Hack21.Learn to Star Hop

Hack 21. Learn to Star Hop

When a Telrad is not enough, find and focus on patterns, and then hop pattern by pattern to the target.

Although the Telrad allows you to locate many objects with amazing speed [Hack #20], it's not a panacea. Some parts of the sky are simply devoid of the bright stars that you need to orient your scope with a Telrad. To find an object in those barren parts of the sky, you need to star hop.

Star hopping is the process of locating an object by beginning at a bright "guidepost" star and then using your optical finder to follow a trail of dimmer stars until you arrive at the object. The secret to star hopping is to plan the hops so that each hop provides a distinct pattern of reasonably bright stars in the finder. When the pattern is right, you know that the finder is pointed exactly where you think it is, and you can then move the finder to locate the next pattern of stars.

Before you attempt to star hop, you need to know the field of view of your optical finder and how objects are oriented in it. You can calculate the field of view precisely by drift testing [Hack #57] or by using the finder to look at star pairs with known separations [Hack #56]. A correct-image finder (rightangle or straight-through) provides an image that is correct top-to-bottom and left-to-right. A traditional straight through finder provides an image that is correct left-to-right, but inverted top-to-bottom. If you are using the latter type of finder, simply invert your star charts to make them correspond to the view in the finder.

You can plan a star hop using either printed charts or planetarium software on your computer. We much prefer using planetarium software because it lets us print out custom charts set to whatever limiting stellar magnitude [Hack #13] we want, and it prints finder circles directly on the charts.

There are two ways to plan a star hop using printed charts. The first method, although it is more commonly used, is less desirable:

Use the original chart

With this method, you use a transparent overlay on the original chart to plan your star hop as you actually do it. There are some disadvantages to using this method, not least that it requires more of your precious observing time. Also, if you use an undriven scope, your last hop is drifting out of view as you plan the next hop.

Some charts come with a transparent overlay with various size circles on it, one of which may correspond closely to your finder's field of view. If your chart does not provide an overlay with a circle of appropriate size, use a laser printer with overhead transparency film to print your own overlay with whatever size circles you need. Ink jet printers are generally unsuitable for this task, as their ink is often water soluble and runs at the first hint of dew.

Use a photocopy of the chart

Using a photocopied chart has several advantages. First, you can plan your star hops beforehand instead of wasting observing time by doing the star hops while you're observing. Second, you can put the finder circles right on the chart for easy reference. Third, because you're preparing the chart ahead of time, you can take the time to write notes on the chartthe magnitudes of the various stars, characteristics of the object you're looking for, and so on.

The best way to use the copied chart is to prepare a transparent overlay with a finder circle that corresponds to the field of view of your finder at the scale of the chart you are using. (Make sure the photocopy is at 100.0% the size of the original, or the finder circles will be the wrong size. If in doubt, photocopy an accurate ruler or measuring tape and compare the size of it on the photocopy to the actual ruler. You can then reduce or increase the size of your printed overlay circle as necessary.)

Use the pointy end of a compass to punch a small hole at the exact center of the overlay finder circle. Then, as you position the finder circle for each hop, punch a hole in the chart. (Make the hole small enough or large enough that you won't mistake it for a star.) When you finish planning the star hop, set your compass to draw a circle of the correct size and use the compass to draw a finder circle at each hole position.

The following four figures show an example star hop from the first-magnitude star Spica in Virgo to the Sombrero Galaxy, M104. To begin the star hop, center Spica in your optical finder. Pivot the scope along the line from Spica to Porrima, offsetting a bit toward the constellation Corvus. When Spica is near the edge of the finder field, you'll see an arc of four bright (5th and 6th magnitude) stars on the opposite edge of the finder field, as shown in Figure 2-18. (This star hop sequence uses a RACI finder with a 5.5° field of view.)

Figure 2-18. The first hop from Spica to M104

These charts show stars only down to 8th magnitude, which is about what you'll see in a 30mm finder from a typical suburban observing site. With a 50mm finder, or at a dark site, you'll see between one and two magnitudes deeper, which means that many, many more stars will be visible in the finder. That's why it's important to select the pattern for each hop from among the brightest stars in the field. Other-wise, it's easy to become confused with the plethora of dimmer stars that are visible.

Continue pivoting the scope on the same line to put that arc of stars on the opposite edge of the finder field, as shown in Figure 2-19. The 5th magnitude star 40 Psi Vir appears in the field of view, as does an arc of three 6th magnitude stars near the edge of the field.

Figure 2-19. The second hop from Spica to M104

Continue pivoting the scope on the same line until 40 Psi Vir is on the trailing edge of the field and the 5th magnitude stars 26 Chi Vir and 21 Vir appear on the leading edge of the field, as shown in Figure 2-20. M104 is now in the finder field of view, although at best it will show weakly in a 50mm or larger finder from a very dark site.

Figure 2-20. The third hop from Spica to M104

Pivot the scope on the line from 26 Chi Vir to 21 Vir until 21 Vir is at the edge of the field. Note the equilateral triangle formed by three 6th magnitude stars, two of which are just outside the finder field in Figure 2-21. When you see that triangular pattern of stars, you've gone just slightly too far. Pivot the scope in the opposite direction until the two stars have just disappeared from the field of view, and M104 is centered in the finder (and the eyepiece). You can confirm the position by the position of the 6th magnitude stars HR4779 (visible here at 6:00) and HR4822 (at 8:00). For further confirmation, note the double star just below the center of the finder field. This 7th magnitude pair is separated by about 5.5 arcminutes, and is quite prominent in the finder.

This star hop is a relatively easy examplein reality, most astronomers would simply use a Telrad to locate M104 geometrically from Spica and Porrimabut it illustrates the process. Rather than 5th and 6th magnitude stars, many star hops use 7th and even 8th magnitude stars. To find an object that's really out in the middle of nowhere, you may have to use 9th and 10th magnitude stars, which are at or beyond the limit of a 50mm finder.

Figure 2-21. The fourth hop from Spica to M104

When that happens, you'll find yourself doing eyepiece star hopping, which is just what it sounds like. You use the optical finder to get as close as possible to the object, and then you begin using the main telescope to locate star patterns dimmer than those visible in the finder. For eyepiece star hopping, it's important to have a finder eyepiece with a wide field of view [Hack #48].