Star Hopping Primer
by Paul Markov, August 1999.
Star hopping is a method used by amateur astronomers to located objects in the heavens. Typically, star hopping is used for finding deep sky objects, however it is just as effective for tracking down asteroids, comets, variable stars and anything else in the sky that is too faint to be readily seen with the naked eye or in a finder scope. For all you LX200 owners, and other computerized telescope operators, put down your keypad and turn off your digital setting circles star hopping is the best way to learn the sky!
Star hopping involves "hopping" from star to star with your telescope, until you arrive at the location of the target object. Your starting point and end point are crucial to determining your "hopping path", and not unlike a car trip across a city, there are many paths you could follow. Obviously you want to take the shortest and simplest path, however, there are times when this approach is not possible due to the lack of reasonably bright stars to follow. This may surprise you, but there are many areas in the sky that are devoid of brighter stars (10th magnitude or brighter) when looking through your small telescopic field of view. If there are not enough starts to guide you to your target, it is very easy to get disoriented and "lose your place" in the sky. The opposite is also true; if there are too many stars it can get very confusing because it will be difficult to create easily recognizable star patters. A perfect example is the Milky Way area between Deneb and Albireo in Cygnus where several dozen stars are visible within a small telescopic field of view. If a simple path is not readily evident, you will have to take "the scenic route" to arrive at your object this may take longer and involve more steps, however it is a good trade off if you can be assured that you will find your object.
In the sections below I will describe the basic equipment required for star hopping as well as a couple of tips and hints that should make your searches easier.
Get a Good Star Atlas
A good star atlas must show enough faint stars to be used for your star hopping trek. I recommend, as a minimum, Sky Atlas 2000. The latest edition of this atlas shows stars down to magnitude 8.5 (total of 81,312 stars), and its oversized (9" x 22") pages often show entire constellations on one chart, making it easier to relate what you see on paper to what is up in the sky. I highly recommend the "desk edition" of Sky Atlas 2000, which shows black stars on white paper. The "field edition" shows white stars on black paper and is actually much more difficult so use in the dark with a red flashlight. Furthermore, the "desk edition" gives you the possibility of writing small notes, add objects, or plot comet positions because the paper is white. One last suggestion; using a pencil (or a thin permanent marker if your atlas is already laminated) add the basic constellation lines to your maps. For example, "connect the dots" in Ursa Major to form the Big Dipper, the ones in Sagittarius to form the "teapot", or the ones in Orion to form the hunters body. I am sure this will be of great help in quickly finding your way around a map, and ultimately the sky. If your main objective is deep sky observing, your sky atlas should also plot as many deep sky objects as possible Sky Atlas 2000 is a good intermediate atlas in this regard, showing over 2500 deep sky objects.
The next best atlas is Uranometria 2000. Once you graduate past the Messier list and the brighter NGCs, you will definitely need Uranometria because many fainter deep sky objects are not even plotted in Sky Atlas 2000. Uranometria comes in two thick volumes showing starts to magnitude 9.5 (total of 332,000 stars) and a total of 10,300 deep sky objects. Because each chart is just 9" X 12" only a small portion of the sky is plotted on one page, so you will find it useful to use this atlas in conjunction with a larger scale atlas, such as Sky Atlas 2000.
The ultimate star atlas is the recently published Millenium Star Atlas. This three-volume modern work of art plots stars as faint as 11th magnitude (total of 1,058,000 stars), and over 10,000 deep sky objects. Just like Uranometria, you will need to use a larger scale atlas together with Millenium because each 9" X 13" chart shows only a very small part of the sky. The main drawback to Millenium is its Canadian price tag of $400.
Finder Scope: The Bigger the Better
A good finder scope with a wide field of view is very important for star hopping. The very minimum is a 6 X 30 mm finder, but 8 X 50 mm is much better, and 9 X 60 mm is best. A finder with a 5 to 7 degree field of view will quickly help you zoom into the correct area of the sky by showing many faint stars at once. There will be several instances where you will be able to use your star atlas and finder to precisely aim your telescope, and then by just looking through your main telescope you should be able to see the target object in the field of view. Many people like using zero-power finders, such as a Telrad, Rigel QuikFinder, or Tele Vue QwikPoint, and although extremely useful for aiming your telescope at naked-eye objects, these are not effective tools for seeing the fainter stars that guide you on your star hop. For the best of both worlds, I highly recommend using both a zero-power finder and an 8 X 50 mm (or larger) finder scope.
Use a Wide-Field Eyepiece
The next tip for successful star hopping is to use an eyepiece that will give you the widest possible field of view. The wider your field of view, the more stars you will be able to see through your telescope, which in turn make it easier to hop from star to star. Never star hop at higher powers as you will only frustrate yourself. You should only switch to higher power once you arrive at your target location and are ready to begin small sweeps of the area of interest.
Whats Your Field of View?
An excellent star hopping aide that you can make at home is an acetate overlay showing the fields of view of each of your eyepieces and finder scope. These can be extremely useful for those instances where there are not enough brighter stars that can be used for star hopping. The way to use an acetate overlay is as follows: place the overlay that matches your eyepiece filed of view on top of your sky atlas, then move the acetate towards your target object and count how many fields of view are required to get there. Next, go to your telescope and move it just as many fields of view as you counted. This will put you fairly close to the target area. Now gently sweep back and forth until you spot your object.
