How to Save Your Back and Time Dealing with Star and Polar AlignmentYet another burden which should be completely automated
When you start as a beginner, steps of alignment appear pretty unusual and unclear. There is a polar alignment to align the RA axis of your start tracker or equatorial mount with the rotation axis of the Earth. If you use an equatorial mount, in most cases you need to run the star alignment which tells the control software how the your lens or the camera is oriented in the sky and which commands are required to change its position to given coordinates. Both alignments just cost time, though they provide essential information for all components to work correctly. Steps are always they same and it becomes a very boring procedure over time. Manual polar alignment requires an additional scope which might not support you well. If you use a portable setup, looking up the right location in the sky ends up quite some creep and crawling.
From my perspective, these steps cost too much time, energy and are not fun at all. The good news is that in our times, there are several neat solutions to simplify all this. In the past I felt so demotivated by these boring processes that I started to rebel and search for a better solution. I hope this article will simplify this for you as as well.
Why do you need Polar Alignment?
Simple: because the rotation axis of the tracker of the mount should be coaxial with the rotation axis of the earth. In the Northern Hemisphere, the right point is very close to the Polaris star. In the Southern Hemisphere, the orientation is more complicated and involves more navigation around Sigma Octanis. Most mounts are equipped with small scope (often also called “polar scope” or “polar finder scope”) with integrated reticle showing a scale marked with years and months. This scale is needed for exact positioning since the point of the Earth rotations axis is moving around around the stars used for alignment over time. Your task to adjust the position of your setup on azimuth and latitude to align the stars with markings on reticle . If you use a star tracker, such a scope might be not included in the package. If you are going to buy one, so not save money on very cheap choices. You will pay much more with your time and patience later.
The polar scopes are simple and it is hard to see the needed stars well through them. The reticle requires backlight. The angle of view is in the Western Europe is around 47 degrees and so requires creeping around the mount or star tracker. It is a very good idea ti buy a polar finder with a 90°-prism which will cost another 100 Euro. Some star trackers provide markers on the casing which help to point the tracker as close to the Polaris start as possible.In the Southern Hemisphere, it is a bit more challenging since the stars around Sigma Octanis are relatively faint.
Prior the actual alignment, you have to make sure that the tripod is placed evenly. For this, most tripods are equipped with bubble levelers. The one I used, was not, and I had to add one. If you need to buy a bubble leveler, it is better to buy one with defined precision. Many cheep bubble levelers on Internet can just distinguish between horizontal and vertical position but not more. A bubble leveler with a precision of 20-30′ is a good choice.
When you found the right spot on reticle, you have to adjust the orientation of the mount or star tracker to align both axis with two screws. One for the RA axis and another for the DEC axis. How well the screws will serve depends on the mount quality and design of you mount or a star tracker. High-end makers, like Lostmandy and iOption invest in good solutions to make make this process stable and comfortable
When you are done, your mount or start tracker is called in the “home position”. The term “home position” also means that the main camera looking along the rotation axis and so facing either the South or the North celestial poles.The initial orientation of the main camera is not expected to be very precise. This will be adjusted during the start alignment.
In the year 2021 all this sounds like a call for a computerized solution… Less dumb work, more time for the actual thing – photography.
Do I Really Need Star Alignment?
Star Alignment stands for a process of provisioning information to the control software of a GoTo-enabled mount which defines how the mount is oriented. This data is also used to calculate the correlation between the information in the stepping controllers connected to motors and celestial coordinates used in GoTo commands. Star Alignment is generally not dependent on exact Polar Alignment, but if you change the mount orientation during the Polar Alignment, you actually need to re-do Star Alignment.
Depending on the software supplied with the mount, various multiple steps are needed. In most cases, you are asked to frame a well-known, bright object in the sky in the middle of the camera preview display by moving the mount with motors and confirm this position in the software. A precise alignment needs three objects spread across the sky. Some mounts provide quicker (and often less precise) solutions with less objects involved. Knowing the coordinates of these objects being confirmed, and the amount of steps which the motors needed to make to go there, the control software calculates settings for the GoTo commands. Some systems allow to re-confirm an object being in the center of view to adjust the position model.
