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Saturday, January 8, 2005

Comet Machholz Passes the Pleiades


The weather was murky with occasional drizzle most of today, so I wasn't optimistic about spotting the passage of Comet Machholz (C/2004 Q2) past the Pleiades tonight, but a couple of hours after sunset the falling temperature forced the ground fog to a lower altitude and the sky was exquisitely transparent. The comet was an easy object for the unaided eye, about two degrees to the southwest of the Pleiades, and with 15 power 50 mm binoculars the inner coma and nucleus were visible and the overall green colour of the coma obvious. (The colour is due to cyanogen (CN) and diatomic carbon (C2) in the coma, both of which fluoresce in the green when illuminated by sunlight.)

After the rather disappointing results of my first attempt to photograph this comet on the morning of the fifth, I decided that with a comet this dim and no clock drive, aperture was king, so I mounted a thirty-year-old Nikkor 50 mm f/1.4 lens on the Nikon D70 digital SLR and made time exposures at f/1.4 with CCD sensitivity set to the equivalent of ISO 1600. The off-axis performance of this vintage lens at full aperture is execrable, so I was careful to centre the Pleiades and comet in the frame. The image above is cropped from the original and reduced by 50%, which almost conceals the small star trails visible in the full scale image.

Both the dust and ion tails of this comet are extremely subtle and I have neither spotted them visually nor picked up the slightest hint of them in a photograph.

Posted at January 8, 2005 00:18