Star Hopping to the Oldies


What is Star Hopping?

Star hopping is the technique of using stars as landmarks to blaze a path across the sky. Many backyard observers use star hopping to find deep-sky objects. You don't need a computerized drive, electricity or special gadgets of any kind. All you need is a dark sky and a good set of star charts. The best way to become good at star hopping is to do it. So, let's get going. There are three rules to live by when doing a star hop.

Get a Good Finder Scope

The job of a finder scope is to allow you to accurately aim your telescope at a celestial object. A finder does this in one of two ways. It either shows the object or the star field where the object is located. The two best options in finder scopes are the traditional straight-through finder and the unit-power finders, commonly referred to as Telrads. The traditional straight-through finder uses aperture and magnification to show the object. And even if the object is too faint to be seen through the finder, it will show enough stars to identify the correct field. The big disadvantage of the traditional finder is that the image does not match your naked eye view of the sky. The view is usually inverted, which can be very disorienting. This is where the unit-power (1X) finder comes into play.

The Telrad and other examples of the 1X finder project a red circle or a red dot against a clear plastic screen. You look through the screen at the night sky and compare the view with that represented on a star chart. If you can identify a pattern of stars in the area of your target and center that area within the circle or against the dot, then your object should be visible in the telescope. Unit-power finders are very intuitive in their use. The view matches your view of the sky and the chart's representation. The disadvantage is that you rely on naked eye stars to aim the telescope. If you observe under light-polluted skies, a Telrad won't be of much assistance. The telescope may not be, either. Even if your aim is dead-on accurate, the object may not be visible through all that sky glow.

Use Good Star Charts

A good set of star charts will do two things. First, the charts will translate easily to the naked eye view. It's important for beginning observers to use charts showing big chunks of sky on each page. If the charts show only a portion of the constellation in which your target resides, it will be difficult to match the chart to the naked eye view. Second, the charts will plot enough stars to make star hopping practical. If the chart only shows stars to 5th or 6th magnitude, some deep-sky objects won't have any nearby reference stars. That's not good!

There are a lot of good beginning star charts from which to choose. I'm going to make one recommendation but please don't misconstrue my intention. The chart set I recommend is not the only good set. It just happens to be the set I prefer and, as we all know, preference can be very subjective. The Sky Atlas 2000.0 charts by Wil Tirion and Roger Sinnott are excellent. Each chart usually includes at least one constellation in its entirety, making comparison with your naked eye view a breeze. Stars to magnitude 8.5 are plotted which means you'll be able to use either your Telrad or traditional finder to aim the telescope. And areas of sky that overflow with bright deep-sky objects are given their own more detailed sections. I recommend you get the SA2000 as your first serious star charts.

Know How Much Sky You Are Seeing

The key to matching naked eye, finder scope and telescope views to the star chart is knowing how much sky each is presenting. The naked eye view is pretty straightforward. The view through a 1X finder is fairly easy to match up, too. However, if you use a Telrad-style reflex finder that projects a circle against the sky, it's important to know how much sky is confined to that circle. A Telrad's outer circle encloses a four-degree area. The innermost circle is half-a-degree across, about the size of the full Moon. You can make a simple tool that will allow you to easily match the Telrad view with star charts. Draw a circular pattern on a clear transparency sheet equal in diameter to a four-degree section of sky on your charts. This overlay makes comparison of the two views very easy. It just happens that a straight-through 8X50 finder also shows four-to-five degrees. Your overlay can be used for either device.

Finally, you will have to match the view through the eyepiece to the chart. Begin by determining how much magnification your eyepiece provides:

Magnification = Telescope focal length (in millimeters) / Eyepeice focal length (in millimeters)

For instance, a 32-mm eyepiece provides 38X magnification in a telescope with a 1,220 millimeter focal length. That's the same focal length as an 8-inch, f/6 Dobsonian. A 16-mm eyepiece produces 76X in that same scope and a 8-mm eyepiece produces a 152X view.

Next, determine the true field of view of that eyepiece. You can determine the approximate true field of view for any eyepiece using this formula:

True Field = Eyepiece Apparent Field (in degrees) / Magnification

For example, a typical Plossl eyepiece has an apparent field of view of 52-degrees. A 32-mm Plossl in an 8-inch, f/6 Dob magnifies 38X. The true field of view (52/38) works out to about 1.4-degree. This is a good, wide field of view to use when looking for objects. Once you find the object, experiment with other eyepieces and magnifications until you find the one that presents the best view.

That's it. If you've got good charts, you know the field of view of your finder scope and eyepieces, then you're set to explore the night sky. Check out the other areas of this section to put into practice these star-hopping tips.


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Revised: March 15, 2002 [WDF]