Sky & Telescope - July 2020 - 36

COMET PHOTOGRAPHY

by Gerald Rhemann

omets are perhaps the oddest
denizens of the solar system. Most
appear with little warning, put on
a modest show, then race back to the Oort
Cloud, not to be seen again for many, many
years (see page 30). Part of their allure is that
each is unique and doesn't always follow
predictions about its brightness and activity.
Some, like 17P/Holmes, defy conventional
wisdom and unexpectedly attain naked-eye
visibility even when located relatively far
from the Sun. Others fail to live up to their
advance billing, with some inevitably disintegrating as they near our star.
All this uncertainty is part of the challenge that has kept
me actively photographing comets for more than 30 years.

Dirty Iceballs
These dusty chunks of ice and gravel are leftover material from
the formation of our solar system. When a comet passes close
to the Sun, the comet's surface can warm enough to sublimate
its frozen gases, producing an expansive atmosphere known as
the coma. Brighter comets typically develop a tail or sometimes
two. One tail, comprising gases that stream away from the
direction of the Sun, is called the ion tail. Such tails usually
appear bluish and can display fast-moving knots that change
appearance over short timespans.

CATCH

t WHIZZING BY Recording many short exposures
and stacking the results registered on the comet's
nucleus produces a sharp image of PanSTARRS
(C/2015 ER61) against a field of trailed stars. In this
case, the author recorded exposures through individual LRGB filters, resulting in colored star trails.

The other tail commonly seen in comets
is made of small dust grains and gravelsized particles that trail behind the comet
along its orbital path and are slowly pushed
away from the Sun by solar radiation. These
dust tails are generally colorless and often
appear featureless, though some contain
visible striations.
On rare occasions a comet may develop an antitail that
appears as a spike projecting sunward from its coma. This
feature is an optical illusion resulting from our Earth-based
perspective. The antitail consists of dust grains trailing
behind the comet along its orbital path.

Different Comets, Different Approaches
Photographing most comets is more challenging than either
deep-sky or planetary imaging. Although comets are solar
system objects, they can't be imaged the same way as, say,
Jupiter or Mars. A comet's nucleus as seen from Earth is
never large enough to be resolved as anything more than
a point source. Indeed, comets rarely (if ever) display sub-

Photographing these icy visitors
often requires a special approach.

a Moving Comet

PASSING BY Comets are fast-moving targets that
often present unique photographic opportunities.
This image of 21P/Giacobini-Zinner as it passed the
Rosette Nebula was assembled using the technique
described in the article.

J U LY 2 0 2 0 * S K Y & T E L E S C O P E

Sky & Telescope - July 2020

Table of Contents for the Digital Edition of Sky & Telescope - July 2020