The Astrotracer O-GPS2 on the Pentax K-3 MarkIII - Recommended settings and photography tipsProduct
What is an Astrotracer and what is it used for?
Astrophotography is a special field of photography. As with other areas of photography, here exposure time in combination with movement also plays a special role. Once consideration with astrophotography is that from a certain shooting distance, we have longer shutter speeds on the one hand and the rotation of the earth on the other. It is hardly visible to us, but from an exposure time of about 30 seconds, celestial bodies are no longer depicted as points, but as lines.
There are limits to increasing the sensor sensitivity because of the sensor noise, so that other tools are needed. Those who delve deeper into the subject will certainly have seen the motors ("startrackers") that can be mounted on tripods to track the camera in order to compensate for the movement caused by the earth's rotation.
RICOH IMAGING is offering a very special solution with the Astrotracer O-GPS2 (as successor of the O-GPS1) for many cameras in the PENTAX series. Using the moving sensor in the camera, which is normally used to reduce camera shake, the Astrotracer detects the earth's rotation by evaluating GPS data and adjusts the sensor accordingly.
This small and compact device, which is mounted on the camera's accessory shoe, allows exposure times of up to 5 minutes to be synchronised with the Earth's rotation, and professional astro-photos to be taken with a simple setup.
The expert author and renowned astrophotographer Peter Mein, describes in this tutorial how to get started in astrophotography and how to use the Astrotracer. Even if he describe the use with regards to the K-3 Mark III, the function with other compatible cameras is similar to this.
1. What do I need for astrophotography?
Only a few things are necessary to enter the field and already achieve remarkable results: a camera with a reasonably fast lens (f 2.8 or better), an astrotracer and a stable trtipod. A remote trigger (infrared or cable, possibly programmable) is also recommended.
For processing the image afterwards, you need a powerful Windows PC. Even if no special software is required for the beginning, for advanced processing freeware programmes are necessary, which are only available for Windows.
2. What can the Astrotracer do?
The Astrotracer O-GPS2, as the successor of the O-GPS1, is compatible with many PENTAX camera models:
KF, K-3 Mark III, KP, K-3, K-5II, K-5IIs, K-5, K-S2, K-S1, K-70, K-50, K-30, K-r, K-01, 645Z, 645D (from May 2022).
*Not all camera models support the full range of features of the O-GPS2.
**K- 1, K-1 Mark II and K-3 II cameras have this feature built-in.
For technical specifications and information about the Astrotracer O-GPS 2, please visit the Pentax website...
Here you will also find an overview of some camera models with the maximum exposure times, depending on focal length and at which height of the sky the camera is aimed. This table shows that the camera sensor can only be moved within certain limits. When it has reached its maximum deflection, the camera stops the exposure.
As you can see there, exposures of up to approx. 5 minutes are possible with the Astrotracer (with a small focal length of less than 50mm). With longer focal lengths, the exposure times decrease accordingly. However, these times are more than sufficient for astrophotography today.
Normally, you do not need more than 30 to 60 seconds per exposure.
3. Areas of application and limitations of the Astrotracer
As we mentioned at the beginning, the PENTAX O-GPS2 is something like a dream solution for astrophotographers. It makes it possible to take pictures in an undreamt-of quality in the simplest way, which otherwise is only possible with complex systems.
However, it should not go unmentioned that this small, portable and very inexpensive device also has its limitations that have to be taken into account in photography:
- A standard (but very stable) tripod is sufficient for the Astrotracer. But: Without an astronomical mount (or even an electronically controlled "GoTo) you have to find the objects to be photographed with the camera viewfinder alone. So you should know your way around the night sky a little, and if necessary you can use a star chart and binoculars.
- As described, the camera stands on a standard tripod and is not moved as it’s the sensor that tracks automatically. But after the picture is taken, it returns to its original position. In many cases this is not a problem, unless you take series of shots to "stack" them later, then the object slowly but surely moves out of the picture. So after a few pictures, the camera has to be readjusted (by hand) to keep the same image section.
