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* Parallel Extraction Subraction *
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09.21.99

Let's say you have a faint object with some line right above or below
a bright continuum source.  Since the object is just a few pix away
from the bright continuum, you can bet that the object's spectrum
will be contaminated by light from the continuum source.  It'll be
sort of like the faint object is riding on a DC bias.  To get rid
of the bias, make an extraction on the opposite of the continuum source,
exactly the same distance away, then subtract this from the spectrum
for the faint source.  Here's how:

1.  Do your usual extraction with apex on the faint object and on the
    continuum source (for the trace).  Using the same trace, also
    do a third extraction, equidistant but on the opposite side from
    the object of interest.

2.  You'll want to use sarith for the spectrum arithmetic.  Sarith (I think)
    can't be used to manipulate apertures within the same .fits file.  So,
    you'll need to make a separate .fits file containing the parallel extraction.
    I'm not sure, but it might be important to make the aperture number of the
    object in the original extraction match that of the parallel extraction
    in the second .fits file, which is to be subtracted.

3.  Sarith croaks unless everything is wavelength calibrated.  Go do that now.
    You could also apply the zero-point shift at this point, but I waited until
    I had the final, subtracted spectrum.

4.  Now you have two .fits files: the original, and the parallel extraction, and
    the aperture numbers of the object and the parallel extraction match.  Go
    to sarith (noao.oned) and just subtract the two.  If you leave "apertures"
    with "*", you should note that the original parallel extraction zeros
    out when the same spectrum in the second .fits file is subracted.