How to Polar Align an Equatorial Mount in the Southern Hemisphere
by: OZScopes - The Australian Telescope Experts
When it comes to the polar alightment of equatorial mounts, there already exists a wealth of knowledge and information on the Internet.
What we've found, however, is a short supply of instructions for astronomers in the Southern Hemisphere - what do you do if you want to polar align your equatorial mount in Australia? Most how-to guides require you to center your telescope on Polaris, or the Northern Celestial Pole. Problem is, Polaris never rises in the Southern Hemisphere, so it never appears above the horizon!
Polar alignment to the South Celestial Pole (SCP) is a little more challenging as well. This is due to the fact that there is no very bright star close to it like Polaris is in the NCP (North Celestial Pole). There are various ways to polar align your telescope and for casual observing, the methods below are adequate enough and will get you reasonably close to the SCP.
This guide thus aims to guide amateur astronomers into the wonderful world of polar alignment to the Southern Celestial Pole.
So - Why Polar Align An Equatorial Mount?
Most amateur astronomers run home as soon as they get their first telescope, pop on an eyepiece, and point it up at the moon. The sheer excitement of being able to see the lunar landscape is enough to keep most going for days - eventually though, you'll want to have a look at planets and other faint, deep-sky objects - and this is where polar alignment is critical. Caveat though - for casual observing, only a rough polar alignment is needed i.e. just point it due North, and you should be good to go.
You really only need more precise alignment for tracking objects across the sky (especially at high magnifications), when you're setting circles to locate hard-to-find objects, and especially astrophotography.
When you're looking up into the sky, it does seem as if the planets, sun, moon and stars move across the sky.
So the theory is...
From the East to the West, it seems as all the celestial objects make a complete circle once every 24 hours. The fact is, however, that it's not the stars that are moving - it's us. The rotation of the Earth is what gives us the illusion that its the stars that are moving instead.
If you watch the skies for long enough, or if you set a camera up with an extremely long exposure, you will notice that it seems as if there's a point in the sky that doesn't appear to move. This point is called a Celestial Pole, and depending on which hemisphere you're in, you'll see either the North or South Celestial Poles. For the purpose of this guide, we'll be looking only at the Celestial South Pole.
Polar alignment relies on one theory as a fulcrum - that the polar axis of your telescope should be parallel to the Earth's axis of rotation.
This simply means that you should be pointing your telescope at a celestial pole so you can cancel out the motion of the sky by turning the axis (by hand/motor drive) at the same rate as the Earth's rotation. By aligning the mount of your telescope with the Southern Celestial Pole (SCP), you can counter the effects of the Earth's movement by rotating in the opposite direction. This enables you to take long exposure photographs on your mount without Star Trails in your picture. The most obvious benefit would be if you were viewing Jupiter, for example - it will stay in your field of view, and you won't have to keep moving the telescope.
So how do I actually polar align my telescope to the Southern Celestial Pole?
Image by Roger Groom.
As you can see in the above image, you're trying to keep rotating your telescope in the opposite direction to the Earth's rotation, so the sky stays still.
Getting it Done
To do this, you should first balance your scope (but not absolutely necessary):
1. Balance the Declination (Dec) Axis
2. Balance the Right Ascension (RA) Axis
And then you can start getting round to polar aligning your telescope in the Southern Hemisphere. There are several ways to do this:
Balancing your Telescope
Balancing the Declination Axis
1. Firstly, you'll need to set the scope on the equatorial mount, and balace it. Put your weight on the mount, then the scope.
2. Now, look at the mount - you'll notice two setting circles on it.
3. The one under the scope tube is knows as the Declination Setting Circle. There are also screws that hole the two major axes tight.
4. Ensure that these screws are tight, then loosen the screw that allows the telescope to turn.
5. Move the scope back and forth in its mounting rings, pivoting on the Declination (Dec) Axis, to the point that you can move it in any position, and it stays where you leave it.
6. Try to do this with an attached eyepiece and/or any other accessories attaches to your telescope, to get a more accurate weight balance.
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Balancing the Right Ascension Axis
1. Rotate the Right Ascension (RA) Axis in the opposite direction to the Earth's rotation. The RA is the axis your weight is screwed into.
2. Now, loosen the screw holding this axis - you want to adjust the weight so the scope can swing from side-to-side while staying in place where you leave it. Balancing the scope this ways saves wear and tear on your mount's gears, and allows for easy movement when looking for celestial objects.
