Polar Alignment of your Equatorial Mount
- 1 Introduction
- 2 Three Things to Understand
- 3 How Hard Is It?
- 4 Quick Polar Alignment for Casual Visual Use
- 5 Better Polar Alignment for Longer-Term Visual Use or Casual Astrophotography
- 6 Precise Polar Alignment for Long-Exposure Astrophotography
- 7 Next
- 8 Comments?
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:
- It allows you to find objects using the standard Declination / Right Ascension coordinates used by astronomers. While this is not, in fact, how you are likely to find objects with an inexpensive manual equatorial mount with manual setting circles, it is essential for computerized "go-to" equatorial mounts, which depend on this system.
- Once you have found an object by whatever means, proper polar alignment makes it easy to keep the object in view while observing - only the Right Ascension slow motion control is needed to track the object across the sky; no further adjustment of Declination will be necessary.
- For longer observing sessions or for photography, a motor on the Right Ascension axis will allow the telescope to automatically track the object you are observing, so no further adjustments will be necessary at all after the object is found.
Three Things to Understand
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:
- What is the polar axis of my mount?
- How do I adjust where the polar axis of my mount is pointing?
- Where is the North Celestial Pole?
Let's go through those now.
|1.|| What is thepolar axis of my mount?
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.
|2.|| How do I adjust where the polar axis of my mount is pointing?
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. 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.
|3.|| Where is the North Celestial Pole?
The North Celestial Pole (NCP) is the point in the sky directly above the Earth's North Pole.
How Hard Is It?
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".
- For casual visual astronomy (i.e. just looking through the telescope for a short while, at a variety of objects) polar alignment doesn't need to be very accurate at all, just a quick estimate.
- Unless you have a go-to mount. Some go-to systems (like older Celestron and Meade systems) require moderately good polar alignment for the go-to to work accurately. Others, such as the Gemini system on Losmandy mounts, can compensate for inaccurate polar alignment.
- For longer observation sessions, where you might want the telescope to stay pointed at an object for many minutes (e.g. studying one object, or displaying an object to a group of people at a demonstration) polar alignment needs to be somewhat accurate - enough to spend 5 minutes or so on the alignment before you start observing.
- For photography through the telescope, polar alignment needs to be very accurate, depending on the type of photography. Serious astrophotographers planning to take long exposures might spend more than an hour on polar alignment before they begin their work. Permanent observatory installations may spend days on polar alignment.
Quick Polar Alignment for Casual Visual Use
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. Neither will take you more than a couple of minutes with practice.
Most Basic: Compass and Latitude Method
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. United Kingdom residents and others can look up their locations at EarthTools, then click on the little teardrop symbol on the generated map to get their latitude & longitude. Google Maps can also display this information, although you may have to turn some options on first.
|Locate the Latitude scale on your mount. Use the polar alignment elevation adjustment to set the elevation of your mount to be equal to your latitude.|
|Here I have set my latitude to 45 degrees for Ottawa, Canada. You don't need to repeat this step unless you move a long distance or take your mount apart.|
|Set up your mount with the top as level as possible. Use a level if you have one available (or there may be a small spirit level built into the head of the mount) and adjust the tripod leg lengths until it is quite level.|
|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.|
|Silva compass is shown corrected for the 14-degree magnetic declination in Ottawa).
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.
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.
Better: Sighting on Polaris
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.
| Many (not all) equatorial mounts have a hole bored straight through the mount along the polar axis. Examples of mounts that I know have such a hole include the Celestron CG-5, Stellarvue M4, Orion Skyview Deluxe, Losmandy G8 and G11, Skywatcher EQ3 and up, etc. Strangely, the expensive Celestron CGE-series mount (original version) does not have this hole, and relies entirely on the software method described below.
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.
|On some mounts the hole is obstructed when the mount is in a certain position and unobstructed in some other position. For example, the hole on this Celestron CG-5 mount is obstructed when the mount is in the normal rest position,|
| but when the Declination is rotated 90 degrees, the obstruction moves aside and you can see through the hole.
