How to polar align a computerised equatorial mount in the Southern Hemisphere (without a polar scope)

This guide is suitable for all telescope owners of Synta computerised go-to EQ mounts, such as Sky-Watcher and saxon.

In a previous blog, I explained how to polar align a simple manual equatorial mount. However, the benefit of a computerised EQ mount is its much higher level of accuracy. It’s definitely worth the extra effort to get it bang on. The following guide will help users of Synta telescope mounts such as the EQ3 go-to, EQ5 go-to, HEQ5, NEQ6, and even the EQ8 to accurately polar align the telescope to reap the benefits for astrophotography and visual observation.

There’s a video on the Optics Central YouTube channel that shows me actually doing all this in the dark with my NEQ6-Pro.

I’ve been asked by a couple of people now about how to get a good polar alignment on a go-to mount (like an EQ3 go-to). These mounts do not come with a polar scope (although you can get one as an accessory if you want).

Other people who have equatorial mounts that do have polar scopes have a similar problem. Finding Sigma Octans in the polar scope is difficult, even in a dark sky site. In light polluted cities such as Melbourne it’s a real challenge. Yes, I’ve done it in my back yard in Kew, and it took about 45 minutes of swearing.

The method I’m going to describe here is good enough (after a few loops – see below) to get you to within 10 arcminutes of the South Celestial Pole (SCP). Other methods are more accurate, but one sixth of a degree is normally good enough for a couple of minute’s photographic exposure without rotation becoming apparent.

One special advantage of this method is that it will get you on the pole, even if you can’t actually see it.

1: Rough alignment

There are a couple of ways of doing this, either one of them is perfectly good to set set up and pointed roughly at the SCP.

Sigma Octantis

Sigma Octantis (σ Oct) is as close as the Southern Hemisphere has to a pole star. Unfortunately, as mentioned above, it’s notoriously dim. However, with a bit of practice, you can use your binoculars to find it within a minute or so. This is a way I’d definitely recommend, although if you can’t actually see the SCP this won’t work for you.

The photo below shows the area, along with the well-known method of finding south from the Southern Cross and the pointers. The photo was difficult to process so you can see all the stars you need to but still have the background black, so you might have to refer to Stellarium or some other program as well. As you can see, finding the pole just using the Cross gets you a bit to the left of where you want to be. There are three triplets (which I’ve circled) that you can use as road signs to help you “star hop” to where Sigma Octantis.

Sigma Octans from the Southern Cross plus a few signposts

Compass and inclinometer

The most reliable fallback method is to use a compass to figure which direction is true south, and then an inclinometer to set your polar axis  to your latitude. You need to know your local magnetic variance, which (for Melbourne) means that true south is about 10.5 degrees to the left of magnetic south. Melbourne’s latitude is 37.6° South, by the way.

This is the method I went through in the previous blog.

2: Three-star alignment

A star alignment is the mount’s way of finding out where the stars are.

The SynScan hand controller offers two different alignment options: two-star and three-star. You can do a two-star alignment, but a three star alignment enables the mount’s brain to calculate the amount it’s actually off the pole. Knowing this error is necessary for this polar alignment method, so a two-star alignment isn’t good enough here.

When the mount is switched on, the SynScan hand controller will ask you to enter things like time and location, it will ask you whether you want to do an alignment. Answer yes and it will jump straight to the point when it asks you what type you want to do. If you’ve done some other bits and pieces before, you’ll have to find the alignment menu yourself. It’s Setup / Alignment.

Scroll down to the 3-star alignment and hit enter. I’m not going to bore you with details about how to do the 3-star alignment, but the important thing is that once it is successful, it will tell you your polar variance. This comes in two parts, an elevation variance (mel) and an azimuth variance (maz).

SynScan handbox showing polar variances

This photo shows that the point around which the mount is currently rotating is 12 minutes and 23 seconds (that’s about a fifth of a degree) higher than the actual pole, and 8 minutes (about a seventh of a degree) to the right.

3: Polar re-alignment

Your choice now is what to do about this. If you want to get closer to the pole, continue on here. Otherwise you can just start your observing session.

How accurate a polar alignment you want is entirely up to you. If you’re a photographer, you probably want to be more accurate, especially if you’re going to be doing some long exposures while the mount is tracking.

This is the menu item just below the 3-star alignment on the handbox.

SynScan handbox showing polar alignment menu item

Press enter and the hand controller will suggest a guide star for you. In this case, it’s suggesting Canopus. You can accept this, or scroll down to choose a different star (if the star it’s suggesting is behind a tree, for example).

SynScan handbox suggesting a guide star

The mount will slew to that star, then stop and ask you to centre the star in the eyepiece using the handbox.

