[AZ-Observing] Re: Changes in 17P/Magnitude

>>  This was CCD photometry with aperture diameter of 8 pixels and
>>  annulus at 15 pixels. The is at f/3.3 with 12" objective., with a DSI
>>  Pro camera.

>>  Need anything else?

     What is the image scale in arcsec per pixel?  Almost certainly
your "sky" annulus is too small.  I'd make it so the inner radius
is _way_ outside the visible comet, like half a degree on the sky
if you can do that.  The typical problem with usual CCD comet
measurements is that people use relatively narrow fields and end
up subtracting off a lot of "comet" flux as though it were "sky",
so the magnitudes are chronically much fainter than than good visual
estimates (a case where the CCD data is wrong and visual data are
right).
     Since you can evidently adjust the sky and target apertures,
make a bunch of them using larger and larger patches centered on
the comet, but keeping the sky annulus fixed and very much larger
than the spots for the comet.  Make a plot of the increase in
brightness of the comet as you include greater amounts of the object.
Once the curve flattens out, then you have a pretty good idea of
the total brightness.

\Brian
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