(IAAC) estimating magnitudes

I would like to start including the magnitudes of some of the field
stars in my drawings.  The problem is I don't know how to go about doing
this.  It seems the online catalogues and megastar only go down to mag
15 or 16.  I was wondering if there is some software out there that
would allow magnitudes to be derived with relative accuracy from dss
images.  If one or two of the stars in the image were of known magnitude
they could be used to calibrate the software so that the magnitudes of
other stars in the image could be obtained.  This would not be the most
accurate method but would certainly beat the estimates I could make
simply by looking at an image.
	How do you arrive at the magnitudes of stars for which there is no
readily available data?
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