What is Polar Alignment?
Polar alignment is the alignment of the axis of your telescope mount with the motion of the sky. This beginners guide explains the concepts of what it is and why without accurate alignment, you will struggle to accuately guide your scope on your target and image well.
When I started out with Astronomy with its miriad of equipment, I struggled to initially fully understand what Polar Alignment was and why it is so important so this article is here for those of you newbies who would like a clear explanation as to what it means and some differing ways of achieving it.
So lets think about the basics and fully understand them – think about the night sky differently. The stars do not rotate or change their position, our earth does. We are are in an orbit around our sun and our planet rotates on a North/South axis around the celesestrial poles as it orbits the sun. Its a bit like a ball (the earth) with a stick stuck through it, rotating on that stick, like a spinning top. Where the stick goes through the ball are the “poles” around which it rotates… each complete rotation is of course a day.
Our seasons come from the tilt of the stick, whichever hemisphere tilts towards the sun gets more direct energy and light and enjoys summer and vice versa. So from the north and southern hemispheres you get to see a different patch of the universe from the ground depending on where you are and what time of year it is.
Alt-azimuth mounts (short for altitude-azimuth) allow a telescope to move up-down and left-right. This movement is parallel and perpendicular to the horizon, making this mount intuitive to point, leaving the eyepiece in a convenient position, and making it well-suited to terrestrial observing. However, it makes tracking objects in the sky more difficult; automatic tracking is only possible with a computerized telescope.
A German Equatorial mount (GEM), on the other hand, has its axes aligned with Earth’s rotational axis, allowing easy automatic tracking of the night sky. Disadvantages include less intuitive pointing, more weight, and sometimes an awkward eyepiece position (but we don’t use them anyway, do we?) For accurate tracking, especially for astro photography, an equatorial mount is therefore essential.
The polar alignment procedure works on one simple principle: The polar axis of the telescope must be made parallel to the Earth’s axis of rotation, called the North Celestial Pole (NCP). When this is accomplished, the sky’s motion can be cancelled out simply by turning the axis (either by hand or with a motor drive) at the same rate as the rotation of the Earth, but in the opposite direction.
As you head further south (something you can’t fail to do when moving away from the North Pole!) the position of the NCP appears to get lower in the northern part of the sky. In fact if you travelled all the way down to the equator, the NCP would appear on the north point on the horizon with its Southern Hemisphere counterpart, the South Celestial Pole (SCP) visible on the southern point on the horizon.
The great thing about German Equatorial Mounts is that once you Polar align and tell your mount exactly where it is and what the time it is, most modern mounts have a GoTo computer that will point your telescope accurately at the chosen object and precisely track it. In addition we can use Guiding (via additional software and camera) to ensure even the manufacturing tolerances of the worm gears or belts in your mounts (backlash) are handled and cancelled out giving nice clean stars with no trails over long exposure times.
Why does Polar Alignment need to be so accurate?
Why does Polar Alignment need to be so accurate?
For imaging, a far greater degree of accuracy is needed than for visual viewing – the accuracy required to track a celestial object an immense distance away for 3 to 5 mins without the image blurring despite our planet constantly rotating! Always get your polar alignment as accurate as you can. However the determing factor is optical resolution – this comes from the pixel size of your camera together with the focal length of your telescope, this makes sense… the higher the focal length and pixels in your camera, the more accurate you need to be as the resolution increases.
Know your axes!
Know your axes!
Make sure you know the correct labels for the two axes of a GEM as they are often referred to in manuals and documentation!
Like many things in life, there are many different ways to achieve the same thing so I present here three ways to do this but I only use two of them depending on which the mount I am using. The intial steps though I follow each time tend to make things quicker and more accurate. I noramlly do not use a fitted polar scope, I tried this a few times intially but found it neither easy to use at my 38 degrees latitude or accurate enough so I recommend you save your cash and don’t fit one (your mount may have one factory fitted).
When I first started to do Polar alignment, it would take me an hour or more of valuable imaging time to get it right but I have now developed a workflow that I can often now achieve this in 10 to 15 mins. Practice!
Before you start…
- Always level your mount with a bubble / spirit level. An uneven mount will not track as well as the angles will slightly change during rotation and accurate guiding will be impaired.
- Most telescopes have a latitude scale on the side of the mount that tells you how far to angle the mount for a given latitude (see your telescope owner’s manual for instructions on how to make this adjustment). This adjustment determines how high the polar axis will point above the horizon. For example, if you live at 40 degrees latitude, the position of Polaris will be 40 degrees above the northern horizon. Remember your latitude measurement need only be approximate at this stage and if you don’t know it Google maps can help you!
- Make sure your mount is pointing due north, use a compass to get the general direction but remember that the metal of your mount will affect compass accuracy. If you do not have a polar scope installed, a good trick is to look though the polar axis of the mount (where a scope is usually installed) and if you can visually see Polaris, then you are on the right track or you can just visually eyeball Polaris up the barrel of the telescope (set on its index marks) to get it very roughly aligned by hand, moving the mount tripod feet.
- To help get a fast rough point at Polaris, I use a green laser pen held along the polar axis of the mount to quickly get rough pointing alignment with Polaris before starting alignment. I adjust my altitude and also gently move my mount feet to get me there so at least I am roughly pointing in the right place. Remember to make sure not to point the laser at people / aircraft and obey your local laws with these – they can be dangerous.
- To help me with alignment, I have bought online a thin sheet of Teflon (PTFE) for use inbetween the mount head and the tripod. Its easy to cut it to shape with scissors and it really helps ensure a smooth azimuth adjustment and prevents “grab” saving more time.
- Make sure the adjustment bolts for azimuth (left and right) on the mount base have been reset to have an equal amount of screw thread available to push the mount in either direction before you start the final alignment process – its really irritating later on if you haven’t
Using a Polar Scope
Click to learn how to Polar align using a polar scope
Drift Alignment software
Click to see learn about drift alignment software options
Polar Align using Tools
Click to see learn about tools to assist you in Polar alignment
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