Astrophotography has become an integral part of my photography. Thanks to incredible developments in sensor technique, it has never been easier to capture the beauty of the night sky.
In this tutorial I will talk about the importance of stacking multiple exposures, and how to use this neat little thing called Vixen Polarie. This is a Star Tracker, a device that compensates for the Earth’s rotation, and helps to avoid star trailing.
Vixen Polarie Star Tracker
Polarie tracks with the motion of the stars to eliminate “star trailing” on photographic images. It allows you to track the camera, mounted on the Polarie, in the same direction as the stars you want to photograph. You can find a detailed description in the Manual. I will focus on the most important steps and show you how I use it in the northern hemisphere with the Mode Dial set to Wide-Field Astrophotography ★
What else do you need?
- A Ball Head (I use the Manfrotto 496RC2)
- A Mounting Thread, 1/4 to 3/8 to mount a standard ballhead on the Polarie (I bought it at teleskop-express)
- A Tilt Head (I use the Sirui L-10)
- A Tripod (I use the Manfrotto 055XProB. Nothing special it but does the job)
- 2 AA batteries
- Optional: Polarie Polar Axis Scope to further improve polar alignment.
It is utterly important that you level the tripod and make sure it is even to the ground. Double check that all screws are fastened fully and everything is tight.
Finding the Pole Star
Polarie works with the so called polar alignment. That means you need to first locate the pole star (Polaris). You can do this by using the compass and tilt meter that comes with the tracker, or in a much easier way. Find the Big Dipper, imagine a line connecting the two outer stars of the “dipper”, continue it on the side where the dipper is “open” to a distance 5 times that between the two stars, and you will arrive at the pole star. (It is also the brightest in this area)
So this is why we need a Tilt Head. Point the device towards the pole star and then use your tilt head and move it up or down until you see Polaris through the sight hole. If you are using a wide angle lens this doesn’t have to be super accurate, but with zoom lenses (we come to this later) there is some try and error involved.
Polaris is very closely aligned with the axis of the Earth but not perfectly. In this Stellarium screenshot you can see the little offset. I recommend using Stellarium beforehand to check the actual position on the day and time you want to take your pictures. You can then try to adjust the alignment accordingly.
The manufacturer claims that the max weight you can put on the Polarie is 2.0kg (4.4 lbs). With my Nikon D750, the ball head and a 70-300mm lens I am probably slightly above that limit, but the biggest problem is the focal length and the size of the lens. The whole system is quite prone to vibrations, so again, make sure that you find a windless position and tighten everything as good as possible. Image stabilisation (VR on Nikon) must be turned off and use ‘Mirror lock-up’ to further reduce vibrations.
Focal Length: As mentioned before it is super easy to shoot at wide angle, lets say between 14 and 35mm. Polarie is not made for heavy zoom lenses and it is recommended not to exceed 150mm. Of course I do. I regularly use focal lengths up to 300mm and yes it becomes more and more difficult, but with a bit of patience good results are possible.
Star Tracking and Time-lapse
If you are using Polarie for time-lapse videos, you can do this in two ways. In this example I kept the Milky-Way in frame and the ground rotates. A quite interesting effect.
The second one would be panning. You can tilt Polarie 90 degrees and it will act like a motorized pan head.
Why Stacking? Very little light reaches the sensor of a camera because the light coming from the stars, nebulas and galaxies is so dim. The more pictures you take the more information you add to the final image. You may think one looooong exposure should do it too, but regardless how well you aligned your system to the Pole Star, it won’t be 100% accurate and at least with longer focal lenght, you will get trailing again, more hot pixels and other effects caused by cosmic rays for example. So, adding many image results in much more detailed and less noisy images.
Even with a Star Tracker like the Vixen Polarie the individual images need to be aligned before stacking, because of the little imperfections I mentioned before with the polar alignment. To do this, I use RegiStar (fast & easy but costs $179.00 US) or DeepSkyStacker which is for free but slower. My workflow is as such that I select the images I want to use in Lightroom, apply some first adjustments and export them as 16bit tiff into a dedicated folder which I usually name ‘stack’. Then I load, align and export the results in RegiStar.
The merging and final image processing I do in Photoshop.
- Step 1: File>Scripts>Load Files into Stack
- Step 2: Select all layers > Right Click > Convert to Smart Object
- Step 3: Thats’s the magic moment! Layer >Smart Objects > Stack Mode > Mean (or Median)
Look at the difference in this 100% Crop – Single exposure vs 20 images stacked
Here are some examples where I used Polarie. All images where taken with normal lenses such as the Nikon 70-300mm VR, Sigma 70mm Macro 2.8, and the Sigma 35 1.4 Art.
More of my night-sky images can be seen in Nightscapes
I very much like the Vixen Polarie Star Tracker. It is light, affordable, easy to use and reliable. The set up is done in minutes but can take a little longer when using zoom lenses. The longer the focal length the longer it takes to find the perfect adjustment. But for me, the results are worth the effort. Once you get used to the entire process, which is really much simpler than it may sound, you can explore new grounds for your photography. The fascinating world of Night Sky Photography.
More on astro imaging in my tutorial: Repairing a Monster image
I hope this was useful. Please keep in mind that I can not do all this for free without your help. If you would like to support me you can do this by purchasing my images, posters or my book. Thank you very much!
A little sneak preview about the upcoming topics:
- Lets talk about taste
- HDR Obsolete – but if, do it right
- Common Mistakes