The key to using an equatorial mount is polar alignment — aligning the mount so that moving the telescope in Right Ascension precisely mimics the motion of the sky. There are several reasons why this is necessary, depending on what you are planning to do next:
Beginners are sometimes intimidated by polar alignment — and, in fact, this fear is often the reason they purchase alt-az mounts instead of equatorial mounts.
Fear not. Polar alignment is actually quite simple. It just means pointing the polar axis of your mount precisely toward the North Celestial Pole. So you really only need to understand 3 things:
Let’s go through those now.
To confirm you have found the polar axis, use your mount’s slow-motion controls to move the telescope in both directions (Right Ascension and Declination). The polar axis is the line through the mount that does not move when you move the telescope in either direction.
Read that last sentence again — it’s going to be important. The polar axis does not move when you move the telescope.
For fine adjustment, there will be controls on your mount to tilt the polar axis up and down, and probably other controls to move it left and right. These are separate from the controls that move the telescope on the mount.
That last point is so important that we’ll repeat it a couple of times. Polar alignment is done by moving the polar axis or by moving the mount itself, not by moving the telescope on the mount. The slow-motion controls you use to point the telescope are not used for polar alignment.
Here’s the repeat. Polar alignment is done by moving the mount, not by moving the telescope.
I see so many beginners making this error — trying to polar align by adjusting where the telescope is pointing, that I have developed an additional recommendation to help form the right habits. It is this:
The first time you set up and polar align your mount take just the mount outside. Don’t attach the telescope — leave it in the house, in a locked room. If possible, release a vicious animal such as a mongoose in the room with the telescope. (A Honey Badger also works well, but it can be difficult to persuade them to leave afterward.) This is so you are not tempted to think that you polar align by worrying about where the telescope is pointing. After the mount is polar aligned, go into the house, free the mongoose, and get the telescope and attach it to the mount.
This is why, if you own an equatorial mount, it is very important that you be able to find Polaris quickly — it’s your main alignment tool.
This is for astronomers in the Northern hemisphere, of course. Astronomers in the Southern hemisphere don’t have the convenience of a star located at the South Celestial Pole, and must use other nearby objects to estimate its position. Sorry, Southern neighbours, I have no experience to help you with that, but I hope the rest of this article is still useful. The “Polar Alignment Scope” method mentioned below, with a polar alignment scope marked for the Southern hemisphere, is your best bet.
In that last point, we talked about the NCP being very close to Polaris, and implied that just pointing at Polaris may be good enough. That raises the question, “how accurate does polar alignment have to be?”
The answer is “it depends on what you are doing”.
Most of a beginner’s observing will be what I would call casual visual use. You will set up your telescope and spend one to three hours observing a variety of objects, then put it away. You’re not planning to be out all night, and you are not doing photography.
For casual use such as this, a very simple polar alignment is sufficient. Let’s discuss the most basic method, and then a slightly more accurate one. With practice, neither will take you more than a couple of minutes.
The simplest polar alignment involves pointing the polar axis of the telescope to where the North Celestial Pole (NCP) “should” be, in theory, without the trouble of actually checking where it is really pointing. This is done by pointing the axis North and setting the elevation of the axis to equal your latitude.
First, determine your geographic latitude to an accuracy of about 1 degree. You probably already know this but if you don’t it is easy to find on the Internet. If you just Google on the name of your city and the word “latitude” you will probably get it. For more official data, Americans can check the US Geological Survey Geonames Server and Canadians can consult the National Research Council Geographic Names Server. Google Maps can also display this information, although you may have to turn some options on first.
These days, the easiest way is with your smart phone. Most have built-in GPS, and can be persuaded to tell you your latitude and longitude. Since they also record this information into photos taken with their built-in camera, taking a selfie, then learning how to query the location information in the photo, is another way, and much more trendy.
Why did we do this? Because the elevation, in the sky, of the North Celestial Pole is, by definition, the same as your latitude on the planet. So now, assuming you have your mount level, the elevation of the polar axis is approximately correct.