SynScan handbox asking the user to centre a star using the direction keys

Use the handbox’s direction controls to centre the star in the field. Use the slew speed controls if you find the star is moving too fast or too slowly. Press enter when the star is in the centre of the field.

a: altitude (elevation) adjustment

Now it’s time to make the adjustment to your mount’s elevation, the up-down angle. The handbox now confirms we’re zeroing in on the elevation variance, and shows you what its current calculated variance is.

SynScan handbox showing elevation variance

You’ll note this should be the same as the mel it showed at the end of the last 3-star alignment. Press enter to show you’re ready to continue.

The mount now slews slightly (you may not hear it) to shift the guide star off the centre of the field.

SynScan handbox slewing the mount

If you look through the eyepiece you should see the star has moved off to one side of the field. (In an extreme case, you may not see it at all, in which case you’ll have to use the finderscope when you’re adjusting the altitude bolts.)

Now the handbox asks you to use the altitude bolts to get the star back to as close as you can to the centre of the field. The altitude bolts are the ones on the north and south ends of your mount. They make the whole thing tilt up and down. The photo here shows altitude bolts on an AZ3 mount and an NEQ6. They’re pretty similar apart from size.

EQ3 and NEQ6 altitude bolts

 

Try to twist the altitude bolts while looking through the eyepiece. It may not be easy, because it’ll involve a bit of gymnastics, and the bolts might be pretty stiff. (Remember to back one of the bolts off before tightening the other, too.) You should see the star drifting in a specific direction in the eyepiece. You’ll notice that it drifts along a line, like in the diagram here. So you probably won’t be able to put it in the centre. Get it as close as you can, like in the diagram. (You’ll get to the centre in the next step.)

Field of vision showing a star moving along a line

Once it’s as close to the centre as possible, press enter. It will tell you what it estimates your updated elevation variance is.

The mel value should be small, and the maz value probably hasn’t changed much.

SynScan handbox showing estimated altitude variance

As you can see, you’re making progress. Press enter to continue.

b: azimuth adjustment

Now it’s time to make the adjustment to the mount’s azimuth, by moving it left and right. This part is very similar to the altitude adjustment part. The mount will slew the guide star away from the centre of the field. Then, it will ask you to adjust the azimuth bolts to get it back to the centre.

The azimuth bolts are the round headed ones on the south end of your mount – if you’re in the southern hemisphere, of course. (And to anyone reading this blog in the northern hemisphere, yes, there is a southern hemisphere!) Azimuth bolts adjust the mount left and right. This photo shows the azimuth bolts on the AZ3 and NEQ6 mounts. Again, they’re quite similar.

EQ3 and NEQ6 azimuth bolts

You should be able to get the star very close to the centre of the field. Once it’s back in the centre, press enter. It will tell you what it estimates your updated azimuth variance is.

SynScan handbox showing estimated azimuth variance

Both these variances should be small – but remember they’re just estimates. It’s a good bet that you’ve aligned your mount a lot better than when you started. However, to actually recalculate the variances you need to do another 3-star alignment.

4: Check the variance using another three-star alignment

This is exactly the same as the first 3-star alignment, but to summarise:

    1. Choose the first star, it slews to the star, you centre it using the handbox and press enter.
    2. Choose the second star, it slews to the star, you centre it using the handbox and press enter.
    3. Choose the third star, it slews to the star, you centre it using the handbox and press enter.

If the 3-star alignment is successful, it will then show you the updated variance (mel and maz) calculations. They should be smaller than you started with.

5: Assess the variance and decide if that’s good enough

Again, how close you want to be to the pole is a decision for you alone. If you’re only using the scope for visual work tonight, you probably don’t need fantastic polar alignment, so a couple of minutes won’t matter. If this is the case, you could stop at step 2.

If you’re only taking short exposures at high gain in order to stack, perhaps you don’t need better polar alignment. In this case, you could do all the steps, but not go back and repeat steps 3 and 4.

If you’re taking photographs and notice your stars are egg-shaped, then a better polar alignment might be appropriate. You might end up repeating steps 3 and 4 a few times, getting better results each time.

As a guide, I hear that this method can get you to under 10 arcseconds with three iterations. That’s pretty good for anyone’s needs.

Bill is Optics Central’s expert on astrophotography, telescopes and bird watching. You’ll find him in the Mitcham store on Fridays and Saturdays. Come in for advice on how to get the best out of your current telescope, what your next telescope should be, how to take photos of the sky, or even how to see some rare birds.

Related posts and further reading

How to set up an equatorial mount

Common disasters in selecting telescope mounts

 

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