Beginners to Astronomy are often disappointed by their second attempt to find an object they want to observe. (Their first target is usually the Moon, which succeeds. Their second attempt is usually something harder, like a galaxy, which fails.) They point the telescope where the star chart says, and when they look in the eyepiece there is nothing there.
Sometimes this failure is a poor choice of target – many interesting objects are beyond the capability of entry-level telescopes. More often, I find that the disappointed beginner has underestimated the difficulty of finding things. The sky is a big place and a telescope magnifies an extremely small piece of it. Chances are slim that a simple point of the telescope will result in it covering exactly the precise spot of sky needed. Just pointing to where a star chart says the object “should” be will almost certainly fail.
Great. So how do we find things then?
If you don’t have a go-to telescope (or you want to learn to find things manually for the challenge or to be independent of your electronics), you have 3 main ways of finding objects:
- Point the telescope to objects you can see with your naked eye. There are several such objects: the Moon, several planets (Saturn, Jupiter, Mars, Venus), and a number of deep space objects (e.g. M42, The Orion Nebula). You will need to have your finder properly aligned for this to work.
- Point to objects that are very near easily-visible objects, in some easy to measure relationship (e.g. exactly halfway between two visible and closely-spaced stars). There are dozens of objects you can find this way. You’ll need a well-aligned finder and a sense of distance and geometry.
- For more elusive objects, you will learn a technique called Star Hopping, which is the subject of this article. Star Hopping involves planning and following a route in the sky using a map and landmarks. It requires some skill and practice, but it is fun and satisfying and you’ll be very proud of yourself when you develop this skill.
Not the Setting Circles?
Nope. The Setting Circles on Entry-Level Telescopes are decorations, not accurate enough, and often lacking necessary adjustments, to be used to point to sky coordinates.
Star Hopping is how most amateur astronomers (without go-to) find their targets most of the time.
A Terrestrial Example
If your friend has a GPS (Global Positioning System) you might just tell them
The marker is at Latitude 45°24’12.06″N, Longitude 75°37’8.49″W.
That would be the equivalent of having a telescope on a “go-to” mount. But if your friend didn’t have a GPS that information would not be very useful.
Instead, you would probably give them step-by-step directions based on easy to identify landmarks. Possibly something like this:
- Go West from the parking lot to a very tall pine tree.
- Go South from the pine tree to a picnic table.
- To the West you will see an old barn. Start walking toward it.
- Before you reach the barn you will encounter a stream. Follow the stream South to the marker. You can’t miss it.
Doing that with landmarks in the sky – stars you can recognize – is star hopping.
Star Hopping consists of
- Planning a route to your target, using a good map;
- Starting somewhere you can find;
- Following identifiable landmarks.
Equipment You Need
You can star-hop with no special equipment other than your nimble mind. But it’s a lot easier, especially as a beginner, if you have the right tools. You will need:
A Finder with a Known Field of View
You will be using the width of the field of view of your finder as a measure in the sky, so you need to know exactly how wide it is in degrees. This information is usually in the specifications sheet for your finder, or you can figure it out by finding two well-known stars that just fit in the field, then looking up their angular separation on a star chart.
- Telrad Finders are loved for star hopping because their concentric circular reticles are accurately graduated at widths of 4, 2, and 0.5 degrees.
- My Stellarvue F50 8×50 RACI finder, as another example, has a field of view of 6 degrees.
- Many other small magnifying finders have fields of about 5 degrees.
For star hopping you will really want a finder that does not reverse or invert the view. Either a Telrad-style zero-magnification finder, or a RACI (Right Angle Correct Image) finder that keeps the sky properly oriented. If you have an inverting or reversing finder, you will be constantly having to account for the view in the finder being different from what is printed on your star charts. (Or, you can get charts, or configure most software, with reversed views so they match your finder.)
Good, Accurate Star Charts with a Known Scale
Since you will be plotting your route from a map, you will need a good map.
The little star charts included in most pocketbook-sized books on Astronomy and Constellations are excellent for many purposes (I have several), but they are not accurate enough for planning Star Hopping. Get a Sky Atlas.
You can download printable star charts for free too. For example, Andrew Johnson has published an excellent set of free star charts on Cloudy Nights, accurate enough to plan star hops to hundreds of objects.
Or, many software programs for your Mac or PC will display star field diagrams accurate enough for Star Hopping.
Starry Night (Mac and PC; inexpensive but not free), Cartes du Ciel (PC; free), and TheSkyX (Mac and PC; moderately-priced, very comprehensive) are examples of packages that produce calibrated displays suitable for planning star-hops.
A Way to Overlay your Finder Field of View on your Charts
Finally, you need a way to visualize the field of view of your finder on top of the star chart you are using.
With a good star atlas, you can draw a circle on a piece of clear plastic, exactly the right diameter to correspond to the field of view of your finder in the scale of the atlas, and lay it on the page you are using. Tirion & Sinnott’s Sky Atlas comes with such a plastic overlay, already marked for the field of view of a Telrad, and with a field of view scale that makes it easy to mark circles on a piece of clear plastic for other finders.
