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

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
>--
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-- 
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)
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