arc

 ARC N -- Fits wavelength scale to an arc spectrum

 Parameters:
        N -- The arc to be fitted. Can be changed inside routine.

 ARC is interactive in nature and has many commands inside it.
 The general aim is to identify arc lines with specific wavelengths
 and fit a polynomial to the result. This can then be interpolated
 onto object spectra later. An outline of the procedure is as follows:

 First use 'NEW' to measure positions of lines in the arc spectrum (uses 
 cross-correlation with a gaussian of user specified FWHM). I normally use
 a FWHM comparable to the lines, but it should not matter. However, you should
 always use the same FWHM for all lines, and sometimes smaller FWHM may be
 less affected by closely spaced lines. Another parameter here allows one to
 avoid having lines too closely spaced. Next 'IDENTIFY' 2 or 3 lines spanning 
 as much of the wavelength range as possible (not very critical though).
 Come out of 'IDENTIFY' and 'FIT' a poly (2 coefficients to start), and then
 go back into 'IDENTIFY'. The routine will then predict wavelengths for the
 lines which helps greatly. If you have a good list of lines then these can
 now be loaded. With more lines you can see if a higher order fit is needed, 
 going back into 'FIT' etc, until you have completed the process. Check for
 blends and dump your line list to disk. You should only use good lines that
 will not be present in one spectrum but not another.

 For similar arcs, load your line list, 'TWEAK' the positions and then refit.
 If during the tweak, all the shifts are similar, then you are OK. If one is
 obviously discrepant, the line may have been a blend or weak, and it may be
 better to discard it, in which case you should re-do the original fit for
 consistency. Sometimes by applying an appropriate guess at the shift it is
 possible to recover such lines. It is however important to use the SAME set
 of lines for all your arcs, even if it means redoing every one because you
 cannot get the position of a line in your very last arc although you could
 in all the others. 

 If you have taken several different arcs at the start of the night which 
 give a good scale but then only observed one of them during the night
 to save time, you can first fit many lines with a high order poly and set
 it as a master polynomial inside 'FIT'. For the nighttime arcs you can then
 just fit a few low order coefficients.
 
 Once you have finished, come out of ARC and dump the calibrated spectra. You
 can use 'DRIFT' to plot the drift of arc scale with time. Particularly useful
 if you are following a single object for a long time.

 Once you are underway wit the TWEAK,FIT cycle, you can use 'COMB' to apply
 them to a succession of arcs, with no questions asked.

This command belongs to the classes: calibration .