Polar aligning equatorial telescope - 1 image attached

[William Bottaci]

I've been 'asked' to provide practical details of a method of polar alignment, specifically that of Drift Alignment, and like all the other methods it's one way of aligning the telescope polar axis (PA) with the north celestial pole (NCP). Clearly applicable to any type of equatorial mount as opposed to any Alt-Az mount.

This method has advantages, such as accuracy and when the NCP region of the sky is not visible, though there is a trade off in that it can take a while, but very suited to aligning a permanent installation. However even when there is just a few minutes now and then the alignment can be progressively improved during a single observing session; better than nothing and often good enough.
It's also a method that can be used regardless of design and is independent of itself so requires nothing else such as equipment or information, apart from an eyepiece with cross-hairs, preferably with a high magnification (the higher the quicker the operation).
As any 'telescopic' star, that is not bright, can be used, this can be done in dark twilight prior to observing.

It requires that you let the telescope track on two different stars in two specific locations in the sky. Seeing how the star drifts relative to the horizontal crosshair tells you how far the mount is offset from true and in which direction.

I've attached some notes I made about 8 years ago at Kenley, it is designed to serve as an aide memoire and depending on previous knowledge and experience may be all you need but below are further notes that help explain the method, an understanding.

Imagine a simple case where the PA, regardless of design, is perfectly aligned, that is with the NCP; everything will stay in view without drift, if driven, else only need to 'catch up' by moving the telescope westwards.
Now imagine in the simplest case the four situations where the PA is pointing either too high or low, to the east or west, that is perfectly in line vertically or horizontally, not any mixture of the two.
It helps to greatly exaggerate when imagining the situation.
Any star will drift and eventually out of the field. We use the direction of drift to tell us in which direction the PA is off, and therefore we adjust it the other way by a certain amount.

First we need to be convinced, is there enough information to do the job? There is, and you can either see this for yourself to take it on trust, but by the end of this explanation it should be obvious.

There are four situations to consider, but as they are in pairs (opposites) there are really just two. Either the PA is miss-aligned vertically or horizontally, with the specific direction being a detail, because one is just an opposite of the other.

Some information beforehand, before we perform the steps, because this is general to all possibilities:-
When a star is chosen it doesn't need to be bright or visible to the naked eye, just find a specific star in the eyepiece. For all stars they need to be on or close to the equator.
For southern meridian stars align the crosshairs vertically and horizontally, for east or west stars then align so that the star roughly moves along one of the crosshairs. Place the star close to the centre, no need to be exact.
We observe the star moving westwards closely parallel to the horizontal crosshair.
We realise that if the PA is not properly aligned this method involves the star drifting either up or down, in declination. So this is also known as the 'Declination Drift' method. Clearly the star drifting sideways is not going to be a lot of use to us.

We should also be aware that this explanation does not consider the orientation of view of the eyepiece, for this explanation the view is as it is terrestrially. Once the process is understood it is trivial to adapt the direction to suit the eyepiece, diagonal, and any mirrors or lenses in the path - after all we only need to know if the star is genuinely moving up or down.

As the star drifts whilst the telescope is undriven that star is expected to move/drift only westwards.

We take the simplest case, because any complication is unnecessary.
So to start, it doesn’t matter which is chosen first, but say the PA is pointing too low. We observe a star as close to the southern meridian and equator as convenient. It is easy to imagine that the star is still only going to move westwards, clearly this combination is inappropriate (the PA being either too high or low), so we either use a star to the east or west, or consider that the PA is instead to the right or left, which will be the case now.

A PA so left or right miss-aligned will effectively rotate the sky, so the star will drift either up or down. Similarly for an east or west star the sky effectively rotates if the PA is too high or low, also resulting in a vertical drift.
The direction of the star, up or down, indicates the direction of miss-alignment.

Now we have an indication for the direction for the error, so we adjust the PA accordingly. As for the measure, the extent, this we will unsure to begin with but with experience we improve, so move by not much.

For aligning the crosshairs, in the first instance view the southerly star as it’s easier to gauge the vertical and horizontal, then when moving to the east or west star the telescope should be naturally oriented correctly, as the tube will automatically rotate by the mount.
If the star is still drifting up or down then this can only be due to misalignment of the PA.

Taking into account any diagonal, mirror, lenses etc, to illustrate with a simple case, the specific example of a refractor, remove the diagonal as it is unnecessary for the star that is low, or at least not high. The view is then inverted so swap up for down and vice versa, the east/west movement is irrelevant.
For a Newtonian it matters in which direction you are orientated with the tube so some experimentation will be required.

I hope this helps, William

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