option for the resulting residuals and various degrees of display
are available.
After fitting all rows of the respective slitlet with fits of the chosen order
the program performs at last a linear fit to get the central wavelength and the
mean linear dispersion necessary to derive a starting wavelength
for the next slitlet from its known offset.
Rows where no fit could be achieved are stored in the table .tbl
with the slit
number -1.
Any selection of slitlets made in the table .tbl will be taken
into account, but all selections of the table
.tbl will be ignored.
If you want those to be respected, too, redo the search for the wavelength
calibration lines with the chosen selection in
.tbl.
After the wavelength calibration you may rebin your frame two-dimensionally to constant wavelength steps with REBIN/MOS. If you prefer to avoid the resampling noise until you have finished the sky-subtraction and extraction of your objects you may rebin the extracted frames with REBIN1D/MOS. This command splits the quasi-2-dimensional frame that is produced by EXTRACT/MOS up into single, 1-dimensional frames. It then takes - separately for each extracted spectrum - a dispersion relation that has been averaged over the CCD rows from which the spectrum was extractedi, and rebins the spectrum to constant wavelength steps with this relation.
If you prefer to avoid the resampling noise at all you can appoint to each pixel of an extracted frame its correct wavelength (table option) without rebinning to constant wavelength steps with the command APPLY/MOS.