Sky and Telescope - September 2018 - 38

Better Alignment

If the title here conjures thoughts of snow,
ice, bitter cold, and dog sleds, don't worry,
there are no dog sleds involved. The prize
sought in this adventure is the north celestial, not terrestrial, pole.
Like several generations of amateur
astronomers, I've turned to the diagram in
Burnham's Celestial Handbook when I've
wanted the accurate location of the north
celestial pole among the stars. Robert
Burnham's chart, reproduced at right,
details more than 300 years of the pole's
precessional drift through Ursa Minor (the
Little Dipper). The chart's limiting magnitude and image scale have always been
sufficient for my needs, which typically
involved my efforts to accurately align
equatorial telescope mounts.
Several years ago I began testing computerized mounts and software that offered
precision alignment routines. And that
got me wondering about the accuracy of
Burnham's hand-drafted chart done half a
century ago. With a bit of effort I convinced
myself that the chart is, indeed, very accurate, but the story didn't end there. In
the process I stumbled upon something
unexpected that bears directly on how
accurately most amateur equatorial mounts
can be aligned. Here's how it happened.
One good way to determine the exact

p In order to assess the accuracy of Burnham's
polar chart, Dennis di Cicco made this 12-hour
star-trail image of the north celestial pole in
December 2011 shooting through an 85-mm
f/7 refractor. While it did verify the accuracy of
Burnham's chart, it also revealed an unexpected aspect of most telescope mounts, which is
explained in the accompanying sidebar.

38

S E P TE M B E R 2 018 * SK Y & TELESCOPE

location of the celestial pole is
to use a fixed camera to make
a time exposure of the sky and
note the point that the stars
appear to revolve around. Digital
photography has made it easier
than ever to create one of these
classic images, but the devil is in
the details. A standard camera
lens lacks the resolution and
magnitude reach to pinpoint the
celestial pole as accurately as it's
shown on Burnham's chart. The
best solution is to use a telescope in place of a conventional
camera lens.
During a moonless spell in
December 2011 I did just that
when the long, cold winter
nights offered more than 12
hours of astronomical darkness. I set up a DSLR camera
p Drafted by Robert Burnham, Jr., for his classic Celestial
attached to an 85-mm f/7 reHandbook, this chart of the north celestial pole's precesfractor on my elevated wooden
sional drift among the stars of the Little Dipper has served
amateur astronomy for nearly half a century.
deck. The camera body was
attached to a sturdy tripod, and
steel pier had shifted everything, including
the front of the telescope rested on the
the mount's polar alignment, by nearly an
deck's railing for additional support. The
arcminute during the exposures. And while
camera made 30-second exposures every
situations vary, this likely happens to some
5 minutes throughout the night. But when
degree with all telescope mounts.
the images were composited together
In practice such a slow, small shift of poin the morning, the stars didn't appear
lar
alignment during the night is of no conto move in anything resembling smooth
cern.
But it does raise the question of how
circles. It turns out my deck was gyrating
accurately
a mount can be polar aligned.
like a hula dancer during the night.
Adjusting
the
position of a computerized
So I repeated the process with the
mount
and
having
software report that your
telescope solidly attached to a German
alignment
is
within
a few arcseconds of the
equatorial mount in my observatory. It
celestial
pole
may
be
very satisfying, but
seemed like overkill since the mount was
it's
likely
true
only
for
the
current condionly acting as a fixed support for the scope
tions.
Metal
expands
and
contracts as the
and camera (the drive was off). This time
temperature
changes,
and
any supporting
the composite image created smooth star
material
involving
wood
can
also vary with
"trails" that allowed me to pinpoint the
the
humidity.
These
small
changes
aren't
pole's location well enough to verify the
big
problems,
but
they
do
serve
as
remindaccuracy of Burnham's chart. But here's the
ers
for
those
of
us
trying
to
tweak
a
polar
rub - on close inspection the trails were
alignment
to
within
a
few
arcseconds
of the
not exactly circular. Once again the camera
celestial
pole.
At
best,
achieving
a
"perfect"
and scope had moved during the night, and
alignment is a fleeting success story.
the reason why was only obvious in hindsight. Thermal contraction of the mount and Dennis di Cicco

CHART: BUR NHAM'S CELESTIAL HANDBOOK

Quest for the
North Pole



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