If you’ve been reading this blog for a while, you probably realize I thread my way through the night sky by star hopping. There was a time when, like many other people, I used a Go-To mount and let it figure out how to get me to where I wanted to go. But I soon realized I really wasn’t learning my way around the sky, which felt distinctly unsatisfying – kind of like relying on a GPS device to get you from point A to point B repeatedly without taking the time to learn the road and street signs you pass every day. Given the fact that the skies above me are pretty darn close to magnitude six on a decent night, I decided there was no excuse for being lost most of the time when I looked up. It took a while, but I now know the constellations as well as I know my eyepieces, and there are parts of some constellations that are permanently etched into the corner of my mind reserved for maps.
But to be honest, if I was laboring under the typical light polluted urban sky, I would most likely have felt differently about the whole matter. My experience with light pollution and telescopes is limited to natural pollution, otherwise known as the full moon. It’s certainly possible to star-hop under light blasted skies, but I’ve found the typical 8×50 finder is limited to stars of about 6.0 to 6.5 magnitude when a full moon is hard at work refracting its rays all over a moisture laden sky. Under dark skies I can see stars as faint as 8.5 to 9.0 magnitude in an 8×50 finder, which makes finding my way around much less of a battle than it is when the moon is in charge of things.
If you use an equatorial mount, you quickly learned it needs to be aligned with the north or south pole in order to track an object in right ascension. If, like me, you have a motorized (tracking) EQ mount without GoTo capability, pointing it at Polaris will provide reasonably accurate tracking for most purposes. But if you stay fixed on an object long enough, you’ll find it drifts from the center of the field of view, which requires you to use the mount’s declination controls to re-center the object. For low-powered viewing, that’s not much of a problem.
But if the object of your attention requires high magnification, you’ll find yourself constantly re-centering it. And if you’re trying to sketch the field of view, that perpetual wandering away from the center can make it difficult to accurately represent the stars in the field relative to each other. Which raises two questions: Why does it happen, and What do you do about it?
The answer to the first question is .75 of a degree, which is how far Polaris is from true celestial north. Which is another way of saying Polaris is really not quite the north star you thought it was. At low magnification, that .75 degree difference is minor, but with each increase in magnification, the quicker the object becomes unlatched from the center of the field. The more magnification you add, the sooner it starts to roam — somewhere around 200x you’ll see the object begin to take off almost immediately. And if you’re a double star observer who finds 300x and 400x to be rather useful on occasion, you’ll find yourself constantly re-centering the object.
The answer to the second question – What do you do about it? – can be summed up in four words: find another north star. Or at least one closer to the actual celestial pole. As it happens, there’s a 9.65 magnitude star located a mere 13.5’ from true celestial north, which goes by several names: BD +89 38, GSC 04661-00002, SAO 3788, and TYC 4661-2-1. We’ll call it SAO 3788 since that’s the name most likely to be used to designate it.
If you’re beginning to feel a bit lost or puzzled, let’s take a look at Polaris in relation to the north celestial pole:
Stellarium screen shot with labels added, click to enlarge.
As you can see, Polaris is a bit shy of occupying the actual north celestial pole, which is the point at which all the lines converge. But as you can also see, there are several stars which lie closer to the actual pole, one of which is the previously mentioned SAO 3788.
So now for the main question: how do you get to it? You star hop to it, of course! Fortunately, we don’t have far to go, so if the idea of star hopping has you on the edge of your seat, despair not. You can do this quite easily from within the field of view of a wide angle eyepiece.
But before we get to the eyepiece view, there are a couple of things we need to pin down, the most important being the positon angle of the ninth magnitude companion of Polaris, which also serves as an important navigational tool. In fact, we might just as well take a quick look at all the pertinent data on Polaris:
Polaris (Σ 93) (H IV 1) (Alpha [α] Ursa Minoris)
HIP: 11767 SAO: 308
RA: 02h 31.8m Dec: +89° 16′
Magnitudes: 2.04, 9.10
Position Angle: 233° (WDS 2013)
Dist: 432 Light Years
Spectral Type: “A” is F8, “B” is F3
Note the declination of +89° 16′ — further confirmation that Polaris is three-fourths of a degree short of celestial north, in case you still had doubts.
But the key number is the position angle, 233°. We’re going to use that as a reference point for determining which direction to move in order to aim ourselves towards the north celestial pole (which is at 360°). Determining celestial directions in the vicinity of the celestial pole is a very tricky business, even for the experienced observer, and it can be downright confusing when you’re gazing into an eyepiece. The old trick of turning off your drive motor and letting a star drift across the field of view to determine which direction is west doesn’t work this close to the pole, so a navigational aid is a necessity.
Let’s start by imagining that the the line running from Polaris A through Polaris B stops at the edge of your eyepiece. We’ll label that point 233°. When we subtract 233° from 360°, we get 127°, which is how far we need to move around the outer perimeter of the eyepiece in order to point ourselves towards celestial north.
