This is the second part of my blog on telescope mounts, and concentrates on the equatorial mount. I look specifically at how to set up an equatorial mount and align it for the Southern Hemisphere. Equatorial mounts are designed to help you track stars. That is, you can keep what you’re looking at in the field of vision while the Earth turns. In this part I’ll show you the steps to set up your equatorial mount well enough to get decent tracking for visual work.
In a previous blog, I set out the two different types of mount, alt-azimuth and equatorial. With the alt-az, you need to adjust both axes to keep a star in view, and even then, the view rotates slowly. With the equatorial mount, once it’s set up for the Southern Hemisphere, you only need to adjust one axis. What’s more, the field doesn’t rotate at all.
In this second part of the blog, I’m going to show you how you do it.
The goal of your setup
OK, so we’ve got our scope and an equatorial mount. What are we trying to do? The goal is to point the polar axis of the mount exactly at the South Celestial Pole (SCP). That means it’s parallel with the axis that the Earth spins on, so we can compensate for that spin. Most of the stuff you’ll find on equatorial mounts is written for the Northern Hemisphere, and talks about the Pole Star. This won’t work for us Down Under, unfortunately.
Remember this wonky drawing from the previous blog?
The other thing to know before we start is that near enough is, in this case, good enough. If you follow the procedure I’m about to describe, you’ll get to within a couple of degrees. This is fine for visual work. If you’re into photography, take it from me, getting it to within 1 arc second (1/3600°) is a whole new level of hell.
Let’s get into it
There are two stages to this process, getting the mount oriented left-right and then setting the altitude up-down.
Before we do that though, figure out where you want your telescope to stand. This might sound silly, but you don’t want trees blocking your target. It’s a pain relocating an equatorial mount as the set up takes a bit of time. There’s a park near my place with an oval. This is perfect, although I’m lazy and normally set up in my back yard.
Note: a lot of mounts can detach from the tripod, and finely adjust left and right. In this blog I’m going to assume you can’t do any of this. If you can, though, it’s a bit easier and you can adjust the mount more accurately.
Stage 1: Orient the equatorial mount (“adjust the azimuth”)
Find how you have to orient the mount. The polar axis of your mount is the one that has to point at the SCP (in the side-on photo two below I’ve labelled it). The first step is to get this pointing due south – that is, to a point directly above the south point on the horizon. Don’t worry about how high it is pointing, we’ll organise that later.
Grab your compass. Leave your tripod where it is, and walk several paces back away from it (er, that’s north, I guess). If you’re close to Melbourne, set the compass so it’s pointing to about 168°, which will point to true south, rather than magnetic south. (That’s roughly 180° minus the Melbourne’s 12° magnetic declination. If you’re not in Melbourne, Google the declination for where you are, or ask me.) Now, move left or right so you can look right over the compass, and due south at your tripod, like in this photo.
Now, rotate the equatorial mount so the polar axis is pointing due south. Better still, get someone else to rotate it while you direct. From the side, it should now look like the picture below (south is to the right in this side-on picture).
Now, take the opportunity to make sure the mount is level. As you can tell from the photos, my back yard is on a slope, so I really need to do this. I use a spirit level, but my iPhone also has an app for it. A lot of scopes have a bubble level built in, which is helpful, but I find they’re not always all that accurate. Take care not to rotate the mount where you’re levelling it.
The polar axis is now pointing due south. We’re not done yet, as that only means it’s pointing somewhere on a line between the south horizon and the SCP.
Stage 2: Tilt the polar axis (“adjust the altitude”)
Now you’ve got it pointing south, we come the second of the two-stage process. Adjust the height of the polar axis so it’s pointing directly at the SCP. That is, you have to set the equatorial mount’s altitude to match your latitude.
Here’s a list of latitudes for major capital cities in Australia (remember that all these latitudes are south):
(if you’re not in any of these places, you can Google your latitude. Alternatively, use a GPS or the compass app on your phone).
There are two ways of setting this angle on your equatorial mount.
The easiest way of setting this is to use the altitude scale, which is on the mount itself. This is accurate enough, assuming you’ve got your mount nice and level. This one is pointing at about 40°, which is probably as good as you can get at that level of accuracy.
