Order location

At present locating the orders is rather an interactive process (there is a program `echfind' that does it automatically but it does not yet work very well in all cases and its use is not recommended).

First decide whether you are going to use the continuum source or the object to locate the orders. It is a good idea first to do a `ystract' / `splot' of the data to get a feel for the width, intensity and profiles of the orders. Sensible commands to use for the GEC chip and the continuum are:

   ICL> ystract cont 185 194 s
   ICL> splot s reset accept

Now display the image using `image':

   ICL> image cont high=hhhh reset accept

The next stage is to use `icur' to define a point somewhere near the peak of each order that you wish to track and extract. You can track orders that are only partially on the image if you wish to but this is not recommended, since it could well affect the wavelength calibration, especially if only a small part of the free spectral range is being covered. It is quite important to choose points close to the peak intensity.

   ICL> icur

Now run `sdist' to track the orders and fit polynomials to them:

   ICL> sdist image=cont columns=8 trace=G width=3 maxdeg=10 softd=no

The two non-obvious parameters are `trace' and `width'. Specify `Gaussian' for `trace' if the profiles across the orders are roughly Gaussian and are not cut off by the dekker. Normally specify `COG' otherwise but if there is a noticeable gradient along the profile you can try `Edge' (they are identical in that both locate the rising and falling edges of the orders, but `COG' estimates the centre by calculating the centre of gravity and `Edge' estimates it simply by taking the mean of the edge positions). For `width', specify an estimate of the FWHM for `Gaussian' and an estimate of half the order width for `COG' and `Edge'. If anything, underestimate it for `Gaussian' and overestimate it for `COG' and `Edge'.

Beware that sky data can confuse `sdist' because it gives rise to profiles that don't fit any of the trace modes. If this appears to be a problem, use `clip' (which sets all data values below a given low value to that low value and sets all values above a given high value to that high value) to get rid of the sky data values that are causing the problem.

There is another program that may be useful here if you have moved to a new object that is not quite in the same place on the slit. `offdist' operates on an `sdist.dat' file and adjusts the constant terms so as to shift the tracked orders up or down by a specified amount.