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Basics of Drift Alignment / Polar Alignment
Submitted: Friday, 15th June 2007 by Geoff Smith

Polar alignment is essentially a simple process that shouldn’t scare anyone.  The aim is to get the polar axis of your mount parallel to the Earth’s axis.

This is done in a two step process:

  • Rough Alignment
  • Fine tuning the alignment

Rough Alignment

You need a compass and maybe some leveling tools, depending on the nature of your mount.  Some have levels built-in, others don’t.  So start by leveling your mount.  This is not actually necessary, but it does make things a bit easier, as will be explained below. Don’t fuss overmuch here.  Near enough is good enough.

You need to make two adjustments when polar aligning.  First, the polar axis has to point N-S (the azimuth adjustment). Secondly, since we are hanging off the side of the Earth, the celestial pole is about 33 degrees above the horizon in Sydney, so we have to make an altitude adjustment.  The altitude of the celestial pole is always equal to your latitude.

Rough altitude adjustment

Most mounts have some sort of latitude scale.  Set the adjustment to be equal to your latitude and you are done.

Rough azimuth adjustment

With a tripod, probably the easiest way to set it up is to orient a long piece of wood in the E-W direction.  You put two legs of the tripod against these, so that the third leg points S.  To do this you need

I’ll say a few words about magnetic declination.  At the time of writing, this is +12.558 degrees for Sydney, so it has a positive sign.  This means that the compass needle points 12.558 degrees E of true N.  If the value for your location has a negative sign, then the compass needle points W of true N.

There are too many compass designs to go into detail here, but the setup you are aiming for looks like this (with the magnetic declination D=12.558 for Sydney):

image1.jpg

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Setup for Sydney

If you don’t use a tripod, for example if you use a pier, then you have to be a little more inventive, but the idea is still to establish true N and point the polar axis in this direction.   Once again, don’t spend too much time here.  Getting within a couple of degrees is probably good enough.  There should be enough adjustment in the mount to take care of any minor inaccuracies.  Once you are roughly polar aligned, it is time to fine-tune your setup.

Fine tuning by drift alignment

In order to drift align, you need an eyepiece with cross hairs and an equatorially mounted telescope.  Altazimuth mounts, driven or not, don’t qualify for drift alignment.  The purpose of drift alignment is to get the polar axis of your mount parallel to the Earth’s axis. 

Step 1: Identify the directions in your eyepiece

First identify the directions in your eyepiece. Start with the West (or Preceding) direction. This is the way the star drifts in RA when you are not running the motor. Directly opposite to this is the East (or Following) direction. Alternatively, you can nudge the scope W and the star will move E.  So this disposes of E and W. 

What about North and South?  This depends on how many mirrors are in your optical train.

  • A Newtonian will have at least two (primary and secondary) and possibly a third (star diagonal).
  • A Cass or SCT will usually have three (primary, secondary, star diagonal)
  • A refractor will usually have one (star diagonal).

If you have an odd number of reflections (1, 3, etc), the field in your eyepiece will look like the left hand diagram below, while if you have an even number of reflections (0, 2, 4, etc) the field in your eyepiece will look like the right hand diagram.

In both cases, use the memory aid “We Never Eat Scraps”.  You can change “scraps” to something else if you prefer. With an odd number of reflections, you start at W and move around anticlockwise round the eyepiece. With an even number of reflections, you start at west and go clockwise around your eyepiece.  As a double check, you could nudge the scope N and the star will move S.

Now that you know where N, S, E and W are in your eyepiece, you can move onto the next step.

image003.jpg

Click to Enlarge
Left image for Odd number of reflections, Right image for Even number of reflections

Step 2: Line up your reticle eyepiece

Select a star, any star, and rotate your reticule eyepiece into such a position, that when you use the slow motions or hand paddle of your mount, the star follows the cross hairs.  In other words, line up the cross hairs of the eyepiece to point N, S, E and W.

Step 3: Adjust the altitude of the mount

Select a star in the West near the celestial equator (that is, with declination near zero) and no more than 30 degrees above the horizon.  Centre the star and watch to see if it drifts N or S.  If the star drifts N, lower your altitude, if it drifts S, raise the altitude.

Alternatively, you could look E to adjust altitude if this is more convenient.   In this case, if the star drifts S, lower your altitude, if it drifts N, raise the altitude.
 
This is all very well, but how do you remember it?  Well, it turns out that the best way is not to remember anything.  You just have to understand what is happening, so that you can just work it out from scratch each time by looking at the sky and visualizing what’s going on. Look at the following two diagrams.  As you look W, the left hand picture shows what happens if the star drifts N in your eyepiece and the right hand one shows the star drifting S in your eyepiece.

image004.jpg

Click to Enlarge
Left image: Star drifting N in the eyepiece. Right image: Star drifting S in the eyepiece

The picture should make it clear that to make the tracks of the mount and star coincide, you have to lower the altitude of the mount in the left-hand case and raise it in the right-hand case.  If you raise your arms to the sky in imitation of these tracks, it should be easy to see what you have to do to adjust the mount.

As an aside, when we talk about the star drifting N, what really happens is that your scope drifts S.

Step 4: Adjust the azimuth of the mount

Now pick a star on the meridian near the equator. If the star drifts N, turn the mount clockwise as viewed from above. If it drifts S, turn the mount anticlockwise as viewed from above. 

Here again, it is best to work things out from scratch each time.  Have a look at the diagram below.  It shows a star drifting S (actually the scope is drifting N).  If you were lying under the mount, you have to turn the mount clockwise as viewed from your position to make the tracks coincide.  However, since you are probably looking down at the mount, you have to turn it anticlockwise.  Again, holding your hands in the air to simulate the tracks of the mount and the star should make it easy to see which way to turn the mount.

image006.jpg

Click to Enlarge
Looking up, star drifting S
Step 5: Repeat steps 3 and 4

Repeat steps 3 and 4 until you get no drift when you swap from one to the other.  How exact you have to be here depends on what you are going to do.  For imaging, you should get no drift for at least 10 minutes.  For visual work, a couple of minutes with no drift is probably good enough. If your mount is not level, you will have to repeat steps 2 and 3 more times, so it helps to have the mount nearly level at the start.

Step 6: Relax! 

You should now be within a mosquito’s whisker of the pole.

Article by Geoff Smith (ghsmith45). Discuss this article on the IceInSpace Forum.

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