- Another limitation results from the accuracy of the GPS module is the combination with very long focal lengths and long exposure times. While the position is determined to within 10m, the accuracy of the compass is given as +/- 5°. In addition, the Astrotracer tracks the sensor in "steps" (step length is approx. 1,5 sec.) and not continuously. This means that if you use the Astrotracer to its full potential at long focal lengths (i.e. at long exposure times), you will still get streaky stars. Here, the "tracking" by the sensor is simply not accurate enough.
In my experience, the limit up to which the Astrotracer delivers usable images is between 300 and 500mm focal length. At 500mm focal length, it is worth taking a few test shots to see whether the tracking is stable or whether the stars are already forming lines. If this is the case, switch the GPS off and on again and recalibrate the camera. If this does not help, set the exposure time to a maximum of 20 seconds. So a 500mm focal length is quite possible, but it may take a bit of trial and error here.
- It’s important to note that long focal lengths, which are needed for images of the moon, the planets or even smaller "deep sky" objects, are not intended for Astrotracer. The maximum exposure times (resulting from the maximum sensor deflection) are simply too short in this case. In addition, the accuracy of the tracking is simply not good enough for Jupiter or Saturn at 1500mm focal length. For the planets, a motorised telescope should be used. And the moon is usually so bright that you can work with exposure times that do not require tracking.
So the moon usually works without Astrotracer.
All in all, it is the individual situation, which depends on the camera model and the location, in which the Astrotracer must be tried out individually.
It works well within its limits, and it can certainly keep up with commercially available camera tracking systems. And if you want to tackle more demanding tasks later on, it is best to buy a small telescope.
4. What can I photograph? (In northern Hemisphere)
Easy to start:
- The big dipper
- The Constellation Cassiopeia (known as the “W” in the sky)
- Constellation Orion in the winter sky
- Constellation Cygnus (also known as the “Northern Cross”)
- The Summer Triangle
- Constellation Leo
Deep Sky Objects:
- The Orion Nebula (also known as Messier42)
- The Andromeda Galaxy (Messier31)
- North America Nebula in the constellation Cygnus
- The Lagoon Nebula (Messier8)
- The Seven Sisters (Pleiades, Messier45)
- Open Cluster Beehive (Messier44)
The Milky Way
- The Summer Milky Way with the constellations Cygnus and Aquila
- The Winter Milky Way near Orion
- Open clusters and nebula near the Centre of the Milky Way, e.g. Lagoon Nebula and Trifid Nebula
- Meteor Showers and Shooting Stars
- The International Space Station (ISS)
5. recommended camera settings
Although all the previous sections are important for the overall result, this is the most important section of this tutorial. It shows the most important settings that differ from the Pentax default settings or are difficult to find in the menu.
- First, the shooting format has to be selected.
Here, as in other contexts, the question "JPEG or RAW?" comes up. In principle, both are possible. JPEG is OK for a start, but for more ambitious photos you should switch to RAW mode.
Our general tip, no matter whether astrophotography or another subject, is that if both formats are captured, then both will be available to choose from after shooting.
The JPEG image data is a compressed version of the RAW and so will depend on the settings at the time. As a result this limits the amount of editing that is possible afterwards. And the RAW file holds the original information captured by the sensor, there is a lot more freedom to edit various parameters before converting to the final output, which again would be a JPEG image.
If the RAW format is used, the general usable DNG format should be selected, as the astro image processing programmes usually cannot process manufacturer-specific formats.
- Before capturing the image, the mode dial must be set to "B" and the display will show "Bulb" once set. Astrotracer only works in this mode, and some functions in the settings menus are also linked to it.
- Next, Astrotracer type 1 should be selected in the camera menu under item "Camera4 - Astrotracer" so that the stars are tracked correctly.
This menu is also available via the settings, which can be accessed via the "Info" button.
*Type 2 is a mixed form with reduced tracking speed for concentration on the foreground of the image, and type 3 is an option for single images without a GPS module).
- In the basic camera settings, the options for noise reduction must now be set. To do this, press the "Info" key and call up the basic settings menu and select the menu "RR for long VZ" with the four-way controller and press "OK". In the submenu that now appears, switch from "NR On" or "NR Auto" to "NR Off" and confirm.
If this is not preset, the camera creates a "dark image" after each long exposure with the same exposure time as the actual shot in order to "subtract" it from the shot for noise reduction. The side effect is then that after each exposure, e.g. of one minute, you have to wait another minute until the camera is ready for use again.
