The story so far Part 2: Polar alignment

Polar alignment (PA) is basically the alignment of a telescopes rotational axis with that of the earth's. The mounts motor system can drive the telescope at a rate that keeps it in sync with the earth's rotation, but if the mount isn't pointing directly at the celestial pole then over time the target object will drift in the scopes field of view. This type of drift is known as field rotation and the less accurate PA is, the more pronounced it becomes. A bad PA is highly detrimental to astrophotography as it results in star trails curving across the image. It's less important for visual observing, as any target can be re-centered with an occasional adjusted as needed, but it's still necessary to have it somewhere in the ballpark otherwise the GoTo system will be way off target. While my immediate concern was just getting PA close enough for GoTo and visual, I knew an effective solution would ultimately be needed before I could do any astrophotography.

So far my PA procedure had just been to plop my mount down on the concrete, try line it up with the pole by eye, then cross my fingers and hope for the best. I had become quite adept at identifying the pole and it's surrounding star pattern with binoculars but translating that perspective to my manual alignment 'technique' proved to be difficult as none of the nearby stars are very visible to the naked eye. In the northern hemisphere there's a nice, bright star called Polaris that sits almost directly in line with the pole. In the south however, we have no such convenient marker. I would need a new method. Thus began a new research effort.

There are several different ways to achieve polar alignment and, as you'd expect, each has their own strengths and weaknesses. It seemed they'd all require a new accessory purchase to be done properly so I really wanted to nail my options down before committing to anything.

The most common and simplest method would be the use of a polar alignment scope. It's a little finder scope that slots into a purpose made housing along the right ascension axis of the mount, that is to say it sits exactly through the line that needs to be pointed at the pole. The scope itself is imprinted with a reticule which needs to be lined up with Polaris and that's pretty much it - job done. It's quite a simple process and the scope itself is also quite cheap; seemingly an ideal solution. As mentioned earlier though, Polaris is only visible in the northern hemisphere. After making enquiries with the local astro gear supplier, it became apparent that no such equivalent exists for the southern hemisphere, or at least not one that's mass manufactured and could be readily ordered. It's still not obvious to me why an SH polar scope is such a difficult proposition. Maybe stars around the southern pole are too dim? Maybe it's just a question of demand more than anything else. Nonetheless I was told not to fret as SH astronomers have a superior and more accurate method up our sleeves - Drift Alignment.

Alignment via the drift method basically involves choosing 2 bright stars and watching for drift in certain directions then adjusting the mounts position until such drift disappears. One of the stars used must be near (within 10 degrees) both the meridian and celestial equator. The mounts azimuth direction should be adjusted until north-south drift of this star is eliminated. The other star must be near both the celestial equator and the eastern or western horizon. Altitude should then be adjusted until east-west drift of this star is eliminated. This is a heavily truncated explanation and there are multiple variations of this method but they all boil down to monitoring and eliminating drift.

The drift method is extremely accurate as you are only limited by the level to which you're able to identify and resolve star drift. No additional equipment is required but having a reticule eyepiece is highly recommended, maybe even essential, as the cardinal directions aren't always obvious when looking through a telescope so having a crosshair to align them against is extremely helpful. The major downside of this method is that it requires constant waiting and adjusting and thus can be quite time consuming. A barlow can help with this to some degree as the higher magnification will help reveal drift direction sooner. Now to me this all sounded great - highly accurate and no large cash expenditure required - but a critical misunderstanding would lead me to discard the drift method as a viable option.

Being that this was early days, I was still yet to wrap my head around a lot of terminology used to describe locations in the sky. Right ascension and declination; arcminutes and arcseconds; celestial horizon and celestial equator, etc - most were concepts that I thought I had a grasp of but in reality I had little idea how they practically applied to what I was doing. When the set of instructions I had referred to the 'celestial equator', for whatever reason, be it a reading comprehension fail or otherwise, I just assumed that meant the terrestrial horizon. The location I observe from is unfortunately surrounded by trees and the northern and southern horizon are obscured to a large degree. That factor, and the aforementioned faulty assumption, led me to conclude that drift alignment could be problematic or inaccurate so I moved on to looking at other PA methods. Later on however, I would come to understand that all I needed was vision of the celestial equator - an imaginary line through the sky that sits in line with the earths equator. The celestial equator is quite easily visible from my viewing spot so the drift method has always been a perfectly viable option. Regardless, despite my mistake, I would end up with something possibly even better.


Continued research would lead me to the discovery of a nifty little product called the Polemaster. It's essentially just a small camera that is attached to the same housing that would otherwise used by a polar alignment scope. The accompanying software uses that view to calculate the center of rotation of the mount, then identifies the exact location of the celestial pole and provides visual cues for the user to make azimuth and altitude adjustments. See below for an excellent video demonstration of the process from Astronz.

The Polemaster is extremely accurate but also makes the PA procedure very quick. As far as PA methods go it's the best of both worlds, the only downside is the cost. I couldn't find one in NZ retailing for less than $500 but searching online I was able to get one shipped from Hong Kong for under $300NZD (including mount-specific adapter and shipping). The makes the Polemaster the most costly of all the PA methods but ultimately a small price to pay for the convenience and accuracy. Assembly was a breeze and the software is pretty straight forward even though the documentation is a bit ambiguous.

With the Polemaster up and running I was quite easily able to get a good GoTo alignment, resolving my original issue. This made my visual sessions a lot more pleasant as no longer had to manually slew around looking for my targets after every GoTo command. More importantly however, having an accurate PA now meant there was no longer any barrier to getting started with some actual astrophotography.