Sky and Telescope - February 2017 - 65

Astrotracer
The Astrotracer is a unique feature that
has been offered on earlier Pentax camera models, but required the additional
purchase of an O-GPS1 module. The
K-3 II comes with GPS built in. Ricoh's
36-megapixel, full-frame Pentax K-1
also includes GPS and Astrotracer, selling for about twice the price of the K-3
II and targets the serious enthusiast. At
just under $1,000, the APS-format K-3
II is more appealing to beginners.
Astrotracer exploits the camera's
ability to shift the sensor in tiny increments, and is also used to provide the
Pixel Shift function described later.
Combined with the camera's knowledge of where it is on Earth (via the
GPS) and where it is aimed in the sky
(derived from its internal compass and
tilt meter), the camera sensor can, in
theory, follow the stars no matter where
it is aimed - at least for short exposures.
I took pictures on several nights,
shooting in all directions, both toward
the horizon and up to the zenith, using
the 14-mm f/2.8 lens.
The results were mixed. In some
directions - and it was not consistent
- star trailing was minimal, except
often at the frame corners. But in other
directions trailing was obvious, and not

S With the Astrotracer on, the Bulb mode switches to offer an extended range of exposures, in
10-second increments from 10 seconds up to 5 minutes. No external timer is needed, a very nice
feature. This 2-minute exposure at ISO 3200 was taken without any separate tracking mount, just
the K-3 II on a tripod with its Astrotracer mode turned on. Stars exhibit trailing only in the lower left
and upper right corners. Distorted stars at the corners are from lens aberrations in the 14-mm lens
when used wide open at f/2.8.

in the direction of the sky's motion,
but usually perpendicular to the eastwest motion of the stars. The trailing
was never consistent across the frame,
indicating the sensor was not turning
around the correct rotation point.

I found that if exposures were kept
under two minutes the results could
be acceptable, and they were certainly
much less trailed than you'd get with
a fi xed camera. But they never compared well to the pinpoint stars across
the entire frame that's possible using a
polar-aligned tracking mount.
The Astrotracer will be most useful
in nightscape photography for achieving
deeper one- to two-minute exposures of
the Milky Way without the significant
trailing of fi xed-camera shots. But for
those seeking the sharpest images of
rich star fields, the Astrotracer is no
substitute for a tracking mount.

Star-Trail Stacking

S When aimed south with Astrotracer on, the Pentax showed considerable trailing, mostly in a
north-south direction but inconsistently across the frame. This is a single 2-minute exposure, with
the ground blurred due to the camera tracking the sky.

Another favorite type of nightscape
image comes from creating star trails
on purpose. The usual technique in
the digital age is to shoot many short
exposures and then stack them later in
post-processing using software. But that
takes more work at the computer.
By contrast, the Pentax K-3 II can do
it "in camera." It has a special "Interval
Composite: Bright" mode that uses its
built-in intervalometer to record up to

s k y a n d t e l e s c o p e .c o m

* FEBRUARY 2017

65


http://www.skyandtelescope.com

Sky and Telescope - February 2017

Table of Contents for the Digital Edition of Sky and Telescope - February 2017

Contents
Sky and Telescope - February 2017 - Cover1
Sky and Telescope - February 2017 - Cover2
Sky and Telescope - February 2017 - 1
Sky and Telescope - February 2017 - Contents
Sky and Telescope - February 2017 - 3
Sky and Telescope - February 2017 - 4
Sky and Telescope - February 2017 - 5
Sky and Telescope - February 2017 - 6
Sky and Telescope - February 2017 - 7
Sky and Telescope - February 2017 - 8
Sky and Telescope - February 2017 - 9
Sky and Telescope - February 2017 - 10
Sky and Telescope - February 2017 - 11
Sky and Telescope - February 2017 - 12
Sky and Telescope - February 2017 - 13
Sky and Telescope - February 2017 - 14
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Sky and Telescope - February 2017 - 17
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