Hello Brian, I did as you suggested: Aperture Annulus Magnitude 8 15 2.362 10 18 2.326 12 20 2.341 14 22 2.346 So I would say the V magnitude was 2.34 +/1 0.02. I'll send you a profile directly. Jeff At 11:38 -0700 10/25/2007, Brian Skiff wrote: >>> 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 >-- >See message header for info on list archives or unsubscribing, and please >send personal replies to the author, not the list. -- Jeff Hopkins HPO SOFT Counting Photons http://www.hposoft.com/Astro/astro.html Hopkins Phoenix Observatory 7812 West Clayton Drive Phoenix, Arizona 85033-2439 U.S.A. (623)849-5889 (623) 247-1190 (Fax) www.hposoft.com -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.