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<H3><A NAME="SECTION0027101300000000000000"> </A>
<A NAME="wavcal"> </A>
<BR>
Wavelength calibration
</H3>
<P>
The parameter <TT>WIDTH2</TT> is the width of the calibration lines.
The parameter <TT>THRES2</TT> defines the threshold above the local background
to detect the comparison lines. A way to estimate this value is to load the
<TT>WLC</TT> on the display. Then choose an order with few bright lines and
extract this order using <TT>EXTRACT/TRACE</TT>. Produce a plot via
<TT>PLOT/IMAGE TRACE</TT> and choose the
threshold level such that 7-12 lines (per order) will be above this level.
To produce a hardcopy of the <TT>WLC</TT> give the commands:<BLOCKQUOTE>
<TT>LOAD/IMAGE image <BR>
COPY/DISPLAY</TT></BLOCKQUOTE>
<P>
Now we can execute<BLOCKQUOTE>
<TT>CALIBRATE/ECHELLE <BR>
or <BR>
CALIBRATE/ECHELLE DONE ! If the order definition is already done.</TT></BLOCKQUOTE>
<P>
<BLOCKQUOTE>
<DIV ALIGN="CENTER">
<B>Note</DIV>
<I>For EFOSC reduction, the recommended method is TWO-D. In this
method a bivariate polynomial is fitted to the initial identifications, instead
of the monovariate polynomial involved in methods PAIR and ANGLE.</I></B></BLOCKQUOTE>
<P>
This command is the first step in the reduction. It finds the positions of the
orders and extracts the comparison lines. The user is prompted for the
identification of two calibration lines on the image display, repeated in
overlapped regions of adjacent orders. For a detailed description of
the wavelength calibration, see Section <A HREF="node472.html#wavcal">D.10.1</A>. This identification
is performed with the cursor. In case the orders are not overlapped, use
the method <TT>ANGLE</TT>:
<P>
<BLOCKQUOTE>
<TT>SET/ECHELLE WLCMTD=ANGLE <BR>
CALIBRATE/ECHELLE DONE</TT></BLOCKQUOTE>
<P>
which performs the wavelength calibration differently, this time prompting
for a minimum of four identifications anywhere in the spectrum.
<P>
Press
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in the control box of the display and provide the asked order
number and wavelengths.
<P>
If the automatic identification procedure does not converge to a satisfactory
solution, use the level 1 command:<BLOCKQUOTE>
<TT>IDENT/ECHELLE [wlc] [lincat] [dc] [tol] [wlcloop] [wlcmtd] [guess]</TT></BLOCKQUOTE>
<P>
to compute new dispersion coefficients. Use commands:
<P>
<BLOCKQUOTE>
<TT>PLOT/RESIDUAL <BR>
SEND/PLOT VERSATEC <BR>
READ/HISTOGRAM line.tbl :RESIDUAL 0.08 -0.04,0.04</TT></BLOCKQUOTE>to display the results. You have to specify all parameters, because you are
dealing with level 1 commands. The definition of the orders should give a
standard deviation of less than 0.5 pixel and the dispersion coefficients
should be fitted to better than 0.2 pixel.
<P>
The extracted orders in the pixel domain can be resampled into
wavelengths with the command <TT>REBIN/ECHELLE</TT>. A method to
verify the wavelength calibration consists of extracting and
resampling the wavelength calibration frame and displaying the
resampled image with a large scaling factor on the Y-axis.
The variation of wavelength coverage from one order to the next
should be smooth. Different regions of the resampled image and
in particular the overlaps can be verified with the command
<TT>PLOT/SPECTRUM</TT>.
<P>
<BLOCKQUOTE>
<TT>EXTRACT/ECHELLE {WLC} &w <BR>
REBIN/ECHELLE &w &r <BR>
PLOT/SPECTRUM &r 4000,4050 <BR></TT></BLOCKQUOTE>
<P>
Solutions are computed by default for each independent order
(<TT>SET/ECH WLCOPT=1D</TT>). They can also be computed using a
global bivariate polynomial (<TT>SET/ECH WLCOPT=2D</TT>).
<P>
Once a solution has been obtained for a particular set-up,
using either of the echelles, you should save the results via<BLOCKQUOTE>
<TT>SAVE/ECHELLE name</TT></BLOCKQUOTE>For all the observations done with the same instrumental set-up, one can
now use in <TT>CALIBRATE/ECHELLE</TT> this dispersion relation as a first guess
to the final coefficients by specifying the parameter <TT>GUESS</TT>:<BLOCKQUOTE>
<TT>SET/ECHELLE WLCMTD=GUESS GUESS=name</TT></BLOCKQUOTE>The main advantage is that
when using the <TT>GUESS</TT> option, no interaction is required
to identify the lines in <TT>CALIBRATE/ECHELLE</TT>. After searching for the
lines which are to be used for the determination of the dispersion relation
the program applies as a first guess the coefficients used in the session
previously saved and then starts the automatic identification.
<P>
The command <TT>CALIBRATE/ECHELLE</TT> has proved to be extremely flexible
and reliable. However, the first-time users might run into some problems.
The most frequent problems are listed below and a suggestion for their
cure is given:
<UL>
<LI><I>The wavelength calibration in a certain wavelength interval is
wrong:</I>
There is probably a wrong identification in the table. Give the command
<TT>LOAD/IDENTIFICATIONS</TT> and proceed as indicated above.
<LI><I>The wavelength calibration is wrong and apparently no
identification error was made:</I>
There are too many lines per order. Try a higher threshold in the
line detection.
<LI><I>There are *** NOT ENOUGH LINES *** per order to fit a polynomial
of order three to get the dispersion coefficients:</I>
The threshold was set too high. Try a lower one and use the <TT>GUESS</TT>
option with the previous solution as the starting point.
If, however, this
problem is restricted to one or two orders this does not have to be
serious because in those orders the dispersion coefficients will be
deduced from a global fit to the whole set of orders. It is also
possible to determine a bivariate solution (<TT>WLCOPT=2D</TT>)
</UL>
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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