Are you having trouble working out how to polar align your telescope in the Southern Hemisphere? Found that all the websites explaining the process are either written for the Northern Hemisphere or incorrectly converted to the Southern Hemisphere?
Well that’s what I found when I started astronomy, so now that I understand it fairly well I’ve tried to put together a good explanation here. One of my problems was visualising which axis (alt/az) I need to adjust when and how, to get good polar alignment. Also being in the southern hemisphere didn’t help, as most good explanations I found on the web were for the northern hemisphere, and I found it hard to convert the ideas to the southern hemisphere. This page is probably most helpful to those people with the German Equatorial Mounts (GEM) and fork mounts. I don’t have much experience with other types of mounts such as tracking platforms.
I teach various polar alignment techniques in my Astrophotography Workshops in Perth Westen Australia. Come along for an in-person demo.
To skip the general explanation on polar alignment scroll down to the Drift Alignment or other section below relevant to your set of equipment.
Here’s an image, and then I’ll try to explain it and how polar alignment works.
So as you can see, you are trying to match the rotation of the earth. Then keep rotating your telescope in the opposite direction to the earth’s rotation so that the sky appears to not rotate. To do this you’re rotating the RA in the opposite direction to the earth’s rotation.
You can see in the diagram that the mount of the telescope is pointing directly in line with the earth’s axis of rotation. I hope that maybe from this diagram you can see what it would be like if the angle of elevation of the mount was wrong, or the rotation was wrong, because it’d take pages of me going on and on and on to explain it in words. 🙂
So in polar alignment you need to get that RA lined up with the earth’s axis. To do this you need to get your telescope pointed up at the right angle for your latitude, and rotated east/west such that it is pointing directly in line with the axis.
If you don’t have an illuminated reticle eyepiece to use for alignment, I strongly recommend you get one. It will cut down the time taken to align your scope by a huge factor. It first took me over 1.5 hours to do polar alignment without the eyepiece, then with the illuminated eyepiece it took under 30mins for a much better alignment than I would have got in 1.5 hours before. I now will spend a couple of hours getting good alignment of the LX permanently mounted in my observatory but this is for a very high level of accuracy that I never attempted on the 4.5″.
Polar Alignment Techniques
There are several ways you can easily achieve polar alignment. All require some practice before you will be fluent in their use, but the right combination of techniques will save you plenty of time and headache.
- Compass Alignment
- Drift Alignment
- Known Star Alignment
- Polar Scope
- Laser Alignment
- DSLR assisted polar alignment
- Polar Alignment cameras and software
Compass Alignment is useful for wide field photography and initial alignment of telescopes to then refine the alignment with a more detailed approach later. Using a compass and knowing the magnetic deviation of your compass from celestial south/north at your location you can roughly align your polar mount to the celestial pole. It’s important to note this alignment method is rough, and so only useful for short exposures at wide angles (for example 2 minute exposures using a 17mm lens). This alignment technique is useful for portable mounts like the Astrotrac and Vixen Polarie. The following basic procedure tends to work well for me:
Tools you need:
- Inclinometer (I use my iPhone with the Measure app, or Clinometer app, but you can buy from Bunnings and tool stores dedicated inclinometers)
- Set up your tripod with mount and camera.
- Level the tripod as best you can to perfect level. Some tripods have a spirit level built in to assist with this.
- Position your mount roughly facing the celestial pole and angled up at the approximate altitude (your latitude).
- Stand back from your tripod/mount, hold your compass out in front of you, and sight “through” the mount to the celestial pole beyond. Position yourself so that the tripod/mount is directly in front of you and celestial pole beyond.
- Look at the rotation of your mount as you hold the compass in front of you and then rotate the mount to better face the celestial pole. Repeat this and sighting “through” the mount until you are comfortable with the alignment.
- Position your inclinometer against a flat surface of the mount which is parallel to the RA axis of rotation and adjust your mount’s elevation to a reading on the inclinometer which matches your latitude.
Known Star Alignment
Known Star Alignment only works if you have a GoTo scope or are familiar with your RA and DEC setting circles and have a good star atlas (computer or paper). The theory here is if you move your telescope to where the known star should be according to RA and DEC, you can then adjust the telescope in Alt and Azimuth 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 or more times, adjusting only Azimuth at the East and only Alt at the Meridian. Doing this can result in a polar alignment that is accurate within about 1 degree.
Polar Scope’s are very useful if you have them. If you can install one on your mount it might be a good investment. Given a good southern (or northern in the Northern Hemisphere) horizon to be able to see the stars that your polar scope utilises you should be able to align quite accurately with the celestial pole. The accuracy will be determined by the alignment of your polar scope and the accuracy of your polar scope and mount.
