Sky and Telescope - June 2016 - 48

OBSERVING

Celestial Calendar

Hunt the Moons of Mars
With Mars now big and close, try for Phobos and Deimos.
The closest planetary moons to Earth, after our own
familiar Luna, are Phobos and Deimos orbiting Mars.
Most amateurs have never seen them. In late May and
early June, with Mars in its close approach to Earth, you
have your best shot at them in 11 years.
The problem with seeing them in a telescope isn't
that they're too dim. You might think they'd be fairly
easy, since they shine at magnitudes 11.4 and 12.5,
respectively, in the week or two around Mars's May 30th
closest approach. (Mars and its moons are a half magnitude fainter at the beginning of May and by end of June.)
The problem is that they orbit close to Mars amid its
dazzling glare. Mars blazes 230,000 and 600,000 times
brighter than Phobos and Deimos, respectively.
I've been able to see Deimos quite distinctly with
my 12.5-inch reﬂector at good oppositions in the past.
But I've never deﬁnitely succeeded with Phobos, which
never gets more than about a third as far from Mars as
Deimos does.
You'll need to use some tricks. The ﬁrst is to choose
the right time to look. You'll want to observe when one
of the satellites is at one of its greatest elongations: at its
farthest Mars. The elongations currently happen to the
planet's northwest or southeast, as shown in the orbit
diagram on the top of the facing page. Find a convenient
elongation time at http://is.gd/phobosdeimos2016, where
timetables are reproduced from the 2016 Astronomical
Almanac. Phobos completes a full orbit every 7.6 hours,
Deimos every 30.2 hours.

Or, in a good sky-mapping program, you can lock on
Mars, zoom way in on it, and then run the time forward
and backward to see when Phobos and Deimos reach
their elongations.
Next: make sure your eyepieces are clean as can be.
Dust on your main mirror matters less because it's
farther from your eye (meaning scattered light from the
dust has more places to go that aren't into your pupil).
But if you've been waiting for an excuse to clean your
mirrors (gently!), this might be it.
In your highest-power eyepiece, add a temporary
occulting bar across the center of the ﬁeld so you can hide
Mars behind it. The usual method is to unscrew the
barrel, turn the eyepiece upside down, and tape a little
strip of aluminum foil across the center of the round
metal ﬁeld stop, the open ring in the eyepiece's focal
plane. Tweezers help. Then, looking in the eye end of the
eyepiece, carefully use a pencil point to maneuver the
foil strip into sharp focus.
The atmospheric seeing and the telescope's focus
must be excellent if you want to see a faint pinpoint on a
bright background. Collimate your mirror so the image
will be as sharp as possible. And it may be best to use a
faint star to set the ﬁne focus, not the edge of Mars.
Unfortunately for northerners, Mars is fairly far south
during this apparition (at declination -21°° or -22° in
May and June). So plan to observe when Mars is highest:
around the middle of the night. And keep coming back
night after night to increase your chance of catching a

Phobos

Deimos

June 2016 sky & telescope

James McGaha in
Arizona took these
images of Mars with
tiny Deimos and
Phobos on August 29,
2003, using a webcam
on a 1-meter reﬂector
atop Kitt Peak. To pick
up the faint moonlets,
McGaha had to vastly
overexpose Mars
(left), so he also made
a composite using
a short exposure of
Mars (right) to show
all three objects well.

http://www.is.gd/phobosdeimos2016

Sky and Telescope - June 2016

Table of Contents for the Digital Edition of Sky and Telescope - June 2016