A more accurate, if fiddly, way of doing it is to (again) use your phone. My compass app also has a declinometer – that’s a little jigger that shows angles of slopes. Make sure you calibrate your phone by checking it on something you know is level. Then, hold the phone against a part of the head that’s parallel with the polar axis on your equatorial mount (see the photo below) to find its angle.
To adjust the altitude, use the two bolts I’ve marked in the photo two above. It’s important to make sure you’re not forcing the bolts together – remember to back one off when loosening the other. If you’re not careful you can bend the bolts. Obviously, winding them one way will point the polar axis higher, the other way will lower it.
It only remains to put the telescope onto the equatorial mount, if it’s not there already.
Once it gets dark, get your target in view (let’s say you’re looking at the Pleiades – awwww). You should find that you only need to turn the adjuster on the polar axis to keep the star there all night. Of course, how accurately the scope is polar aligned will determine how often you need to adjust the declination axis. This depends on a number of factors, including the quality of the equipment and how well you’ve done the job. But, like I said at the start, if you’re a visual astronomer, near enough is good enough.
Short cuts for aligning your equatorial mount next time
Here are some ways to save time setting up your equatorial mount. You’re less likely to actually get out and astronomise (if that’s a word) if you’re faced with a lengthy setup time. This goes double if you have young astronomers who want it done right away. Do these things and you’re up and running in 5 minutes.
From now on, unless you’re taking your telescope a long way away from home, you can leave the altitude set.
- Always set up in the same spot (eagle-eyed readers may have noticed I have some small pavers set into my lawn).
- If you are set up in the same spot, you can leave the leg lengths set so you don’t have to level the mount each time.
- Take note of some landmark you can use for orientation. I know that the fence post in the background is roughly due south of my telescope. You can see this in the photo above where you can see my hand holding the compass.
- Learn how to find Sigma Octantis with a pair of binoculars. This is for the more advanced astronomers, especially astrophotographers. I’ll get back to this in a later blog.
Now you’re aligned, not only can you track stars, but you can get find targets using catalogued co-ordinates. This is especially useful for comets, because comets move. You can find out what their co-ordinates are at any given time, and zero in using those.
Once you’re set up, don’t move the equatorial mount. When you want to move to a new target, unlock the clutches of both axes and move them to roughly the right area. Lock the clutches and then use the adjustments to get closer to the target.
Good luck with your equatorial mount. You’ll find that once you’ve got some practice setting it up, and you remember my short cuts, you’ll finish the set up and start observing in minutes. Plus, tracking is way simpler, and you can use catalogue information to find that nearly-invisible comet, planet or supernova.
Once you’re done, if you’ve got a mate who’s using an alt-azimuth mount, try not to look too smug as you listen to her swearing while she gropes around in the dark trying to find the right one of two adjusters.
If you don’t have an old-school compass, your smartphone will probably have a compass app. In fact, my iPhone’s compass says it can point to true south, if I can trust it.
You’ll need quite a lot of practice to get the scope centred on an object close to the pole. The axes just don’t do what you want them to. Try to visualise where the target is in relation to the pole, and this will give you an idea of where the right ascension should point (the counterweight should point at a right angle to the right ascension). Errrm, it’s complicated, and takes practice. In an old astro club I was in, there was one kid who just had the knack. It was awesome.
Don’t try to align an equatorial mount during the night, before sunset is way easier if you can. Yeah, it’s possible, but it’s so much more difficult if you can’t see things like trees in the distance. Take it from me – I once spent 45 minutes up at Mount Burnett roughly aligning my big heavy NEQ6-Pro. This included mounting the scope, counterweights, cameras, electrical cabling and all. Then, of course, I found I was just out of adjustment range. I had to tear the whole thing down just so I could move the tripod. Hours wasted, and what was worse, was there were other people there watching me!
Helpfully, some tripods have an “N” marked on one leg. The one I was photographing for this blog certainly did. Manufacturers make them for the northern hemisphere, of course, so the “N” leg has to point south. Duh.
The Astronomical Society of South Australia has a nice easy web page that provides some more detail on polar alignment, including how to use Sigma Octantis.
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.