The Manual Method: Polar Scope Alignment
It is the classic, tried and true method that is basically included in most entry level star trackers like the Sky Watcher Star Adventurer. It uses a little telescope called a polar finder scope which is directly attached to the mount.
The Gear
A polar scope is a miniature scope which aims directly along the mount’s Right Ascension axis. The reticle in the scope is a small piece of glass etched with a pattern usually a circle with smaller marks to indicate the position of Polaris.
The Process
Level and Point: One of the very first things you do is to place your mount on a level surface and point it approximately towards the north. Then you move the latitude scale of your mount to your current geographical latitude.
Clocking Polaris: Polaris is not at the exact North Celestial Pole (NCP); it moves around the NCP in a very small circle. There is an application on your phone, like Polar Scope Align Pro, which tells you where polaris should be in the reticle pattern at that time. Time and location are the two things you must know.
The Adjustments: After looking through the polar scope, you use the mount’s altitude (up/down) and azimuth (left/right) adjustment knobs to actually move the mount head until Polaris is the closest to the position on the reticle that was calculated.
Lock it Down: If it is there that you want it to be, you then proceed to tightening the adjustment knobs making sure you do not change the position of the mount.
The Accuracy
It’s a quick and simple method that can result in good to great alignment. In the case of wide field lenses (up to approximately 135mm), the accuracy level is sufficient for long exposure shots. On the other hand, the method depends on being able to see Polaris clearly, and obtaining that extremely precise, sub arcminute accuracy to be suitable for telescopes with a long focal length is quite challenging just with the human eye.
The Electronic Method
This is the contemporary, online method of alignment, which is utilized by computer controlled mounts and units such as the ZWO ASIAIR or a dedicated polar camera like the iOptron iPolar or QHY PoleMaster. It operates on the principle of celestial plate solving.
The Gear
With a help of an electronic camera (it can be a small, dedicated camera or your main imaging camera with a guide scope), a picture of the sky near the pole is taken. The camera sends this picture to a small computer (such as the ASIAIR) or a laptop with software (such as SharpCap) running on it.
The Process
Initial View: A picture of the sky is taken by the camera.
The Calculation: The software employs plate solving recognizing the stars in the picture to figure out the exact location your mount is pointing at.
Measure the Error: Afterward, it compares your current pointing position with the North Celestial Pole (NCP) and determines the error in arc minutes.
Live Correction: The program offers you live, on screen assistance. It shows a target where the real pole is and where your mount is pointing. By turning the mount’s altitude and azimuth knobs and following the movement of the red circle (your mount’s axis) which is going directly into the green circle (the true pole) on the screen, you are actually aligning your mount visually.
Achieving Perfection: Being given instant, highly magnified, digital feedback, you can very easily reduce the error to less than 0.5 arc minutes.
The Accuracy
This is the most accurate method by far. Basically, to do anything with a focal length exceeding 200mm, this method is strongly suggested since the tracking error resulting from a slight misalignment is almost completely eliminated. The reason being that if you cannot see Polaris then it is using any other stars around to determine the pole’s position, so it also works in this case. The disadvantage is that you have to carry more equipment, batteries, and a computer is required most of the time.