[daip] ISPEC question

[Gary A. Welch]

Dear Aips Helper,

I'm using ISPEC to produce the spectrum of HI emission in a faint source.  The 
input cube has been cleaned in IMAGR to remove the effects of HI emission in a 
much more intense source a bit over 9 arcmin away.  The velocity ranges partly 
overlap.  I have no zero-baseline flux values.

I produce spectra in 5 rectangular areas, each of dimension roughly 1.6 x 6 
arcmin.  One area includes the faint source.  The other 4 "sky" areas have 
similar dimensions, are contiguous with the one containing the source, and 
surround it on all 4 sides.  The sky areas contain no detectable emission.  The 
spectrum of each sky area scatters around zero, with an (eyeball) rms of 3-5 
mJy.    

The question:  The spectrum of the faint source itself shows emission peaking at 
10 mJy.  On each side of the line, though, the flux drops to clearly negative 
values, near -5 mJy by eye, and remains negative to the edge of the plot.  None 
of the sky spectra show such a systematic decline to negative values.

I am wondering why that might be so.

Regards,

Gary Welch