To make the overlay you will need to know the exact field of view of your eyepieces.
The quick method for determining this is to aim your telescope at any known part of the sky, then compare what you are seeing in your eyepiece against a star map. Next, using the maps declination markings (shown in degrees), figure out how much of the sky you are seeing in your eyepiece. Another method requires two pieces of data your eyepieces apparent field of view and its magnification in a given telescope. Heres an example: say you have a 25mm Plossl eyepiece that has a 50 degree apparent field of view and your telescope is an 8-inch f/10 Schmidt-Cassegrain telescope, with a focal length of 2000 mm. First calculate the power of this eyepiece for this specific telescope, which equals to 80 (2000 / 25). Then take the eyepieces apparent field of view and divide by the power, which gives you an actual field of view of 0.625 degrees (50 / 80). If you dont know the apparent field of view of your eyepieces, check the ads in astronomy magazines for same eyepiece, as they often state this value, or check with a telescope dealer or visit the eyepiece manufacturers web site. The most accurate method for determining your eyepieces fields of view is to time how long it takes a star to cross your field of view. Start by aiming your telescope at any star that is within 10 degrees of the celestial equator. Next place this star at the edge of your field of view, then turn off your telescope drive (if you have one) and time how long the star takes to cross the entire field of view using a stopwatch. To obtain the field of view in degrees, divide the time in seconds by 240. This calculation is derived from the fact that a star near the celestial equator moves one degree every four minutes (i.e. 240 seconds).
Now that you know your eyepieces fields of view, you can create the overlay by simply drawing a corresponding circle on an overhead projector acetate with a permanent marker, or if you have the equipment, you can use a computer and a laser printer to print circles right on the acetate. Note that you will need a different overlay for each of your sky atlases since the charts will have different scales.
Inverted and Reversed Images
Depending on your telescope type, what you see through the eyepiece will be an inverted image, or a reversed image. I am sure all of you have experienced the confusion this can lead to! As a rule of thumb, remember that an even number of reflections will give you an inverted (upside-down) image, such as a Newtonian telescope, since primary and secondary mirrors amount to two reflections. An odd number of reflections will give you a reversed (mirror) image, such as a Schmidt-Cassegrain, since primary, secondary, and star diagonal mirrors amount to three reflections, or such as a refractor with a star diagonal , since that amounts to one reflection.
Inverted views in your telescope are easy to cope with - all you have to do to match what you are seeing in the eyepiece is to simply turn your sky atlas upside down. When dealing with reversed images unfortunately there is no easy solution. One thing you can train yourself to do is to always remember that whatever you are seeing on the map will be mirror reversed in the eyepiece. This is not too hard to master, but if similar star patters are evident on either side of your target object, things can get very confusing and you may end up star hopping in the wrong direction. Another solution is to flip over your sky chart and shine a light behind it to view a mirror image of the printing through the paper. This only works if the paper is not too thick and if your map is printed only on one side, such as Sky Atlas 2000. This trick will not work with Uranometria or Millenium since they have printing on both sides of each sheet. If a reversed view is really confusing you, you could temporarily remove the star diagonal from your Schmidt-Cassegrain or refracting telescope. Drawbacks to this solution is having to extensively turn your focussing knob to reach focus, and possible uncomfortable viewing due to the eyepiece being in an awkward position.
Suppose you are looking for a galaxy that is just a couple of degrees north of a star you are viewing. You know you have to move your telescope north, but you just can figure out which way that is when looking through the eyepiece. Should the star in the field of view disappear towards the top or the bottom of the eyepiece? Well, it depends on many factors telescope type, use of a star diagonal, and the orientation of your eye. The method I use to bypass all these issues is to simply move the telescope tube up (i.e. north) and forget which way the stars end up moving in the eyepiece. This works perfectly each time, assuming your telescope is equatorially mounted. If it is not, this will still work, except you will have to make a slight adjustment is azimuth. The same method will work for east or west movements. If your target is east of your current location, just move the telescope to the left, and if its to the west, move your instrument to the right. Again, this assumes an equatorially mounted telescope, but it will work with an alt-azimuth mount by just making a small adjustment in altitude. I like this method because I find it easy to relate what I see in an atlas to the sky. For example, the globular cluster M15 is to the right of Epsilon Pegasi, as seen in a sky map. My star hopping path would be to start by aiming the telescope at Epsilon then slowly move my telescope right until I find M15.
The above trick will work just fine if youre facing east, south, or west, but what if you are facing north? Well, if you are facing north and you need to move your telescope to the west, you should still move it to the right. This will appear as if you are actually moving it towards the eastern horizon, but in fact you are moving it west. Another way to look at it is as follows: your motor driven telescope moves from east to west (left to right). No matter which way your telescope is pointing, its still being driven from east to west, meaning left is always east and right is always west. This may be a rather unorthodox methodology since most observing books and articles will teach you to always think in terms of east, west, north, and south, however the method I described above for determining which way to move the telescope (left, right, up, and down) has never failed me. Give it a try as it may work well for you, too.
I hope all the above information will help you with your star hopping adventures. If you have tips and hints of your own, I would like to hear from you via email at firstname.lastname@example.org.
Copyright (C) 1999 by Paul Markov
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