Southern hemisphere drift alignment
Fork mounted telescope on an equatorial wedge.

Introduction

Whilst it is not the most comfortable for observation, polar alignment is necessary for accurate astrophotography.

Many amateur astronomers, (me included) have been confused on drift aligning for years. There is so much text out there that assumes you know so much. Hopefully this step-by-step tutorial will help clear things up for some of you.

My particular telescope is a Nexstar 11 GPS however this tutorial is relevant to all fork mounted telescopes. The basic steps can also be translated to the movements of equatorial mounts.

This tutorial will use two methods (In order) to achieve a perfect alignment.

1. Basic Polar Alignment (South). Must be done first.
2. Drift Alignment. (Fine tuning)

A few things to know first

Right ascension. (RA)

This is the motor drive and movement of your telescope that rotates the telescope and its fork arms left and right or from north, around to south, east and west. When set up flat and level without a wedge, it is known as azimuth. (AZ)

Declination. (DEC)

This is the motor drive and movement of your telescope that moves and points the outer tube assembly (OTA) up and down. That is, in an arc rising on any angle from the horizon to the opposite horizon, not necessarily passing through the Zenith (Straight up).

When set up flat and level without a wedge, it is known as altitude. (ALT) In ALT mode it ALWAYS passes through the Zenith on its traverse from horizon to opposite horizon.

*Handy hint* Put a sticker on the RA and DEC drive points marked with big black marker to help you remember until it becomes second nature.

The Meridian

This is an imaginary line that passes from the north horizon, straight up and overhead to the Zenith, then back down to the southern horizon. This is the same no matter where you are on the planet. There is no line in the sky nor is there a star positioned on this line to mark it. The stars always move, the line over your head stays there. Pointing your telescope tube up, using the RA (Right ascension, left-right) drive, places you somewhere on the meridian. It doesn’t matter what angle your telescope is at in DEC.

The Celestial Equator

This is the line that the stars due-east on the horizon, travel overhead and back over to the west. This line would be straight up if you lived on the earth’s equator. For us here in the southern hemisphere, that line starts due-east on the horizon, but instead of passing straight overhead, it arcs on an angle towards the northern sky before heading back to the west. See the procedure and pictures for drift alignment west to get a better picture. It becomes very simple.

South Celestial Pole

First and foremost, you need to be able to find the SCP easily. If you can’t identify it, then you can’t do the rest that follows. Now this can be difficult to find as we southerners don’t have the luxury of a bright polar star like Polaris in the north. Forget alignment for now. Once you know where it is and what it looks like in any rotated position throughout the night, it will be a breeze and you will never forget.

The SCP is roughly just over halfway from the Southern Cross to the Large Magellanic Cloud (LMC). It can also be found by drawing a line along the long axis of the Southern Cross roughly four and a half times its length. From there, you are in the right area. Then all you have to look for is the Chinese hat and Little Corona. The SCP is right next to them. (See diagram)

If you still have difficulties, get someone to show you one night. Try your local astronomy club. Use binoculars. Print maps off the internet. Persist. Just find it!

Below are a couple of maps I have drawn to help you. There are many more available on the internet if you want to be more accurate, but these should do. (These are simple)

Illuminated reticule eyepiece

This will make the job much easier. If you do not have one, then a normal eyepiece with crosshairs can suffice on a night where the sky is illuminated with the moon. If you live in a light polluted city, then you may already have enough ambient light in the eyepiece to be able to see the crosshairs without any help. Alternatively, you can place a small lamp inside the rim of your telescope aperture. This will enable you to see the black crosshairs against the dark sky.

Basic Polar Alignment

Ok. You have found the SCP. What’s next?

First things first. Set the wedge up on your pier or tripod and get the base plate perfectly flat and level. A good wedge should have a bubble spirit level built-in. If not, there are cheap ones at your local hardware store.

To get the wedge close to polar aligned, you need to work on one direction at a time. Drift alignment comes later.

We will start with basic east-west alignment, (Left-right)

Once you have identified the south celestial pole and its whereabouts, set your telescope up on its wedge, pointing south as accurately as you can by eye. Make sure the bolts that hold the wedge down are loose enough to allow you to move it. Draw a line out from the pointy end of the Southern Cross approximately four and a half times its length and then drop straight down to the horizon. That is south. Stand behind your telescope and wedge and rotate the wedge to point south.

*Note* Don’t use a compass. Magnetic south is totally different to celestial south and it moves every year!

Wind up your wedge to the angle of your latitude. E.g. Canberra is -35 degrees below the earth’s equator, so the wedge needs to be wound up to 35 degrees above the horizon.

*Note* Some wedges have cheap stickers with the increments marked on them. They are useful as a guide, but cannot be considered gospel as stickers are inclined to move.

Rotate your telescope in RA so that the fork arms are side by side horizontal, pointing up on the angle to the SCP, with the telescope tube pointing in the same direction.

Looking through the finder scope to acquire and the eyepiece in turn to fine tune, move the WEDGE LEFT AND RIGHT ONLY to bring the vertical centre line in your eyepiece to the SCP.

*Note* You can move the tube in DEC to bring the SCP into the field of view, but this has no bearing on alignment yet.

Next, we move on to basic north-south alignment, (Up-down)

Rotate your telescope in RA so that the fork arms are one above the other vertically, pointing up on the angle to the SCP, with the telescope tube pointing in the same direction.

