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Constellations:
Star-Hopping: the Basics

I am going to do something this month that I should have done in the beginning, a year ago. I have dug out all of the information that I could find to help describe the technique of star-hopping.

To begin to find your way around the sky you need to have an idea of the main constellations. During the last year I have taken you through the Zodiac constellations. These are the constellations that lie along the apparent path of the sun, also known as the ecliptic. Since these constellations are on the path of the sun they are visible to both the northern and southern hemispheres. If you are looking for planets, this is also the path that they follow, staying within two or three degrees of the ecliptic.

One of the best tools you can get for starting your hops and learning how to find the different constellations is a planisphere. With a planisphere you can get a full view of the night sky all in the palm of your hand. All planispheres work pretty much the same: you set the observing time (adjusted for daylight savings time) against the observing date. Hold the planisphere overhead with the N arrow pointing to the North. The view in the planisphere window should match the sky view.

The planisphere will also give you the location of the ecliptic, the celestial equator (the Earth's equator projected straight out into space/celestial sphere). It will also show the coordinates of right ascension (R.A.) and declination (Dec.). (Right ascension and declination are the sky equivalents of longitude and latitude on Earth.) It will show the names of the constellations and, perhaps, a few prominent deep sky objects. The coordinates are a great help, for beginner and advanced alike, for quickly locating an area.

Once you find the constellation that an object you want to see is in, you need to narrow down your search area. If you don't have setting circles there are a couple of ways to find out where you need to go in a constellation to hit your object. One way is to ask how to find it; this works very well with our group since we are always ready to help. However, there is far greater satisfaction in knowing how to find an object yourself.

The challenge lies in the telescope's very limited field of view, which is much smaller than any constellation. The higher the magnification, the smaller the field of view, making recognition of the tiny patch of sky seen in the eyepiece quite difficult. "Star-hopping" with your finderscope, or a low power eyepiece, will enable you to take measured steps from a nearby, easily recognized bright star to the precise location of the faint, deep-sky object you seek. The technique is easily mastered with a little practice.

Star-hopping is a visual approach that requires a more detailed sky map than the overview provided by a planisphere. It also requires familiarity with the size of your finderscope's field of view relative to the scale of the star chart you use. For instance, if your finderscope has a 5° diameter field of view, a 5° diameter circle on the star chart will enclose the same area on the chart as is seen in the finderscope.

There are many star charts available on the market today. Look for one that has a magnitude limit compatible with your telescope. For example, if your scope is a 60mm refractor, a fifth- or sixth-magnitude star atlas will provide adequate detail. If you have a much larger telescope, you will need the expanded detail of an eighth-mag. star atlas such as Tirion's Sky Atlas 2000.0 or even the ninth-mag. Uranometria 2000.0.

(One great alternative we have available to us is computer programs that can be tailored to show what the sky will look like through our own personal scopes, and then print out charts that we can take into the field.)

Once you have a chart to go by there is a neat little trick that works very well. Instead of figuring out those 5°-or-whatever-diameter circles, get yourself a plastic template for drawing circles, available at any office supply store. Take your scope out to a dark site and look, first through your Telrad, then through your finderscope. Now study what stars you see in your fields of view through the finder and Telrad; on your chart, use the circle template to mask all but what you see, and note the size circle that shows just your field of view. For my 9x60 finder it is a 1¾" circle on my Sky Atlas 2000.0, and my Telrad outer ring is 1½". (Special note here: what you see in a Telrad depends on where you place your head, so it helps to pick a spot that you can keep going back to.) This same method can be used for the inner rings on a Telrad and for your favorite eyepieces. I have my circle template marked with tape and labeled for my finders, but the way I first heard to do this was to make metal rings and keep them with your charts. Either method works well. Just remember that your set of "calibrated" circles can only be used with the chart for which they were selected, or charts printed to the same scale.

After you have your circles, you can hop from any object to another just by overlapping the circles on your chart, then looking for key objects in each circle until you get to the object you are searching for. If the deep sky object is faint, it won't be visible in the finderscope. However, by using the pattern of stars shown on the chart to center the location of the targeted object in the finderscope's field of view, the object should then be viewable in the main telescope with a low power eyepiece. Once the object is centered in the telescope's field of view, you can switch to higher magnification if desired.

The example shows how to set up a star-hop from Mizar/Alcor, the double star in the handle of the Big Dipper, to M51, the Whirlpool Galaxy. Hundreds of deep-sky objects can be found using this system of working from naked-eye stars and hopping one or more finderscope fields to targeted objects.

The end result of using this technique is a working knowledge of star fields. You may be surprised at how familiar the star patterns become with a little practice.

Happy Hopping...Eric

Copyrighted © 2000 by Stockton Astronomical Society
Lasted Updated: 11/29/2000
http://www.stocktonastro.org/Newsletter/CON_StarHopping.html