Using a Map at the Telescope

By Alan M. MacRobert

A Practice Star-Hop

An observing session should begin with some indoor planning. Let's use Gemini for a dry run. If you know the constellations, you can find its two bright stars, Pollux and Castor, in the sky. These will be our starting point.

A few centimetres south of Pollux on Chart 5 of Sky Atlas 2000.0 is the planetary nebula NGC 2392, the Eskimo Nebula, as shown above. It is indicated by a little green circle with four spikes. This looks like it might be a nice item to check out! We look it up in Burnham's Celestial Handbook and learn that it is a small, round 8th-magnitude glow, which is bright enough to show up in most telescopes. So far so good.

The next step is to plan how to get there by star-hopping. This just means following a trail of star patterns to move the telescope from a place we know, such as Pollux, to some place we don't, such as the location of the nebula. The trick is not to get lost on the way.

Take the wire ring that corresponds to your finder's field and centre it on Pollux, as shown above. Several fainter stars are in the circle, just as they would be if you were looking through the finderscope at the sky. The bright star closest to Pollux is 75 or Sigma (ε) Geminorum, to Pollux's north (in the direction of Castor, which is out of the field of view). Near the southwest edge of the field is the star 69 or Upsilon (υ) Geminorum. It forms a long right triangle with Pollux and 75; Pollux is at the right angle. This triangle confirms 69's identity in the sky, where there's no convenient label next to it!

Now shift the wire ring to centre on 69; this corresponds to moving the telescope. Pollux moves off to the eastern edge of the finderscope's field, and two new pairs of stars enter the opposite edge a little north of due west centre: 64 and 65 Geminorum and 59 and 60 Geminorum. These four stars form a distinctive elongated shape.

Shift the ring to centre on 60 Geminorum. The fainter star (59) just to its southwest will confirm that you've got the right one.

Star 57 has now come just inside the south edge of the finder's field. Shift south to centre on 57; bright Delta (δ) Geminorum is now waiting just outside to the south. Shift south again an equal amount; Delta quickly appears and can be centred just after 57 leaves to the north.

See how Delta forms an equilateral triangle with 56 and 63, to its south and east? With 63 now identified (aided by two fainter stars on either side of it) we're less than 1° from our prey.

Note the flat triangle that 63 and 61 form with the nebula. The shape of the triangle allows us to target the correct position even if the nebula is invisible, as it may be in the finder.

If we do this outdoors at night and move the telescope to match each step on the map, NGC 2392 should now be visible in the main eyepiece: a small, dim, eerie round glow quite unlike the pointlike stars, grayish-green in colour and with a very faint star at its exact centre— a prize worthy of the rather complicated chase.

The star-hopping route may seem like a lot of trouble to the beginner, whose impulse is just to sweep from Pollux "about the right distance that way." But most deep-sky objects are many times dimmer than the faintest stars on the chart and won't catch your attention even if, by luck, your tiny telescopic field happens to sweep right across them. The only way to succeed is to know exactly where you are at all times. If you suspect you're lost, go back and start over. Have patience. You'll speed up later as practice increases your skill.

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