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Polar Alignment in the Southern Hemisphere
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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 and fork mounts. I don't have much experience with other types of mounts such as tracking platforms.
To skip all this info and just get the essential quick cheat sheet for polar alignment, get it here (PDF file).
Here's an image, and then I'll try to explain it and how polar alignment works.
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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 permenantly 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.
Techniques:
- Drift Alignment
- Known Star Alignment
- Polar Scope
- Laser Alignment
Drift Alignment is covered in detail further down this page. 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 one of the other alignment techniques to gain a rough polar 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.
Drift Alignment in detail
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 eq 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 eq 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".
- 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.
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You should repeat this procedure a few times to get a very accurate polar alignment. Doing step 2 may mean you can now get step 1 more accurate. Sometimes you will be making adjustments that are very minute, but important if you want to do astrophotography or anything like that and just plain helpful if you're doing viewing.
As a general rule, for the 4.5" newtonian: if the star stays within the centre 1/6th or so of the eyepiece for 30mins, I'm happy with it for doing mounted (piggyback) photography. This is subjective to what eyepiece you use ,etc. but for my one it is a Meade Illuminated Reticle eyepiece, which gives a magnification of about 70x in my telescope. You'll get used to how accurate you need to be.
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. |
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