Astrophotography – Star Tracker Basics

In Astrophotography, General - Tips and Tricks, Learning Center by jfischer2 Comments

As a landscape photographer the words ‘High ISO’ might as well only have four letters in them. For the cleanest and best images, I always strive to have my ISO settings as low as I possibly can. Even with today’s excellent sensor technology, there is still a difference at the end of the day between shooting at ISO100 and ISO400 when you start looking at the files at high zoom or printing at a large scale. So cranking the ISO on my Canon 5D mkIV or 6D to 1600 or higher certainly does make that day-time shooter part of my brain cringe more than just a little.

So why the need to turn up the sensitivity so high? It’s not just that the night is, well, dark – it’s also because of the rotation of the earth. While it might not seem like it while staring up at the stars for a few seconds or even a minute, but we really are spinning at an incredible rate. One that you’ll notice in your images if the shutter stays open for even 30 seconds or less. Compounding this problem is today’s ultra-high resolution sensors. In the past there was something known as the 600 rule, which then became the 500 rule, which later became the 400 rule and so on. Basically the idea was to divide this number by your focal length and you’d get the length of time you could leave the shutter open before star trails started becoming obvious. To keep the math simple, let’s assume a 20mm lens on a full frame camera – even the now obsolete 400 rule means only 20 seconds at this very wide focal length. The problem is the size of each individual pixel on the sensor has become smaller and smaller. So there’s a much shorter amount of time before the light from a star hops from one pixel to an adjacent one. The result is not a circle, but a line, a star trail. The only ways to combat this is to either get more light on the sensor faster with a wider aperture – which comes with other trade offs – or turning up the ISO. Or maybe there’s another way.

Enter Star Trackers.

I first wrote about star trackers last year in my ‘Off the Deep End‘ article where I went into some depth about how I started the slippery slope of deep sky astrophotography. All of the star trackers I’ve researched all follow the same basic design of the German Equatorial Mount, or GEM for short. The basic concept is to counter the rotation of the earth by means of a high precision counter rotating axis that keeps the camera in exact alignment to the stars within the frame. To achieve this you need two things – your point of rotation to be in exact alignment with the center of rotation in the sky, and for your tracker to be moving in perfect sync with the rotation of the earth. From the inexpensive options such as the Vixen Optics Polarie to monster sized units from Orion and others costing as much as a high-end full frame camera body or more, they all aim to achieve this one simple job. The primary difference is how well they do that job and how much weight they can handle while doing it.

Back to the two things you need to make a GEM work, alignment with the rotation of the earth, and the ability to stay in sync. For the first requirement, it’s going to take some effort on your part. You have to get the axis around which the camera turns aligned with the north star, Polaris. For this all trackers will come with some sort of an alignment scope as part of the package that helps you position the mount correctly. Depending on the tracker you’ll typically have two sets of adjustment screws to fine tune the up/down and left/right position of the mount until it’s in alignment. The second part is also up to you, or more, up to how much you want to lighten your wallet. Staying in perfect sync with the heavens for minutes or hours requires precision gearing and motors and the weight you place on the mount directly impacts the ability for the mount to do it’s job. Just like asking a Honda Civic to tow a travel trailer up a mountain is likely going to cause some difficulty, putting a heavy camera and lens on a mount that isn’t rated for it might not give you the results you are after. A good rule of thumb is not to go over 50-60% of the stated weight limits for good tracking for long exposure astrophotography.

So what tracker is right for you? The first question to answer is what are you really needing to achieve with the tracker. For the sake of this first article for ‘Tracker Basics’ let’s assume you just want to mount your DSLR camera with a wide field or perhaps short telephoto prime lens to capture high quality shots of the night sky at much lower ISO than you could without the tracker. Full frame camera bodies weigh in at around 2 lbs, the lens let’s assume a mid-range zoom that you’ve probably already got but isn’t the lightest thing around at another 2 lbs. Throw in the battery, an L-bracket, for sake of argument say 5lbs of weight you’re trying to keep in sync with the stars. That $400 Vixen has a stated weight limit of 7lbs and might do the job if you’re using a small Sony alpha series compact mirrorless or a Canon Rebel with the kit lens and has the advantage of being super compact and easy to throw into your camera bag, but if you’re using prosumer grade camera equipment or expect to get minimal to no trailing, I’d personally look at something a bit more robust.

The SkyGuider Pro – I was so excited I didn’t even close the coat closet door for the photo.