3. Don't loosen the screws all the way up, just enough so that the scope moves with only slight resistance on both axes.
4. Typically, once you've balanced the RA axis, the weight needn't be readjusted. If you need to take the scope down, simply unscrew the weight (shaft and all), and store it as is.
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Polar Aligning your Telescope
1. For rough alignment, hold or mount a laster pointer on your telescope such that it's directly in line with your telescope's RA axis, pointing at the celestial pole.
2. Look along the laster beam, and move the telescope mount until your laser is pointing directly at the celestial pole.
3. Once roughly aligned, you can further refine the alignment by looking through the scope itself and ensuring that your laser beam is set on the celestial pole at medium power/magnification.
4. If you can align the laster well with yotu telescope axis and sight it through the scope as well, you'll achieve a very accurate polar alignment this way.
5. NB: Australia's actually banned the astronomical lasers that are required to perform polar alignment in this way. Although OzScopes does stock a small supply, you will have to fill out a form (downloads form) on the Victorian Police Website (or state equivalent), and contact us and we'll be able to organise to have one delivered to you. They're banned because these lasers are very powerful, and can cause permanent retinal damage if they're pointed at someone's eyes.
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Known Star Alignment
1. This method only works if you have a GoTo or Computerized telescope, or if you're very familiar with your RA and DEC setting circles and you own a good star atlas.
2. With known star alignment, you have to move your telescope according to where the known star is, according to RA and DEC.
3. You can then adjust the telescope until the star is in the correct place. Repeat alternating between a known star on the East Horizon, and a known star at the Meridian 3 (refer to your star map).
4. Doing this will result in a polar alignment accurate to about 1 degree.
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Polar aligning your telescope via drift alignment will render adjusting your RA unimportant - drift in the RA is just a case of your motor drive being too slow or fast, it's not an alignment problem.
1. Direct your telescope to a bright star that's low-ish (about 20 degrees) on the Eastern Horizon, near the Celestial Equator (i.e. 0 degrees DEC). The Orion nebula would be good to look at in mean time (given the right time of the year - as it may be too high or not visible).
2. At this point, the east/west rotation of your mount has little effect, leaving you to correct the angle of elevation of the mount.
3. Now, if the star drifts NORTH, your polar axis is too low - change the axis to angle further up in the sky (i.e. the latitude knob on your eq mount)
4. But if the star drifts SOUTH, your plar axis is too high - change the axis to angle further down toward land (i.e. the latitude knob on your eq mount).
5. You can tell if the star is drifting north or south by allowing the star to drift for a little bit - now, move your telescope to "catch up with the star" using the DEC control. If you see it heading north, then it's drifting north and vice-versa!
1. Now, point the telescope to a bright star on the Meridian and on the celestial equator i.e. right above your head somewhere. This is so the elevation of the mount (as corrected in the steps above) will have minumal effect, allowing you to accurately correct the east/west rotation of the mount.
2. Now, if the star drifts NORTH, your polar axis is too far EAST - so rotate your mount further West (i.e. rotate your mount clockwise while looking down at it).
3. Now, if the star drifts SOUTH, your polar axis is too far WEST - so rotate your mount further East (i.e. rotate your mount anti-clockwise while looking down at it).
1. Repeat these two steps a few times to increase the accuracy of your plar alignment - repeating Part 2 will allow you to get Part 1 more accurate.
NB: Important rules to make drift alignment easy:
a) Time your "drift times" to ensure that they're consistent - or proportional, at the very elast.
b) Determine star direction by moving your telescope to "catch-up" with the star - i.e. move it fast enough that you can tell which way you're moving the scope. You can slew it much further than necssary to be sure you're going either North or South, then go back to the star.
c) Try to watch and measure how far you move the star while adjusting for Alt and Az, and remain consistent.
d) The amount of time necessary for drifting is dependent on your telescope's Field of View. Try 5 munites and make it shorter or longer, depending on your equipment.
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Bear in mind that polar alignment will only be as accurate as you want it to be. If you want to take long exposure photographs, the alignment has to be very good or you'll see streak in your pictures. That said, if you don't plan to look at any particular object for an extended period of time, it probably won't really matter if the alignment's not exactly right.
In addition, as with most things - the process of polar alignment will become much faster and easier as your practice. Hopefully this guide was helpful - if you needed any additional assistance or clarification, you can contact us. Clear skies!