At Little Mtn, Utah, outside Salt Lake City, differential photometry - not color index corrected - was repeated as an experiment on Comet 17P on 2007-11-01 6:15 UT. (See prior obs rep 2007-10-27.) Using a wide-field camera consisting of a Meade DSI adapted to a fish-eye lens, a wide field (~14 deg) image of the region of Perseus around Comet 17P was taken. The image was dark flatted but not white flatted. Using lam Per as a reference star and AIP4WIN's single star photometry tool, a zero-point was adopted that yielded the Tycho V catalogue magnitude for
lam Per.
Differential photometery was then tried on Comet 17P resulting in an estimate of 2.56+-2%. The same method was used on 2007-10-27 9:15UT, at a higher altitude near the zenith of 2.59v+-2%.
http://gallery.utahastronomy.com/d/10555-3/20071101_17P_Photometry.PNG
The rough photometry image was taken in a mag 5.0 sky, just after a 3rd quarter moonrise on the eastern horizon at 6:00UT.
Before moonrise, 17P had a faint extended object appearance to the naked eye. In 10x50 binoculars, a faint outer halo could be detected outside the diffuse coma. The comet continues to present an end-on annular appearance without a tail.
17P appeared dimmer than del Per to the naked-eye and after moonrise, Comet 17P had a stellar appearance to the naked-eye. But these were contrast illusions.
In 10x50 binoculars, the circumference of the diffuse coma of 17P had the same angular size as the arc seen in the Pleaides defined by 17-16-19-20 Tau. Based on this comparision, the diffuse coma of 17P had a size of approximately 20 arcmins. On 2007-10-27, it was less than one-half this size. The continuing stellar appearance of Comet 17P appears related to its bright central nucleus - which has more contrast to the naked-eye as compared diffuse coma.
While the total brightness of 17P appears to have remained the same between 10-27 and 11-1, it appears dimmer to the naked eye due to the rapid increase in its true and apparent sizes.
- Kurt
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