The three software packages I mentioned all have the ability to draw a circle of a given exact field size on the display, so you can have your own customized finder overlay. Starry Night even has pre-defined overlays for several standard Finders (including the Telrad, the Rigel Quickfinder, and several magnifying finders).
Strangely, the excellent and free Stellarium software lacks this feature at the time I am writing this – there is no way to get a circle of defined size overlaid on the field. No doubt they’ll correct this oversight eventually.
Skills You Need
Finally, before you tackle star hopping, you should have some basic skills:
- You need to be able to read a map. If a standard map on Earth confuses you, a star chart will too;
- You need to be able to locate major constellations and some of the brighter stars;
- If you are using an Equatorial mount, you need to be able to accurately polar align it;
- Finally, you need to have enough experience with your finder and telescope that you are comfortable with the directions in which things move in their fields of view (which may be reversed left-to-right or top-to-bottom, depending on the optical design).
A Worked Example
Developing the Plan
Let’s work through an example of planning, and following, a star-hopping route.
It’s a late Winter or early Spring night, and we’re going to try to find the Messier 47 (M47) open cluster. We have a star chart of that part of the sky, and a calibrated circle that exactly represents the 6 degree field of view of our finder on this chart (in fact, I’m using the Starry Night software to generate these charts).
Note that, in this example, we are in a rather light-polluted area (like my suburban driveway), so there are a small number of stars visible. Strange as it may seem, this can actually simplify star-hopping to basic objects, as the view in the finder is less complex and the bright stars are easier to remember. Although dark skies are far better for observing, they make the finder view much more complex and require that you have more detailed plans on how to locate landmarks in the busy field.
First, let’s have a look at that general area of the sky, to get a feel for our route.
M47 is East of Orion – and we note that brilliant Sirius in Canis Major is about halfway in between Orion and the cluster.
Sirius is so easy to find that it seems like a good place to start our star-hop; let’s explore that.
We have a plan. We can start at Sirius then go through 3 steps with the finder – in each case having an easy to identify star that we will move to a specific location in the finder field.
Note that I sketched the location of Orion onto the edge of the chart too, to help me remember how these stars are oriented with respect to the sky.
Write such notes in blue pen on white paper for maximum visibility when you are reading them by a dim red flashlight in the dark. (Don’t write in red pen! It will be invisible in red light.)
Using the Plan
Now let’s go to the telescope and walk through this plan at the eyepiece.
The first thing to note is that, by the time the telescope is rotated around to the correct part of the sky and the finder is mounted wherever it may be on the telescope, it is unlikely that the 12:00, 3:00, 6:00, etc., positions on our plan will correspond to those positions in the actual finder. So we’ll have to do a quick calibration manoeuvre as we get started.
(This is another reason Telrad finders are so popular – you are looking at the real sky, not through mirrors or lenses, so there is no confusing change of orientation.)
We get the telescope close to Sirius, then scroll Sirius into the field of view of the finder, being careful to approach it from the direction of Orion (i.e. moving almost directly East to scroll it into view.) This is how they were oriented on our star chart – Orion was to the right of Sirius. The edge where Sirius first appears corresponds to the “3:00” position on our charts. Mentally make that adjustment for all of the remaining steps. For the images below we’ll continue to show the “3:00” position in that spot, for simplicity.So, for step 1, we put Sirius just inside the edge of our finder, at the 3:00 position. At the other side of the field, note the star Muliphein.
Step 2: keeping our eye at the finder, we use the telescope’s slow motion controls to carefully move Muliphein to the edge of the field, at the 3:00 position where Sirius was.Note the next star in our hop at the other side of the field, HIP35951. We selected it to be easy to find – it is slightly brighter than the others, and we note the pattern of stars around it.
As our final hop, we carefully move the telescope until HP35951 is at the edge of the finder, at about the 4:00 position (relative to the calibration check we did at the start).According to our plan, that should have brought M47 to the centre of the field. In this case, M47 is a rather bright object, and we can, indeed, see a faint patch of light just above the centre, and that’s probably it.
But the beauty of Star Hopping is that it doesn’t matter. We know from our charts that, with HP35951 at this position in the finder field, M47 is in the centre. We can use star hopping even for objects that are too dim to see in the finder scope, because we are positioning relative to things (stars) that are bright enough to be seen.
In fact, you might have noticed on our chart that M47 was right next to M46, another open cluster. And if we checked the distance between them, and knew the field of view of our telescope with various eyepieces, we might even know if we could expect to see both of them in the same field. Let’s try this.Putting a very wide-field eyepiece into the scope (a 22mm Nagler in this case), we find that we can, indeed, see both M47 and nearby M46.
This concludes our introduction to the concepts of Star Hopping. The idea is to use accurate charts, and a finder with a known field size, to plot a route to your target, using visible and identifiable stars as landmarks along the way.
There are no “right” or “wrong” routes – anything that will get you to your target is a good route. You’ll find favourite star hopping routes to favourite targets is a common topic of discussion among serious amateur astronomers, so you can compare notes and pick up tips from your friends at star parties.