If you don’t know what that means, don’t worry about it; just find North as accurately as you can. Polaris, the North Star, is a good reference. The advantage to the compass is that you can do your alignment before it is dark while Polaris, being fairly dim, requires that you wait until dark.
In this photo, the mount is pointed too far to the right, so I just pick the whole thing up and turn it to the left, then check again with the compass, repeating until the polar axis is pointing straight North.
Reader Ian Wood has pointed out that, if you are not on level ground, picking up the whole tripod and rotating it will spoil the leveling you did a moment ago. He’s quite right. So, if you are not on level ground, you may need to cycle between pointing the mount North and leveling the tripod legs a few times, until you achieve both: the mount is pointing North and the head is level. Good catch, Ian, thanks!
That’s it. Your polar axis is now pointed to approximately where the North Celestial Pole should be (if it knows what’s good for it). This isn’t a very accurate method — it depends on the accuracy of your levelling, your latitude setting, and your North direction. But it is good enough for a couple of hours of casual observing.
This next method is both more accurate and easier than the compass-and-latitude method described above, if your mount has a simple but important feature: a hole through the polar axis.
Note that, for now, we are talking about mounts where this hole is empty. On more advanced mounts there may be a small telescope permanently mounted in the hole, and that case is covered below.
The photos in this section are of a Celestron CG-5 (AS-GT) mount. Note that the telescope is not attached; polar alignment is done by moving the mount, not the telescope.
(This feature is so that you will rotate the telescope, if it is mounted, to a position where you are unlikely to bump it with your head when standing up after looking through the hole.)
That’s it. If Polaris is centred in the hole through the polar axis, then the polar axis is pointed quite precisely toward the North Celestial Pole. This is much more accurate than the compass method above since you are actually pointing at the NCP, not just at where it “should” be. This is plenty of accuracy for an evening of casual viewing.
For longer-term visual use you need good polar alignment. For example, when I am participating in public outreach star parties, I like to take the time to do a good polar alignment. Then I can centre the telescope on an object and know it will stay centred (with a motor drive) for a long time, allowing a lineup of interested visitors to look without me having to interrupt them to re-centre. The polar alignment described in this section is also good enough for “casual astrophotography”, meaning photography with video cameras or with short exposure times (several seconds but not several minutes).
To get to this level of accuracy, we will be correcting for the fact that the North Celestial Pole is not precisely the same place as Polaris, but is offset slightly (about 1 degree).
You need the help of a tool to get good polar alignment. We’ll review two such tools here: a polar alignment scope, and the polar alignment software built in to many “go-to” mounts.
Many equatorial mounts have an option to permanently mount a small telescope inside the hole bored through the polar axis. This polar alignment telescope is used to magnify the view of the sky around Polaris, and will contain an etched reticle to help you point your mount to the North Celestial Pole at its appropriate offset from Polaris.
There may be several lines and labels that are for use by Southern Hemisphere astronomers. Use your instruction manual to determine which markings apply to you, and ignore the others. For the rest of this discussion, we’ll use just the Northern Hemisphere markings.
(On the Losmandy, the polar scope can be rotated in the mount housing. On the Skyview Deluxe and Stellarvue M4, the polar scope is fixed in place, so you rotate the entire mount on the Right Ascension axis.)
Get it reasonably close to matching the position of the two constellations in the sky, but don’t obsess over it. This will be accurate enough for most purposes and, if you have a dark sky, you will be able to fine-tune using two stars in the polar scope (below).
If you have the constellations lined up with their approximate locations in the sky and Polaris in the spot indicated, this is quite a good polar alignment already, and you may wish to stop here.
There may be several lines marking different spots for Delta. This is because of the slow movement of the stars near the NCP (because of the wobble of earth’s orbit called “Precession”). The lines will be labelled by year, and you should choose the target location closest to the current year.
You continue to make fine adjustments to the up-down and left-right of the mount, and fine rotations of the polar scope, until these two stars are in the marked spots.
This gives an excellent polar alignment, good for any application except long-exposure astrophotography.
If you have a modern mount equipped with a “go-to” feature, or digital setting circles, you probably have another option to improve the accuracy of your polar alignment. Most such systems have a “polar alignment” routine available in their software.