Laser Alignment is really just an alternative to the Polar Scope if you don’t have one. What you can do is hold or mount a laser pointer on your telescope at a place where it is pointing directly in line with the telescopes RA axis, pointing at the celestial pole. You then look along the laser beam and move the telescope mount in Alt and Azimuth until the laser is pointing roughly at the celestial pole. Once roughly aligned you can refine the alignment using this method by pointing your telescope at the celestial pole also, looking through the scope at medium power/magnification and sighting the laser beam, checking that the laser beam is pointing at the celestial pole. If you have your laser aligned well with your telescopes axis, and sight it through the telescope at medium power, you can achieve a very good polar alignment using this method.
This method requires reasonable time to perform 30mins – 1hour but achieves a very accurate alignment. I would usually choose to do this once I have already performed a compass and inclinometer alignment to gain a rough polar alignment.
While drift alignment may seem a bit “old school” understanding drift alignment in detail gives a good foundation on which to understand all other forms of polar alignment. The other techniques all tend to build upon the basics of Drift alignment.
The explanation of how to polar align using the drift method is as follows. In this procedure, adjusting the RA isn’t important, just leave it, drift in the RA is just a case of your motor being slow/fast, not an alignment problem.
- Point the telescope to a bright star low-ish (20 degrees say) on the eastern horizon, near the celestial equator (the line of 0 degrees DEC, something like the Orion nebula is fairly close, and good to look at in the mean time (given the right time of the year – it may be too high/not visible.) At this point the east/west rotation of the mount has minimal effect, leaving you to correct the angle of elevation of the mount.
- If the star drifts NORTH your polar axis is too low. So you have to change the axis to angle up into the sky more. (the latitude knob on your equatorial mount.)
- If the star drifts SOUTH your polar axis is too high. So you have to change the axis to angle down into the ground more. (the latitude knob on your equatorial mount.)
What do I mean by “drifts NORTH” and “drifts SOUTH” ? In the field of view or what??. Well this is what got me stuck for ages. The way I do it now, is you let the star drift for a bit, you then move the telescope to ‘catch up with it’ by using the DEC control. If you find the telescope is heading north, then the star is drifting north! 🙂 and that’s what they mean by “drifts North”.
- 2. Point the telescope to a bright star on the Meridian and on the celestial equator. So this is basically straight over your head somewhere. This way the angle of elevation of the mount (corrected above hopefully) will have minimal effect, leaving you to just correct the east/west rotation of the mount.
- If the star drifts NORTH your polar axis is too far East, so rotate the mount west. (Rotate clockwise looking down on the mount).
- If the star drifts SOUTH your polar axis is too far West, so rotate the mount east. (Rotate anti-clockwise looking down on the mount).
Again, What do I mean by “drifts NORTH” and “drifts SOUTH”? Well even though we are testing the rotation not the angle of elevation, it’s the same as I described above basically.
As a general rule for the LX200 mounted in my observatory: If the star drifts at all within 10 minutes then I make corrections otherwise I am usually happy to leave it as is. In this setup I use the 12″ at F/10 (3048mm) with the Meade illuminated eyepiece resulting in a magnification of 254x. Note that this is a much higher level of accuracy than what I attempt to gain with the 4.5″. This is because of the limiting accuracy of the 4.5″ mount and because of the type of work I attempt on each telescope.
DSLR / Camera Assisted Polar Alignment
I have a procedure for this in my workshop course notes, yet to translate to here.
This approach uses your DSLR or similar camera to measure the amount of drift seen over a period of time, essentially a quicker and more accurate equivalent to drift alignment.
Polar Alignment cameras and software
I have a procedure for this in my workshop course notes, yet to translate to here.
In reality these days, with a modern telescope, this method is the most efficient and easy, once you have the right combination of hardware and software configured and functional.
This approach is used by products such as:
- SharpCap (using a CMOS guide camera on a small guiding scope/refractor.
- ASIAir Plus (using your main imaging camera)
- iOptron iGuider (using it’s built in camera and software installed on a PC or Mac
The common approach with these tools is to:
- Take a photograph of the sky
- Plate solve (match to a database of stars) that image to know it’s position and geometry
- Rotate the mount by an amount, often 60 degrees
- Take another photograph of the sky
- Plate solve that image
- Suggest Alt/Az refeinements to make
- Repeat steps 4 through 6 until you have refined the polar alignment.