Now we’re faced with the big question: which direction do we go? Clockwise or counterclockwise? The answer is that it depends on the kind of telescope you’re using, so pay close attention — it means the difference between success and frustration. Assuming the use of a conventional inverting star diagonal (NOT an erect image diagonal), if you’re using a refractor or an SCT, north is clockwise from the 233° mark. If you’re using a reflecting telescope (Dobsonian, Newtonian), north is counter-clockwise from 233°. If moving by degrees around the outer perimeter of the eyepiece field is confusing, you can draw a line perpendicular (90°) to the line running between Polaris A and B and then add another 45° to that to point you toward north.
Here’s a diagram which should make all that more clear:
This may look simple when north and south are straight up and down, but that will seldom be the case when you peer into the eyepiece! Click to make the image easier to read.
And here’s how that move looks when superimposed on our previous chart:
Stellarium screen shot with labels added, click to enlarge.
Now that we’re directionally oriented in the eyepiece, there’s one more thing we need to discuss before going any further, which is how to move to where we want to be. If you’re new to this, it may come as a surprise, but it’s the mount that needs to move, NOT the telescope.
When you bought your mount, the instructions for it should have covered the procedures for lining up on Polaris. The first thing you would have done is set up your mount so that the declination axis was pointing toward Polaris, and then you would have used the altitude control to move the tilt of the declination axis so that it matched your latitude – in my case, that just happens to be 45 degrees. Then you would have centered Polaris in your finder by using both the declination and azimuth (horizontal) controls on the mount, and then fine-tuned that further by making final adjustments to center Polaris in your eyepiece. During that entire procedure, you would have left the telescope untouched.
So that’s what we’re going to do here.
With the counterweight shaft pointing straight up and down, and the declination circle lined up with the ninety degree mark, lock the declination and right ascension clutches on the mount and then use the altitude and azimuth controls to center Polaris, first in your finder, and then in your eyepiece (if your mount has locks for the altitude and azimuth controls, make sure to unlock them now – and don’t forget to lock them when we’re done!). Here are photos of two frequently used equatorial mounts, which will give you an idea of what to look for in the way of altitude and azimuth controls:
This is a Celestron CG4 mount. The altitude and azimuth controls are unlocked when one of the pairs of control devices is loosened, and locked when the two opposing devices are turned so they’re pressed tightly against one another. Other mounts, such as the CG5, have similar controls. Click for a larger view.
To unlock the azimuth and altitude controls on the Losmandy G11, just loosen the wing nuts shown in the photo — note there are two, one each on opposite sides of the mount. The Losmandy G8 uses the same setup. Click for a larger view!
All the movements we’re going to make from this point forward will be made using the altitude and azimuth controls on the mount.
Now if you look carefully at the chart above (here it is again), you’ll see there’s actually an easy way to star hop to SAO 3788. Notice that the Polaris A-B line points almost directly at 6.45 magnitude HIP 7283, which is distinctive because it’s bright and because of the ninth magnitude star located next to it (that star has a name by the way, SAO 223). And if you move towards true celestial north from HIP 7283, you’ll see it leads you past 8.10 magnitude HIP 3128. And if you extend that line an equal distance, it just happens to lead you to our goal, 9.65 magnitude SAO 3788.
And here’s how that looks when diagrammed, minus the celestial grid in our previous chart:
Stellarium screen image with labels added. (Both this and the previous image portray the scene as it would be seen in a refractor or SCT, meaning east and west have been swapped).
Here are those moves once more, but this time plotted on our earlier grid:
Stellarium screen image again, click to enlarge.
So how does all that look in the eyepiece?
Click to enlarge – note, east and west are reversed here to match a refractor or SCT view.
This is the view with a 40mm Celestron Plössl in a 9.25 inch SCT. I’ve moved Polaris to the eastern corner of the field in order to pull HIP 7283 and HIP 3128 into the field of view.
If you now move toward the north until HIP 3128 is in the same position in the eyepiece as HIP 7283 was, you’ll see our goal, SAO 3788, come into view at the opposite (north) corner of the field of view:
Click to enlarge!
Center SAO 3788 in your eyepiece and you’re now a mere 13.5’ from true celestial north. Once you’ve done this a few times, you’ll find it’s quick and easy to do.
I’ve found parking SAO 3788 in the center of my eyepiece is more than sufficient for a 400x view — I can barely detect any motion unless I stay on the object for ten minutes or so. Remember, we’ve gone from being 44’ off center from celestial north to 13.5’, which is a huge improvement.
Lock up your altitude and azimuth controls, unlock the declination and right ascension clutches, grab a 4mm eyepiece, and go split that pair of sub-arcsecond stars that’s been on your list for the past year without having to chase it across the field of view!
Happy star hopping and clear skies!
Filed under: 2. Observing Tips and Tactics, 4. Choose a Constellation:, Ursa Minor | 5 Comments »