Looking through the finder scope to acquire and the eyepiece in turn to fine tune, move the WEDGE UP AND DOWN ONLY to bring the horizontal centre line in your eyepiece to the SCP.

*Note* At this point, if you move the tube in DEC to the previous east-west centre line onto the SCP, you can assume that the tube is now parallel with the fork arms, and you may want to mark a line across the DEC pivot joint. It comes in handy for next time.

That’s it! The telescope is now pretty well aligned. You can go back and repeat both procedures to confirm that you haven’t knocked it out with all your wiggling if you wish.

To tune up on this and get it perfect for astro-photography, drift alignment is the next step.

Drift alignment

Now that the wedge is in the ball park, it’s time to fine tune

Take it one step at a time. It is time-consuming, and there is no getting around it. Don’t worry, this gets easier (and quicker) as you do it.

East-west drift alignment

Position the tube at right-angles to the fork arms. Now watch and rotate the telescope in RA from the east horizon, all the way to the west horizon. This line traveled is the celestial equator.

The imaginary line that travels from the north horizon, straight up and overhead to the Zenith, then back down to the south horizon, is the meridian. With the tube at right angles to the fork arms and RA rotated so that the tube is pointing upwards, puts you right on the intersection of the meridian and the celestial equator. This point in the northern sky is where you want to be for an east-west drift alignment.

Pick a star near this intersection. It doesn’t have to be right on it, but the closer, the better. You can use a little bit of DEC to acquire one if needed.

Switch on your telescope so you can use the direction drives in RA and DEC, but do not engage any automatic tracking.

While looking through the eyepiece, use the RA drive and wiggle the star left and right. Rotate the eyepiece in its holder so that the horizontal crosshair is parallel with this movement.

Using the motors in RA and DEC, centre the star anywhere along the horizontal crosshair. Now watch and wait. You will notice the star will drift along the horizontal crosshair. If the alignment is good, the star will stay on the horizontal crosshair all the way. When the star drifts all the way off the eyepiece, bring it back on using RA ONLY. Keep repeating this process for ten to fifteen minutes. If the star is still traveling along the horizontal crosshair after this time, you east-west alignment is spot on.

If the alignment is bad, the star will also drift off the horizontal line as it travels along, either above or below it. If it is really bad, you will notice big drift in the first pass.

Now read carefully. Remember this….”UPRIGHT!”

If the star moves UP above the horizontal line as it travels sideways, carefully ROTATE the WEDGE so that the star moves RIGHT IN THE EYEPIECE.

If the star moves DOWN below the horizontal line as it travels sideways, carefully ROTATE the WEDGE so that the star moves LEFT IN THE EYEPIECE.

You can do fairly bold adjustments to start with, getting finer as drift away from the horizontal crosshair becomes less apparent.

North-south drift alignment

Position the tube at right-angles to the fork arms as you have done previously. Again, watch and rotate the telescope in RA from one horizon all the way to the other. This line traveled is the celestial equator.

Rotate the telescope in RA toward the eastern horizon, (where the stars are coming up) and pick any star along the celestial equator roughly 20 degrees, (2 Hand spans) above the horizon. Pick a nice clear one. Again, it doesn’t have to be exactly on the equator but the closer, the better. You can use a little bit of DEC to acquire one if needed.

Switch on your telescope so you can use the direction drives in RA and DEC, but do not engage any automatic tracking.

While looking through the eyepiece, use the RA drive and wiggle the star left and right. Rotate the eyepiece in its holder so that a crosshair is parallel with this movement. You probably shouldn’t have to move it at all, considering you have just rotated the telescope along the equator to get to this point.

Using the motors in RA and DEC, centre the star anywhere along this crosshair. Now watch and wait. You will notice the star will drift along the crosshair. If the alignment is good, the star will stay on the crosshair all the way. When the star drifts all the way off the eyepiece, bring it back on using RA ONLY. Keep repeating this process for ten to fifteen minutes. If the star is still traveling along the crosshair after this time, you north-south alignment is spot on.

If the alignment is bad, the star will also drift off the crosshair as it travels along, either above or below it. If it is really bad, you will notice big drift in the first pass.

Now fixing this one is EASY.

If the star moves away from the crosshair as it travels along it, carefully RAISE OR LOWER the WEDGE so that the star moves back onto the crosshair.

It’s that simple.

As before, you can do fairly bold adjustments to start with, getting finer as drift away from the crosshair becomes less apparent.

From here you can go back to the east west drift alignment once more if you wish, just to confirm that you haven’t bumped or moved anything on the way. Don’t forget to tighten all your bolts so that nothing moves again!

You are now done! Happy astrophoto’s!

Conclusion

Hopefully, this tutorial has slowed down some of the more overwhelming instructions available out there. I know I struggled with the concept until I got out there and gave it a go. An understanding of the movement and tilt of the Earth stars and your position on the surface of a round, spinning ball is helpful. Drawing can help if you have trouble visualizing.

Print this tutorial off, set up your telescope with the view of aligning rather than hurrying to observe or photograph. Take your time and go one step at a time.

I pray this helps and you come to understand that this complex issue can be very simple and you can move straight on to imaging without any undue complications.

Questions?

If you have any further questions, please do not hesitate to ask, using the A.S.I.G.N. observatory contact page. Similarly, if you see a quicker or easier way to do this, please let me know!

God bless,

Barry Armstead
A.S.I.G.N. Observatory
Canberra, A.C.T.