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Pentax Astrotracer Review: Astrophotography Made Easier
by Jim Keenan -  12/30/2013

Over the past several years, I've reviewed a number of Pentax DSLRs for this website, including their K-30, K-50, K-5II and, most recently, the K-3. These Pentax DSLRs have impressed as being feature-rich cameras at attractive price points, partially because most of them offer weather sealing at MSRPs hundreds of dollars less than competitive brands. But while doing the K-5II review it was another feature that caught my eye - when equipped with an optional Pentax O-GPS1 unit, the K-5II offered a capability known as Astrotracer. When enabled, Astrotracer utilizes the Pentax in-body camera stabilization system to move the camera's sensor during lengthy exposures so as to mimic the earth's rotation and capture celestial objects such as stars and planets as points of light rather than elongated trails. As Mr. Spock might say, "fascinating".

Build and Design
The O-GPS1 unit measures approximately 1.875 x 1.75 x 1.25 inches and utilizes a single AAA battery as a power source - weight is approximately 2.1 ounces. Pentax recommends removing the battery from the GPS if it is not going to be in use for a long period of time; also, battery changes should be undertaken with the GPS not mounted on the camera. The unit attaches to the camera via the hot shoe, and this location combined with unit's light weight have no significant effect on the camera's handling when shooting handheld. The obvious downside to the hot shoe attachment point is that the GPS and a flash cannot be used simultaneously. It is important to note the camera must not be picked up by the GPS unit alone as the unit and/or camera hot shoe are likely to be damaged.

In addition to Astrotracer, the K-3 also offers an electronic compass display that includes latitude, longitude and elevation along with date and time; a circular compass rose display including a continuous digital readout of the exact azimuth (horizontal direction or bearing) the camera is pointed at is also provided. Azimuth readings are based on true north, not magnetic north. Here's a look at the electronic compass display.

Overall construction is of composite materials and the unit was assembled in the Philippines. Pentax lists an MSRP of $249.95 for the unit, which can also be had at reputable New York internet vendors for $199 as this is being written.

Setup And Shooting Performance
The GPS unit is simple to use for the electronic compass feature. With both camera and GPS unit powered up, the GPS will automatically acquire satellites, displaying a steady blue light adjacent to the satellite icon on the unit when three or more satellites are detected - three satellite acquisition is the minimum required for GPS data to be recorded to captured images.

The monitor status screen also displays a satellite icon whose color indicates the general number of satellites acquired: a yellow "2D" icon indicates three satellites; a green "3D" icon is four or more and, not unexpectedly, a red icon signifies GPS data is unavailable. You can see the icon in the top right of the electronic compass shot above.

Acquisition time on the GPS can vary depending on the amount of open sky overhead--buildings and terrain can block signals from satellites--but was generally a minute or less.

Utilizing the Astrotracer feature is a bit more complex, but not onerously so. With the camera and GPS on, "precise calibration" must be performed. Precise calibration involves rotating the camera in excess of 180 degrees along three separate axes once satellite acquisition has been obtained. Sounds more complicated than it is--airplane pilots will quickly recognize the axes as corresponding to pitch, roll and yaw. For all you non-aviators out there, consider this your first, very basic bit of ground school. The rotations don't need to be in any particular order, but the sequence must be completed within 1 minute after initiating the process.

"Pitch" involves rotating the camera and lens above and below the horizon (say, from pointed at your feet up and past directly overhead to a point behind you); "yaw" rotates the camera from right to left (or vice versa) along the horizon and "roll" rotates the camera in place from upright and horizontal to past fully inverted and horizontal, like the motion of turning a doorknob or unscrewing the lid of a jar. How precise is "precise"?  Very precise, it seems--when I first tried precise calibration in my driveway, the camera consistently would not complete the calibration satisfactorily. This location featured the garage about 15 feet behind me, and a large, bushy tree about 15 feet to my left. Moving about 50 feet to the front lawn, away from tree and garage and into more open sky all around me, precise calibration was acquired on the first try. The good news here is precise calibration should be easily accomplished in the wide vistas that typically are part and parcel of night sky work; if you're trying to shoot a band of sky from the bottom of a slot canyon, you may have trouble. If you forget the nature of the rotations, the K-3 gives you a diagram at the onset of the calibration sequence as a reminder.

For Astrotracer use, precise calibration is required at the exact location you'll be using the feature before each use. Switching locations or shutting off the camera/GPS at the present location and powering up again later requires a new precise calibration. As a practical matter, I hand held the camera while performing the precise calibration sequence, then mounted the camera onto the ballhead/tripod at the same spot. As with all night sky photography, have your camera lens focused to infinity, tape the focus ring so that it can't be rotated by accident during shooting and set the camera and lens for manual focus before initiating captures.

During Astrotracer operation, the camera must be in bulb (B) mode, and the Astrotracer menu page allows you to set an exposure time from 10 seconds to 5 minutes--a "maximum trace duration" readout tells you the actual length of time Astrotracer will be available at any location. The following 4 shots show the menu sequence that takes you into the Astrotracer feature to the point where you set the exposure time.

We've got the camera set to shoot with Astrotracer, let's go find some clear night skies and see what happens.