And here again, in the year 2021 all this sounds like a call for a computerized solution… Less dumb work, more time for the actual thing: photography.
Polar and Star Alignment Helpers
My take on the alignment is not unique and not new. Some manufactures offer different helpers. In many cases, these helpers are not perfect, since their implementation is limited by low cost. This does not seem to run very well given complex image processing and calculations.
Celestron provides the StarSense add-on with includes a camera and controls for the mount. I once was reading a review of StarSense 3 which did not sound very exciting to me.
iOptron provides BrightStar Polar Alignment and Star Alignment in the GoToNova control software available for iOptron mounts.
The EKos from the INDI project also provides a solution for Polar Alignment and Star Alignment. Both are based on a powerful approach called “plate solving”. This will be discussed in the next section. The Ekos’s Polar Alignment requires a exact, and narrow view on the Polaris star which makes it hard to use, since it requires something you might now have in a portable setup: good initial alignment. For stationary setups (and this is where most Ekos users come from), this solution should work well.
Integrated Plate Solving as a Killer of Manual Alignment. ZWO ASIAIR Shows How This Has to Work
Plate Solving is known since a very long time. Its name actually explaines where this approach comes from. In the past, specially educated people used paper maps of the sky and manual calculations to “solve” an astrometry equation telling if the telescope is actually pointing where it is though to be pointing to. Today, there are several software solutions for this. All of them are based on data created and provided by the http://astrometry.net project. Astrometry.net also provides an own service which allows to calculate celestial coordinates for a picture of the sky. It is also possible to adds labels with object names as a layer on top. Astrometry.net is available for not a long time. the project brought first results in 2005 and in 2011 it has been launched in the first beta version of a web-based service. There are 2-3 open source software components which use the data from Astrometry.net and provide API for plate solving. More and more mount control software uses plate solving provide new user experience. In my opinion, it is not just yet another features, plate solving changes the user experience!
ZWO ASIAIR integrates plate solving and demonstrates how users can benefit from this and save time and energy for the actual thing they want to do: photography. The best way to describe this is to list steps during the gear setup. The below list assumes that the main optics is in focus and ASIAIR communicates directly with the mount, e.g. via an EQMOD cable bypassing any other mount control software.
- You start the ASIAIR app on a smartphone and connect to the ASIAIR device
- On initial connection, the time and location information from the smartphone is synchronized with the mount. No user input is required here.
- The mount and the main scope are expected to roughly point to the Northern or Southern celestial poles. No exact positioning is required. A visual judgment with a view on the Polaris star, for example, is sufficient
- The main camera takes an initial picture of the sky, the system uses it for plate solving
- If the first plate solving was successful the mount rotates on RA axis (the axis pointing to a celestial pole) 60 degrees and takes another picture for plate solving
- Upon a success, the adjustment process can start. The software shows a picture where the RA axis currently points to and where it should be
- The user is asked to tilt and rotate the mount in specific directions. This is done with adjustment bolts on the mount.
- On each step, another picture is taken, plate-solved and next corrections are shown
- Depending on the experience, the whole process from the first point above takes 3-5 minutes until the final total error of around 1′ is achieved. The user can continue to make it better as long as desired.
- After the Polar Alignment, in the Preview mode, the mount can be moved with a GoTo command to any object. It is good idea to select one you know and can see well. Alternatively you can go straight to the object you want to photograph.
- After the move, a new preview picture is taken, plate-solved, the mount position is corrected and the final picture is taken to verify the position once more
- That’s it. Your gear is ready to go.
When using photo lenses, you often have a situation that the lens is not focused yet as you start. In ASIAIR, you can use switch the guide camera to be used as the main camera. You can do the initial alignment with it and than point to a bright start to focus the photo lens on the main camera.
All of the above presumes a view to region around the Polaris or Sigma Octanis stars which might be not given in your location. The Drift Alignment method requires an obscured view to the South and either to the West or East. It is based on the live visual observation how stars are moving and correcting the mount orientation. With a focal length of 200mm and more, stars move fast enough that this method is doable in a reasonable time. This are the two best descriptions I found so far:
The PHD2 guiding software provides a wizard for this process. Unfortunately, it is far from perfection.
I’m waiting for better solutions for drift alignment. Maybe it is time to make one myself…