Sky and Telescope - September 2015 - 69

Comet

Freeze
Here's a great technique for stacking your comet photos in ImagesPlus.
Comets are rare visitors to our night
sky. Often arriving with little warning, they put on a brief show and then
slowly fade as they recede from the Sun.
The brighter ones grab the public's
Tim Jensen attention, while amateurs grab their
cameras. A bright comet is easy to shoot
- simply point your camera or telescope at it, open the
shutter for about 5 minutes, and you're pretty much done.
Imaging the fainter ones can present an interesting challenge; they require much longer exposures to reveal any
streaming ion or dust tails, which leads to more complex
issues when assembling your image.
Perhaps the most appealing images of comets portray
them as a brilliant greenish coma with a long, streaming
tail that trails away against a field of sharp, round stars.
But since comets move noticeably with respect to the stars
in only a few minutes, long exposures tracked on the
background stars are limited to only a minute or so before
the comet begins to shift against the star field. The resulting photo often shows a nice star field with a blurry streak
that was the comet as it moved during the exposure.
One fi x to this issue is to guide on the comet itself
with a separate guidescope. This produces a deep image
of the comet surrounded by long streaks of trailed stars.
So what can you do to get an image that has the best of
both worlds: a deep, detailed comet with gossamer ion
streamers against a perfectly tracked star field? My solution to this dilemma is to do both, and then combine the
result using post-processing software. Here's how I do it
with ImagesPlus (mlunsold.com).

Double Alignment
Part of the difficulty of freezing a typical comet's motion
in deep photos is the fact that you need to take exposures
tens of minutes long to bring out any ion or dust tails

LOVEJOY'S JETS Deep images of comets are surprisingly challenging to achieve. Because they move against
the background stars, telescopic exposures are limited to
just a minute or two before producing a trailed image. In
the photo on page 68, author Tim Jensen has "frozen" the
motion of Comet Lovejoy (C/2014 Q2) against background
stars using images taken in mid-January with an Orion
EON 110 refractor and Canon EOS T2i DSLR.
Above: Stacking many short exposures aligned on the
moving comet produces a crisp, detailed view of the target. But some of its fainter details, including a wide dust
tail, are hidden by long star trails. All images are courtesy
of the author.

Sk yandTelescope.com September 2015

69



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Sky and Telescope - September 2015

Table of Contents for the Digital Edition of Sky and Telescope - September 2015

Contents
Sky and Telescope - September 2015 - Cover1
Sky and Telescope - September 2015 - Cover2
Sky and Telescope - September 2015 - 1
Sky and Telescope - September 2015 - Contents
Sky and Telescope - September 2015 - 3
Sky and Telescope - September 2015 - A
Sky and Telescope - September 2015 - B
Sky and Telescope - September 2015 - 4
Sky and Telescope - September 2015 - 5
Sky and Telescope - September 2015 - 6
Sky and Telescope - September 2015 - 7
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Sky and Telescope - September 2015 - Cover3
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