If you shoot with normal ISO values (up to ISO 1600 with the K-3 Mark III), this "dark image" can be turned off for single shots. If you take continuous shots, you can't use this function anyway. However, if you take a series of shots in post-processing ("stacking"), you can add these so-called "darks" separately.
- In the next step we make an adjustment for the function of the remote release in the menu under "C5 B-mode options".
Depending on the remote trigger used, this needs to be adjusted. For wired remote releases, e.g. programmable, also for serial and continuous shooting, "Bulb" must be set here. For an infrared remote triggers, please set this to "Time". In this case, the long exposure is started with the first press of the shutter release button and ended with the second press.
The activation for the remote control is integrated to the four-key control. Press the "bracketing/time preselection" button and then activate the IR remote shutter release.
- The next step is to deactivate the autofocus on the camera and the lens. This is particularly important because the camera cannot find any contrast for focusing in the dark sky at night, and understandably the AF auxiliary light of the camera is of no use at this distance. And since the camera will not release without the "ready" message from the autofocus, please switch the switch on the left side of the camera from "AF" to "MF", and if the lens also has a switch, deactivate the AF here as well.
- And finally, the GPS system must be calibrated (Actually it’s only the acceleration sensors for determining the position in the GPS module has to be calibrated).
To do this, first switch on the GPS module (if you have not already done so). For astrophotography you need the "exact calibration". This can be found in the menu item "Camera 4 > Astrotracer".
Please select the item "exact calibration" and perform the "Pentax dance" until the message "data processing completed" appears.
This procedure is well explained in a small video:
After the procedure has been completed by pressing OK, the camera menu can be closed. The camera is now optimally prepared for astrophotography.
A small note about the “Green”-button on the back of the camera. By pressing it, you can decide whether the camera should be used in a real "B-mode" (the shutter remains open as long as the remote release button is pressed) or whether an exposure time should be set for the shot. In the second case, continuous shooting could also be programmed. However, you should know that once started, these cannot be interrupted during the series of shots until the series of pictures has been completely taken. It is not possible to check the image after each exposure.
6. Focusing correctly
Manual focusing on the starry sky is unfortunately not as easy as one might think. And unfortunately, accurate focusing is a must for good photos.
After mounting the camera on a stable tripod, I recommend turning off the monitor on the back of the camera. To do this, press the info button twice and select "Display off".
Now aim at a bright star (any star) through the viewfinder. When the star is well centered in the viewfinder, the "Live View" is switched on again. The star should be clearly visible there. Now switch the Live View to magnification mode by pressing the OK button and set the magnification to 8x by turning the adjusting wheel.If the star slips out of the monitor image, you can change the monitor image accordingly with the arrow keys.
Now turn the focus until the point of light has the smallest possible size (usually other, not so bright stars appear on the display). Switch off the Live View again and make sure not to adjust the focus ring. Now switch off the camera display on the back, otherwise it will dazzle.
Some cameras, such as the K-3 Mark III, are equipped with the "night view function". In this case, the basic colour of the monitor changes to red and very easy on the eyes.
7. Take pictures!
Now we can finally get started! Only two more quick important settings are necessary:
Firstly, an ISO value of 1600 is recommended for the K-3 Mark III (later, higher values can also be tried out in order to approach the "noise limit"). You should also gain your own experience with the aperture. To begin with, it is advisable to open the aperture completely. The extent to which the sharpness of the edges changes when the aperture is stopped down depends on the lens.
Now hands off the camera! And expose for 30 seconds.
A correctly exposed image could then look like this or something similar:
We can find more examples on Peter Mein's website, or in various tutorials on Youtube.
With the RAW images you can then start the image processing.
With this tutorial we have gathered some of our own experiences as a beginner’s guide. There are many different factors that go into getting a successful result, but if at first you don’t succeed, keep trying and you’ll be up to speed in not time.
But most importantly have fun and enjoy photography!
We would like to thank Peter Mein for this Tutorial and the images of this, so please check out his website for additional inspiration.
He wrote this tutorial with regards to the PENTAX K-3 Mark III, but it could also be used for the other compatible cameras.