Capturing a comet

Posted: January 7, 2013 in Comets, Imaging

I’ve been out of touch with astronomy for various reasons in the past few weeks.  But a brief message from a friend alerted me to a comet in the sky.  A quick check on SkyTools revealed the comet to be C/2012 K5 LINEAR – a small object passing relatively close to Earth at the moment.  Although this comet is too faint to be seen without good binoculars or a telescope I thought it would be worth trying to get some images.

The orbit of the comet around the Sun is tilted to more than 90 degrees to the plane of the Earth’s orbit.  This basically means that K5 LINEAR is moving from celestial north to south in the sky.  The following chart shows the rapid motion of the comet during the next week or so.


K5 LINEAR is tracking south through a busy region between the constellations Orion and Taurus.  I set up the 80mm Skywatcher refractor on the HEQ5 Pro mount without much hope of getting a great picture.  It was overcast and raining at one point so me and Malcolm had to throw a cover over the whole setup.  But the cloud was quick moving and eventually I got a clear spell lasting more than half an hour.


This image is a composite of 10 one-minute subexposures taken with the Nikon D80 at prime focus of the Skywatcher telescope.  The processing was carried out in DeepSky Stacker and GIMP.

The green colour of the comet is from jets spewing cyanogen (CN) and diatomic carbon (C2) into space.  Both have a distinctive green glow under the action of sunlight in space.

The sub-exposures actually show the rapid motion of the comet across the field of view.  Here is an animation showing the comet as it tracks across the sky at a rate of 10 arcseconds per minute.

K5 was about 0.3 astronomical units away (about 28 million miles) when the pictures were taken.  That rapid motion corresponds to an orbital speed of about 80,000 miles per hour!  (Since the comet is passing through the plane of our orbit, the radial component is almost zero).

Comet K5 LINEAR is already fading as it heads south and away from the Sun.  But 2013 looks like it will be a spectacular year for other comets!  In late February we’ll have a naked eye comet called PANSTARRS in our sky and towards the end of the year – a more dramatic visitor called ISON might give the moon a good run for it’s money.


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