Sky & Telescope - July 2020 - 37

A LL IM AG ES BY THE AU THOR

arcsecond details in their inner-coma, so you don't need
to shoot them with long focal-length optics as you would a
planet. Instead, comet photographers typically use deep-sky
techniques such as long exposures, stacking multiple images
and processing the results to display the wide range of brightnesses in the coma and tail.
On very rare occasions a truly great comet swings through
the inner solar system, producing a huge tail spanning tens
of degrees or more. You can image these using the most basic
photographic setup of a camera with a wide-angle lens on a
tripod. A single, short exposure of less than 30 seconds can
produce fantastic results. Comets such as these appear every
decade or so, but with very little warning. The last one to
grace our skies was Comet McNaught (C/2006 P1) in 2007,
which was best seen from the Southern Hemisphere.
Most comets are far more modest in appearance and as
such require a unique imaging approach to be photographed
successfully. The problem is that comets move relative to the
background stars. This motion is particularly apparent when
they are nearest to the Sun and Earth. An exposure of around
5 minutes with the camera tracking the stars results in an
image with nice round stars, but a comet stretched into a
blurry streak.
One solution is to track the comet with a telescope
equipped with a guidescope and an autoguider that is locked
onto the comet. Another approach for those with advanced
computerized mounts is to program the mount to track the
comet's apparent motion rather than the stars. Both techniques produce photos with a sharp comet nucleus but a ﬁeld
of trailed stars. While acceptable, some ﬁnd these results aesthetically disappointing. After all, we usually can't see a comet
moving against the stars when we observe it visually.

Freeze Frame
With a bit of planning we can create a photo that "freezes"
the comet's motion relative to the background stars. It
involves taking many short exposures that are just long
enough that the comet's nucleus doesn't trail in a single
shot, and neither do the stars. The results are then combined
in software to produce a deep image of the comet against a
background of pinpoint stars.
First, with your rig set to track the stars, make a series of
test images to determine the longest exposure you can make

p REMOVING STARS The PC software program Straton automatically removes stars from most image files, including FITS files. You
can fine-tune the aggressiveness of the program by adjusting the
Parameters slider at lower right.

p STARLESS COMET Once the stars have been successfully removed from the aligned FIT files, imagers shooting through monochrome cameras equipped with color filters can combine the images
into a starless color composite using their favorite astronomical
image-processing program. The author uses MaxIm DL 6.

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Sky & Telescope - July 2020

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