Using a Map at the Telescope

By Alan M. MacRobert

From Map to Sky: Know Your Directions

The biggest pitfall in going from map to sky is keeping your directions straight in the eyepiece.

Remember that in the sky, celestial south is not up but toward the South Celestial Pole, no matter how cockeyed this direction may be in the eyepiece. To find the south side of your eyepiece view, just nudge the telescope a bit toward the South Celestial Pole (only a handspan from the Southern Cross). New stars will enter the view from the south side. Turn the map around accordingly, so south on the map is oriented the same way.

This south-nudging trick will become such a habit at the telescope that you'll forget you're even doing it.

If you have an equatorial mount, turn the eyepiece of the finderscope so the crosshairs line up with the telescope's motion as you sweep north-south or east-west. The crosshairs will now mark the four cardinal directions no matter where you point the scope.

Okay — you've found north and south in the eyepiece. East and west can be a bit trickier, depending on your telescope.

East is 90° counterclockwise from north if you're looking at a "correct" or right-reading image, just like on a map. A correct image is given by any optical system that has an even number of mirrors. Examples are a Newtonian reflector, which has two mirrors, and a straight-through refractor, which has zero.

But east is 90° clockwise from north in a mirror image, which is what you see in any optical system that reflects light an odd number of times. The usual culprit is a star diagonal on a refractor or Cassegrain-style telescope. A mirror-image view will not match a correct-image map.

Note that this is not the same as the image merely being upside down. In that case you could just turn the map upside down too. A mirror image cannot be made correct no matter how you turn it.

To get a correct image you can simply take out the diagonal and reinsert the eyepiece to view straight through. This is especially important to do to your finderscope, if it came with a diagonal. But straight-through viewing with your main telescope is rarely practical unless your target is near the horizon.

Alternatively, you can photocopy your map, turn the photocopy over, and shine a flashlight up through the paper from beneath to view a mirror image of the printing through the back side. Better yet, photocopy maps onto clear overhead transparency sheets, turn the sheets over, and tape them to a red background.

Some people simply get good at the mental gymnastics of flipping a mirror image left-for-right in their heads. Others set up a small mirror on their chart table in which to view their maps. Perhaps the best solution is to buy a diagonal that is made with an Amici prism, which employs two reflections instead of an ordinary diagonal’s one. At the cost of possibly diminishing image quality a bit, the Amici prism will provide a view of the sky that can be directly compared to your charts.

When star-hopping, always think in terms of north, south, east, and west — never up, down, left, or right, or you'll quickly get lost in trackless wastes of space. Once you get the hang of it you'll always be mumbling as you turn from map to scope: "Starting from that bright one in the north of the kite shape . . . half a finder field east to the pair in the skinny triangle . . . then a quarter finder field south to the one at the west end of the flat triangle . . . " Triangles are the most basic units of star-hop patterns, and you'll be seeing a lot of them!