Portable Tracker Recommendations

The two models that I would recommend for a Landscape photographer who has invested in enough time and money to be taking the next step to improving the night-sky half of their images is the iOptron SkyGuider Pro and SkyWatcher SkyAdventuer Pro. For a very small increase in budget over Vixen I was talking about earlier you get a lot more for the money. Both have a stated 11 pound weight capacity, easily enough to handle larger gear, especially when using the included counter balance weights. You can set up either of these trackers without the counter balance like the Vixen if using small and light gear or only need slightly increased exposure times after a lengthy hike in, or using the counter balance to greatly improve functionality when using longer lenses and/or longer exposures. Also included in both of these units is a wedge base, something that the Vixen does not. The quality of the wedge (or whatever is being used to move unit into alignment with Polaris) has a significant impact on tracking. Standard tripod ballheads, even the best ones, can have some slight movement when tightening them up, as well as not being able to independently control the vertical from horizontal movements in very minute adjustments.

As seen above, I ultimately bought the iOptron SkyGuider Pro after several days of intensive research and reading of reviews across the internet. I know both units have a lot of fans, and either will most likely perform just as well as the other for you. If you have read my ‘Off the Deep-end’ article, you know that I did ultimately buy a SkyWatcher mount when moving up to my telescope setup and have been extremely happy with everything I’ve bought from this brand. One thing to note on the iOptron, since the photo of it above was taken, I’ve flipped the central bracket that holds the camera and counter balance around so the counter balance weight attaches to the longer end. This makes setup and tear down a little more finicky, requiring me to install the ballhead first, before putting the bracket on the mount, but gives the counter balance a little extra leverage for dealing with larger lenses. If you plan to use heavier lenses such as a 70-200 f/2.8 and full frame camera or larger, this is a modification that I recommend looking up, I’ll try to find the link and supply it here if I can find it. iOptron also sells additional weights if you find you are unable to balance your desired lens and camera setup sufficiently with the included weight.

None of the mentioned trackers include a tripod. And as anyone coming from landscape photography knows, it is a vital piece of the equation for tack sharp images at longer shutter speeds. When I bought my SkyGuider Pro, I knew I was likely going to be using it in addition to shooting more traditional non-tracked night photography at the same time so didn’t want to dedicate my carbon fiber Induro to the tracker, and I knew I was not likely going to be walking all that far from the vehicle with the unit so ultra-light weight was also not a real big factor for me. To that end, I decided to buy the Slik 700DX Pro for the mount, which now also doubles as my tripod when storm chasing since I’m not worried about throwing it around in a mad dash back to the truck to stay ahead of a hail storm. Any sturdy and solid tripod will do, the bigger and heavier the better.

How to get your Tracker setup.

Now that we know why you want a tracker, and what trackers might best fit your needs, let’s finally discuss what is needed to get a tracker ready for action. We’ve already discussed the need for polar alignment, but here is my step-by-step procedure for getting the iOptron SkyGuider from box to shutter release.

Before leaving for your first trip out with your tracker, set it up, understand how it goes together, and practice balancing your intended camera and lens setup. Remember, the mount will be facing north, while the Milky Way will be most likely to the South-Southeast, so make sure you’re able to position the camera in the desired direction and orientation with your setup. Second thing to do once you get a handle on the physical setup of the mount is to find a good app for your phone that will assist in the polar alignment step. I use both PolarFinder and SynscanInit on my Andriod, I suspect both are also available for IPhone. What you want is an app that will give you both your coordinates via GPS as well as a visual of your mount’s polar scope. PolarFinder has versions for SkyWatcher, iOptron and many others. Another useful app to find would be an Inclinometer as you’ll see in the first step below.

