Polar Alignment without Polaris

Polar Alignment without Polaris

An updated tutorial by Vince Legge

I am referring throughout to the use of a German Equatorial Mount (GEM).

You will need a magnetic compass, one that you can read down to 1° increments, a device for measuring angles, and a little basic handyman skill but we’ll get to that as we work through the technique.

Also you will need to determine the Latitude and Magnetic Declination of your location. These can be found here http://www.pangolin.co.nz/almanac/magvar.php, or also Latitude is commonly shown on most road maps, and in Atlases.

Overview:

The term Polar Alignment refers to the alignment of the Right Ascension axis of a mount so that it is parallel to Earth’s axis. This then enables the mount to track stars by offsetting Earth’s rotation. For many in the Northern Hemisphere this is usually achieved by aligning to the star “Polaris” (with date and time offset) but in the Southern Hemisphere we can’t see Polaris, and the equivalent is the constellation Octans. Unfortunately Octans is not easy to see and likely impossible from the Light Polluted skies of our major cities, but you can still accurately Polar Align without any stars at all. Here’s one way

Step 1

Find South by compass and lay the tripod so that the forward leg (usually labelled “N”) is facing South. At this stage just standing back with the compass and getting the tripod roughly in line is sufficient. Now level the tripod head. Here I am using an old carpenter’s level that I cut down so that it fits in my accessory case. Check level over three points.

PA1

Step 2

Here’s the part where you need to make up a simple tool (or get someone to make it for you). Here I have an aluminium bar screwed to the centre of the tripod head, and centred over the Azimuth adjustment lug. Note it has a scribed centre line and is about 600mm or 2ft long.

PA2

You don’t need to be as fancy as this. A nice piece of timber or even a two or three foot wooden ruler held in place with a weight will do the trick. Just make sure that you don’t use any ferrous metal, and the bar is centralised to both the tripod head centre bolt and the Azimuth adjusting lug.

Now you need to put a bit of good old boy scout skill to practice. Earth’s magnetic field is variable and the compass does not indicate the true line of North/South. Now you need to apply the Magnetic Declination for your location that you looked up earlier. Mag Declination will be described as a number of degrees either East or West. If it is East you need to subtract that number from 180°, if its West, add it to 180°.

For example the magnetic Dec at my current location is 11° East and therefore I set my compass to 169° which will therefore point me to true South. Simply move the tripod a little left or right until the compass is so aligned.

PA3

Step 3

Fit the mount head ensuring that the azimuth adjusting screws as indicated by the red arrow are centralised, and you’re halfway there. The mount is now reasonably parallel to Earth’s axis in one plane

PA4

Step 4

All that remains is to either elevate or depress the mount head to your Latitude. Note that there is likely a Latitude scale on the mount head for this purpose but I am advising you to forget it. Very rarely have I seen a mount that actually has an accurate latitude scale, in fact my EQ3 is out by 3° and my EQ6 by 4°.

Following are three different was that you can easily set Latitude accurately. The first is with a protractor and plumb line. I have fitted a string that is too thick to be practical simply so that it shows up in the photo. A 180° protractor is probably a better idea than the 360° version shown.

PA5

The second is using and engineer’s protractor, which has a spirit level in it.

PA6

And the third is using a digital inclinometer. Note that it doesn’t make any difference whether you align from the mount head saddle or the counterweight shaft.

PA7

Summary

Now your mount is polar aligned accurately enough for all night visual observing sessions, although you may need to make a minor adjustment in Dec occasionally. If you are using “go to” then now is the time to go ahead and complete a three star alignment. With a little practice you’ll find that you can align this way in only a few minutes. If you always observe from the same place then all you need to do to set up subsequently is mark the positions of the tripod legs on the ground. However if your goal is long exposure photography your initial Polar align will not be accurate enough and you will need to proceed from here to Drift Alignment Here are two excellent Drift Alignment tutorials http://www.andysshotglass.com/DriftAlignment.html http://www.petesastrophotography.com/ Any questions or points that I have not explained clearly enough? Feel free to ask.