Personally, I find polar alignment with a polar alignment scope faster and more convenient than a mount’s polar alignment software, and sufficiently accurate for most purposes, so I rarely use the software feature on mounts that have a polar alignment scope. On my permanently-mounted observatory mount, on the other hand, this is how I establish a “pretty good” alignment before going on to more precise and more labour-intensive methods.
The specifics of these systems are quite different among brands, so I won’t try to give specific instructions here — read your manual. The general idea, however, is:
In fact, you don’t really need software polar alignment features in your scope. You can do this manually with any go-to scope.
This generates quite accurate polar alignment, depending on how carefully and precisely you centre the target stars (use high magnification and an eyepiece with a target reticle if you want to really get this right).
If you want to do long-exposure astrophotography, or are setting up an observatory with a permanently-mounted telescope mount, you need polar alignment that is as close to perfection as can be obtained. There is really only one way to get this level of accuracy: Drift Alignment. It’s not hard, but it’s time consuming, so it’s probably not something you’ll do unless you need it.
Drift alignment sounds very complex when you read instructions on how to do it. Conceptually, though, it couldn’t be much simpler. The idea is that, if your mount is perfectly aligned and you have an accurate motor drive, stars should remain perfectly still in the field of view of your telescope. Drift Alignment is just watching if this is happening (i.e., if stars are drifting in the field) and, if it isn’t, correcting your alignment until it is.
You do this by polar aligning by some other means, then pointing at a star that will show the most drift if alignment isn’t perfect (that is, a star that is far from the North Celestial Pole).
Usually, we start with a star in the Southern sky, close to due South and at an elevation near the Celestial Equator. This maximizes the drift caused by left-right errors in polar alignment. We observe it at high magnification, with an eyepiece with a reference reticle, for a long time — 5 minutes to much more depending on how picky you need to be. Drift of this star can be corrected with fine adjustment of the mount’s left-right polar adjustment controls.
Then we use a star near the horizon close to due East or due West, let it drift, and correct by adjusting the mount’s up-down polar adjustment controls.
There are many well-written descriptions of drift alignment already on the ‘net — Google will find hundreds, so I’m not writing another one here. Some of the guides I recommend are:
The real secret to drift alignment, however, is to use software and a camera to assist you.
Since you wouldn’t be bothering with drift alignment except for photography purposes, it’s reasonable to assume you have a camera and computer connected to your scope. Under these circumstances, very precise drift alignment becomes simple: you use software that uses the attached camera to monitor the drift and guide you in adjusting it.
(“Camera” in this context means a camera that can be operated by the computer — either a webcam, a dedicated CCD camera, or a DSLR with appropriate computer control. A DSLR that you must operate manually will not work for software drift alignment.)
I tried a few such packages and quickly settled on PEMPRO. Its primary purpose is measuring and tuning the periodic error of an equatorial mount, but it also has a polar alignment feature that is really easy to use.
After a bit of calibration, you just let the software watch a star for a while and, based on that star’s drift, it determines how you need to adjust your azimuth alignment. Then it helps you to make the necessary alignment by giving you a target on a star field, and you use continuous camera exposures to track your adjustment. You then repeat this with a different star to adjust the altitude alignment. It really couldn’t be much simpler, and I find I can do a very good drift alignment in about 45 minutes. PEMPRO isn’t free, but I find it is worth the price ($149), there is a free trial, and there is excellent support (by which I mean a user group in which the actual software author responds to questions and makes helpful suggestions).
WCS is another inexpensive package with a free trial. For me, since I needed the other functions of PEMPRO, the choice was easy, but WCS was also recommended by many users when I searched for recommendations.
At the very high end, specialized software that improves the pointing accuracy of the sky model on a Go-To mount (TPoint for TheSkyX and MaxPoint for Maxim DL) can produce a very accurate analysis of the quality of your polar alignment, and give very accurate instructions for adjusting it. (If you have a mount capable of being adjusted this way, and one of these software packages, you don’t need the advice in this article.)
