Polar Alignment

Finding Polaris
[title type=”modern-h2″ align=”center”]Polar Alignment[/title]

Polar alignment is a phrase that sends shivers down the spine of any newcomer to astronomy. For those who have yet to get engaged in this activity it essentially refers to the act of aligning one of the axes of the telescope, the polar axis, so that it points in exactly the same direction as the axis that the Earth revolves around. The reason astronomers get bothered about this is that it makes following objects across the sky much easier.

Its of minor benefit to the visual observer but a necessity to the astronomer trying to take images of the night sky. Once a telescope is polar aligned and an object centred in the eyepiece, then assuming a motor is attached to the telescope, the object will stay centred. The better the polar alignment, the longer it will stay there. If no motor is attached then simply nudging the telescope around one axis only will bring the object back to the centre of the eyepiece again.

A little about telescope mounts then before looking at how to polar align. Mounts come in two broad categories, alt-azimuth and equatorial; an alt-azimuth (often referred to as alt-az) mount simply has an up and down (altitude) and left-right (azimuth) motion. A standard camera tripod is considered to fall into this category. It is not possible to polar-align this type of mount however it is possible to apply motor’s to both axes with some whizzy electronics to drive the telescope to track objects across the sky. Systems like this can be shop bought but they do have their limitations. The main problem relates to the image you see through the eyepiece which will slowly seem to rotate as the object moves across the sky. This isn’t a problem for visual observers but for those wishing to take images through the telescope, this is a big but not insurmountable problem.

The equatorial mount also has two axes but it is possible to tilt the whole lot to an angle that equates to your latitude. The result of this tilting is that one of the axes must be pointing in exactly the same direction as the Earth’s axis of rotation and its this process that we refer to when talking of polar alignment. The beauty of this is that just one axis needs a motor to follow objects across the sky and the image doesn’t appear to rotate in the eyepiece as the telescope ‘rotates’ with it.

The two mounts on the equatorial mount are termed as the Declination and Right Ascension axis. Its the Right Ascension axis (we’ll call it the polar axis from now) which must now be aligned to the Earth.

There are two steps in polar alignment, rough polar alignment which is fine for visual work and precise which is essential to make imaging easy and stress free. Before starting, be sure that your finder telescope is aligned to the main telescope [LINK]

Rough Polar Alignment

The object of rough polar alignment is to get the mount setup so that it is approximately aligned, this will generally be enough for visual work but is also a necessary step to make the precise polar alignment phase easier.

Identify your telescopes declination axis and note there is a scale around it. Rotate the telescope about this axis until it’s at 90 degrees. It should now be pointing along the polar axis. You now need to adjust the angle of the whole mount so that it is the same as your latitude and for that, equatorial mount’s have a latitude scale so the next step is to adjust that to equal the latitude of your observing site. Your telescope should be reasonably well setup now to get roughly polar aligned.

Identify Polaris, the pole star, in the sky which is easy to find by following the two pointer stars in the bowl of the Plough (part of the constellation Ursa Major).

Finding Polaris

Now move the entire telescope mount, telescope and all so that the tube is pointing toward Polaris. If you have setup the mount described above then Polaris should be visible in the field of view of an the finder telescope. Now you just need to make minor adjustments to the mount by adjusting its left-right position and your latitude setting to centre Polaris. Assuming your finder telescope is aligned well, then you should now see Polaris in the field of view of an eyepiece.

Your telescope is now roughly polar aligned and will be enough for visual observing, if you are hoping to take images through your telescope then you will need to move on to precise polar alignment. If visual is the key, don’t waste your time on the next step, just start enjoying the sky.

Precise Polar Alignment

Assuming your goal is long exposure imaging then it is well worth spending time getting your mount precisely polar aligned. The first time you do it expect to spend an hour or two fiddling around but you will soon get it down to a fine art. Time spent here is very worth while and the benefits will be gained in the quality of your final images. The technique relies on observing the drift of stars through the eyepiece and slowly fine tuning your polar alignment.

You need an extra piece of equipment to perform this task, an illuminated reticule eyepiece. This is an eyepiece that has either a cross etched into the lens or thin wires forming a cross. These are then illuminated by a faint bulb inserted into the side of the eyepiece. They can be bought from most astronomical suppliers.

To start, identify a star that is roughly due south, or preferably a little to the left of due south and within 5 degrees of declination from the celestial equator (the celestial equator is the extension onto the sky of our own equator). Centre this star in the field of view of the telescope so that it lies on the illuminated cross. Now using the slow motion controls of the telescope, move it east and west in right ascension. Adjust the illuminated eyepiece so one axis of the cross follows that line and the star moves slowly back and forth along it. With the motor running, observe how the star moves, ignoring any left – right movement, just looking at up and down.

if it moves down, the polar axis of the telescope is too far to the west (or to the left of Polaris)
If it moves up, the polar axis of the telescope is too far to the east (or to the right of Polaris)

Using the mount’s azimuth adjustments, make appropriate changes to correct. Now recenter the star and perform the same step again. It will take a few goes but eventually there will be zero drift up or down for a good 5 minutes or more.

Now find a star near the eastern horizon, ideally it should be 20 degrees above the horizon, no more and fairly close to the celestial equator. Center it on the cross like before and align the cross so that east-west right ascension movement takes the star along one axis of the cross. Now monitor the star (with the motor still running).

If it drifts down, the polar axis of the telescope is too low
if it drifts up, the polar axis of the telescope is too high.

(note if you don’t have a good eastern horizon, a star in the western horizon will work fine but the notes above regarding adjustments required will need to be swapped)

Make adjustments to the elevation of the polar axis as appropriate, re-center the star and check the drift again. Once drift is eliminated, go back and check with the star due south and once that has been rechecked, your telescope will be very accurately polar aligned and objects will stay in the centre of the eyepiece or more importantly your camera.

Take the time to really nail this and get it accurately polar aligned and with some practice you will achieve alignment with quite incredible accuracy. If you want to increase your accuracy and the speed of the process, insert a barlow lens (which usually doubles or triples the magnification of the eyepiece) so drift will be noticeable much quicker. You will also be able to polar align to a greater accuracy.

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