(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.)
|Set the latitude on your mount as described above.|
|Roughly level the head of the mount as above. Then pick up the mount, tripod and all, and rotate it so the polar axis is pointing as close toward North as you can manage.|
| Now, wait until dark and find Polaris, the North Star. (That is, find it with your naked eye, not through the telescope.)
|Crouch down behind the mount and look upward through the hole in the polar axis.|
|Continue to adjust the up/down and left/right position of the mount until Polaris is centred in the hole through the polar axis.|
|Reminder: use the polar alignment adjustment controls to do this adjustment. You polar align by moving the mount, not by moving the telescope.|
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.
Better Polar Alignment for Longer-Term Visual Use or Casual Astrophotography
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.
Polar Alignment Scope
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.
|As an example, here is a Stellarvue M4 mount with the polar alignment telescope mounted. You can see the eyepiece of the scope poking out of the bottom of the polar axis.|
|The polar alignment scope on this Losmandy mount has a small LED illuminator to make its internal reticle more visible against the dark sky.|
|If you look through a polar alignment scope, you will see some kind of pattern displayed on top of the view.
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.
|Note that the place for Polaris is not in the centre of the scope. Since it's not in the centre, it matters which side of the centre it is on.|
|That's what the small pictures of the Big Dipper and Cassiopeia are for. You will rotate the polar alignment scope (or the mount, depending on your equipment) until those two constellations are in the same position in the scope that they are in the sky, then put Polaris in the marked spot.|
|For example, tonight if the Big Dipper is to the left of Polaris, and Cassiopeia is below and to the right, you would rotate the polar scope to match that configuration.
(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).
|Manually align the polar axis (by moving the tripod and then the fine up/down, left-right controls) until, crouching down under the mount, you can see Polaris in the field of view of the polar finder scope.|
|Then adjust your mount's up/down and left/right fine controls until Polaris is in the spot indicated.
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.
|If you have very dark skies, you may be able to see a second, very dim, star in the field of view. This is Delta Ursa Minor, and most polar scopes also give you a marked spot for that star.
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.
|Chances are that Polaris is in place but Delta is not. This indicates the rotation of the polar scope to match the position of the sky is not quite right, so you will need to rotate the Polar scope (or mount) slightly.|
|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.
Polar Alignment Feature in Go-To Mount
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.
This method is the sole exception to my suggestion about locking your scope in a distant room when polar-aligning: it does depend on looking through the scope. Because you will get confused about which controls to use (scope slow motion vs. mount adjustment) I don't recommend beginners start with this method - use one of the more basic techniques described above for a while first.
Personally, I find polar alignment with a polar alignment scope faster and more convenient than my mount's polar alignment software, and sufficiently accurate for most purposes, so I rarely use the software feature.
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:
- Start by polar aligning your mount in the best way you can without the software - the more accurate you are to start, the faster the rest of this procedure will go.
- Do whatever sky alignment your mount's go-to system requires. This usually involves slewing to, and precisely centring, two or three known stars at different locations in the sky.
- Invoke your mount's "polar alignment assistance" feature, whatever it is called. Since use of this feature will require moving your polar alignment controls, which will cause the mount to lose the sky model it has established so far, there are usually some "are you sure?" questions to answer before the software will start this feature.
- The mount will slew to where some star should be if the mount is perfectly polar aligned. With some mounts, this is Polaris, and with others it will be a star elsewhere in the sky (or you may be asked to choose a star first).
- You then use the mount's polar alignment controls (not the electronic controls on the hand controller) to make fine adjustments to perfectly centre this star.
- Some systems repeat this with more stars.
- Once you indicate to the mount that you are done, you will need to re-do the go-to system alignment.
This generates a very 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).
Precise Polar Alignment for Long-Exposure Astrophotography
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:
Drift Alignment Software
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.
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