  • First step is finding a suitable location to setup. You will need a location that gives you an at least a view of Polaris when looking north. Depending on your location, the elevation of Polaris above the horizon changes. This is why having your coordinates are important. Polaris will be at an elevation matching your Latitude. I’m at 33* Latitude here in Texas, but if I travel to Colorado which is closer to 40* Latitude it will be higher up in the sky. If you are somewhere that has trees or other obstructions to the north, having a way of determining your general horizon angle during the day can be useful, which is where that Inclineometer app comes in handy. Sight down the edge of your phone looking north to where the horizon is and note the angle read on the app. This lets you get everything but the final polar alignment done during the daylight hours.
  • Setup and level the tripod. You want to pay decent attention to getting the tripod level, but the fine tuning of your polar alignment will correct for any minor off-angle.
  • Install wedge onto tripod, then mount the tracker body to the wedge. Some wedges have inclination markers that will allow you to get in the ballpark for elevation before nightfall.
  • Depending on your setup, this is where you might have to skip back and forth a tad, if your setup allows use of the polar scope with counter-balance and camera attached, I would recommend mounting everything at this step and balancing the weight with the camera pointed in the general direction you wish to be shooting. If you slightly knock the tripod while doing this, it’s no big deal since you’re not aligned anyways. If by chance the full setup does block the alignment scope – such as if not using the counter balance on the iOptron.
  • Once the sky is sufficiently dark to locate Polaris, make sure your mount is in general alignment before looking through the polar alignment scope. No use trying to find Polaris through a narrow view alignment scope if you’re not close to begin with.
  • Next step is final Polar Alignment. Depending on how wide of a lens you’re using, and how long of exposure you want to run will determine how accurate you need this to be. Also, at least on my iOptron, I have to rotate the axis of the camera/counterweight until an internal light comes on and the alignment scope is properly oriented to see the reticle lit. Another case for practicing setup at home first! Since Polaris isn’t exactly due north it moves slightly throughout the day, if I want a really accurate alignment I need to know where to place the star in the field of view. For really wide lenses ‘middle of the alignment scope’ might be enough, but for really good alignment you’ll need to refer to the PolarFinder app. Based on the current GPS location, elevation, and time of day it will display the exact position to place the star.
  • Using the adjustment screws on the wedge, get Polaris into position. Remember, the view you are looking at is upside down and backwards so left is right and up is down. It’ll take a few times to remember which screw to adjust in what direction to get the movement you want. If you are having difficulty seeing either the reticle or Polaris, make sure the focus on the alignment scope is adjusted, and if you wear glasses, I often find it easier to remove them and adjust the scope to my uncorrected vision.
  • Carefully lock down the adjustment screws on the wedge, watching for any shift in your alignment. Congratulations, you’re now aligned and ready to start imaging.
  • I like to image with the camera and counter-weight at an angle. I find the gears in the tracker mesh better than when it’s upright to start and I get better tracking. What angle works best for you will depend on your camera setup and where you’re aiming, you want to try for a consistent light weight against the counter-clockwise rotation of the mount.
  • Don’t forget to get your camera focused, take a few test frames at high ISO/short exposure to make sure you have the composition you want and start tracking and imaging at long exposures you could only dream of before!

Limitations of Small Portable trackers

The small trackers covered here do have their limitations that you need to consider going in. The first one has already been hinted at already, and that’s payload. The two recommended trackers have payload limits of 11lbs, and the closer you are to reaching that, or the less optimal your ability to balance the camera and lens the less reliable your tracking will be and thus reduce the length of time you can image. They are also not the must sturdy when setup compared to larger units, so wind and other introduced vibrations are also going to have an effect at longer focal lengths. I love my iOptron SkyGuider Pro for use up to around 200mm focal length in good conditions. Beyond that I start finding fewer good images when I get home. I’m sure there are people who have successfully used their small trackers up to 400mm and beyond, but for my limited time under the night sky, I chose to not push my luck.

The other feature that small portable trackers lack is something called ‘GoTo’. This is a feature that will automatically position the mount pointing at a set of pre-defined known interest points in space. Galaxies, nebula, star clusters, all things you might really want to shoot with a longer lens or telescope, and all things that might be near impossible to see with the naked eye let alone get your camera pointed at as well with any degree of accuracy without dozens of trial-and-error attempts. If your goal is wide field astrophotography, capturing wide sections of the sky at focal lengths at or below around 135mm or so, GoTo has little or no real value and adds weight and cost to the mount. Mounts of comparable cost with and without GoTo, the mounts without will likely be able to handle more weight, be smaller and less complex. I started with the iOptron SkyGuider Pro, and still use it frequently, however after one night in the cold trying to find Andromeda I quickly came to understand the usefulness of a GoTo mount and by spring had purchased the SkyWatch EQ6-r Pro and a telescope. But that will be a discussion for the next article for deep sky astrophotography tracking.

Comments

  1. Thanks, I’ve wanting to get a tracker but just don’t really know which one to get. There is some good information here.

    1. Author

      Find me on FB or IG if you have any specific questions. Still working on the rest of the series that follows on to this for bigger mounts and telescope setups for deep sky imaging.

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