[AZ-Observing] Re: 17/P Holmes Annular or Not?
- From: Jeff Hopkins <phxjeff@xxxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx, amastro@xxxxxxxxxxxxxxx
- Date: Sun, 28 Oct 2007 14:51:29 -0700
When I took my V band photometric image I also did a 3D profile of
Comet Holmes.
See http://www.hposoft.com/CometHolmes.JPG
Jeff
At 14:22 -0700 10/28/2007, Tom Polakis wrote:
>In all of the views through the eyepiece, I have been convinced that
>there is a weak, thick ring structure to the main, bright disc.
>Properly exposed images suggest a ring as well. In this image by
>Jim Cormier...
>
>http://media.skyandtelescope.com/images/Holmes-1.jpg
>
>...the disc appears to be brighter from the edge inward to about 80%
>radius, then fainter inward to 50% before becoming very bright.
>That's a ring, right?
>
>Last night I overcame my fear of through-the-telescope imaging, and
>put my 20D on my TV101 refractor. An exposure of 10 seconds at ISO
>400 was about right to begin to show the outer halo and just barely
>not saturate the core. That image again showed to my eyes what
>appears to be a ring.
>
>Lacking any proper image analysis software, I used brute force in
>Photoshop, and measured a 3x3 average pixel value in the three color
>channels along a horizontal slice passing through the center of the
>nebula. They are plotted above my image of the comet here.
>
>http://members.cox.net/tpolakis/astro/holmes_photometry_20071028.jpg
>
>If it were really a ring, there would be a dip in the brightness
>curve as you approach the center. In fact, there is only a change
>in slope (on the east side only) before it becomes steep again. So
>it appears that it is not really a ring, and there is some sort of
>illusion at play. It seems to have a lot to do with the very bright
>core. I wonder how many times I have called planetary nebulae with
>bright central stars rings due to the same illusion.
>
>As I have already heard a couple times, "don't believe everything you see."
>
>Tom
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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)
www.hposoft.com
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