Image Quality
In the article I wrote on Night Sky Photography for this website, I mentioned that trial and error had led me to limit my shutter length to 20 seconds when shooting at 24mm in order not to produce star trails. Since then, I've received my winter Really Right Stuff catalog which included an article on night sky work that included reference to a "500" rule which I was unfamiliar with. (Really Right Stuff is a California company specializing in a variety of camera support equipment. I'm not affiliated with them but use some of their equipment including tripods, ball heads, panorama shooting rails and large telephoto lens support brackets). The rule postulates that to estimate the length of an exposure you can use at any particular lens focal length for unguided night sky work without producing star trails that you divide your effective focal length into 500, with the result being the answer in seconds. The results using the rule correspond to my observations in the field, so I used the 500 rule as the basis for the unguided night sky shot that follows.

When I reviewed the K-3 it mounted the 18-135mm kit lens, but night sky work was shot using the Pentax HD 20-40mm f2.8-4 DA Limited lens.

With the K-3's APS-C sensor, the 20-40 shot at 30mm at the wide end in 35mm equivalents and application of the 500 rule gave a 16.66 second maximium exposure time, which I rounded down to 15 seconds. Ideally, you'd set your exposure time for the Astrotracer, lock up the K-3 mirror and fire the shutter by wireless remote to minimize vibrations. Lacking a remote, I fired the shutter via the camera's self-timer, using the 12 second delay. The baseline shot was made at 800 ISO, f/4 for 15 seconds.

Now the fun began--with precise calibration accomplished and ISO remaining at 800, I set the Astrotracer for 3 minutes, well over the threshold time limit for no star trails. All the night shots (including the baseline unguided shot) were post processed using Photo Ninja software. A 3 minute exposure should have resulted in an image with pronounced star trails, but the Astrotracer did an excellent job of maintaining the stars as points of light.

Next, I pointed the K-3 more toward the eastern horizon to capture the constellation Orion (the hunter), along with some ridgeline, airplane trails and light from desert communities in southeast San Diego county. This shot demonstrates one characteristic of the Astrotracer that has to be considered if you opt to place terrestrial subjects in the frame--the sensor is being moved by the camera to follow the apparent motion of the sky, which results in terrestrial subjects being blurred by the sensor movement as they do not shift position relative to the camera.

I set the ISO to 400 and shot a 2 minute exposure looking north towards the constellation Cassiopeia, primarily as one of its neighbors is the Andromeda galaxy, M31. In the shot that follows about 1/4 way down the frame and just a bit left of the centerline is an elongated fuzzy blob with some haze extending to the upper left and lower right away from the central blob--that's Andromeda, some 2.5 million light years away. Andromeda is the nearest major galaxy to our own Milky Way and is visible under good conditions with naked eye or binoculars.

Moving back to Orion, I zoomed the lens in to 40mm (60mm in 35mm equivalents) and set a 90 second exposure time at 400 ISO. In this shot Orion takes up much of the frame; the three fairly bright stars arranged vertically are Orion's belt--his sword extends down at about a 45 degree angle from a bit right of the lowest star in the belt, and includes another fuzzy blob, the Orion nebula. The red giant star Betelgeuse forms one corner of his torso to the left of the belt. Orion is about 1344 light years away. The significance of this shot is that increasing the focal length of the lens creates a narrower field of view, which causes the apparent motion of the stars across the frame to increase for any given period of time. Guiding errors are more apparent with narrower fields of view, but the Astrotracer did a very good job with the longer lens.

Finally, I zoomed back out to 20mm on Orion, set the ISO to 200 and captured a 3 minute shot that includes the planet Jupiter, the bright object near the left edge in the bottom third of the frame. The ability to use lower ISO numbers with long exposure times allows you to capture fainter objects without the high ISO noise penalty exacted by having to ramp up ISO to produce acceptable exposures during short, unguided exposure times.


Conclusion

The Astrotracer feature on select Pentax DSLRs is a wonderful addition to the toolbox of Pentax shooters wishing to go beyond basic, unguided and short exposure night time photography. With the addition of a relatively modest-priced Pentax GPS, Astrotracer offers up to 5 minutes of celestial tracking utilizing the camera's built in shake reduction system. Since the shake reduction is built into the camera, Astrotracker is available with longer focal length lenses that can provide more detail on distant objects such as the Orion nebula or Andromeda galaxy. Besides the ability to lower ISO sensitivity, another bonus of longer guided night exposures is the ever-present possibility that a bright meteor may choose to fly through the frame during your exposure. If you look closely at the 3 minute/800 ISO shot at 100% magnification, across the top about 1/5th of the frame is a long, faint satellite trail. The Astrotracer's strong suit is celestial photography with just sky in the frame--terrestrial objects will tend to blur due to the tracking motion of the camera sensor.

There are aftermarket manufacturers that make guiding equipment that can accommodate your DSLR if you want to make long, guided exposures, but the price floor for this equipment starts around $500 and goes up from there. One advantage of this equipment is that your guiding time is not limited by sensor shift parameters; the entire lens and camera are moved in coordination with the apparent motion of the stars. You can also spend some serious money and get a dedicated astronomical telescope and piggyback your DSLR onto it for long exposures. But a $200 Pentax GPS is sounding like a real bargain to get a modest guiding capability--the Astrotracer may be limited to a 5 minute exposure, but what it does it does very well.

In fact, I've been so impressed with the performance of the Astrotracer and the photo potential of longer, guided exposure times that I'm one of those folks who's now in the market for some of that high-priced aftermarket guiding gear...

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