Eyepiece cheat codes: Determining directions in the telescope

In the Space Walk Among the Stars audio guides, I frequently refer to compass directions or position angle. Sometimes I say "right" or "left" from a path we're following in the scope, but that really only applies to Newtonian reflectors, and for that I apologize.

Directions in the telescope can be confusing for beginners and even long time observers. It all depends on how many mirrors your telescope has. Generally, an odd number, and your telescope will mirror-reverse the view. An even number and your telescope will rotate the view 180 degrees.

A Newtonian reflector has two mirrors, the primary at the bottom of the tube and the secondary, the smaller one which directs the light to the eyepiece. In this case, an even number, therefore the view is rotated 180 degrees. (The view is rotated additionally because your focuser is usually located off to the side and your eye is positioned differently throughout the sky, so don’t assume south is always “up.”)

Most people with refractors and Cassegrain telescopes use a 90-degree mirror star diagonal before the eyepiece to give a more comfortable viewing position. That counts as one mirror in a refractor and three in a Cassegrain (which has a primary and secondary mirror plus the diagonal). An odd number, therefore these telescopes will keep the image correct side up, but mirror-reversed. (How your diagonal is rotated will affect what direction is actually "up" in your view.)

If you use an “erect image prism diagonal,” such as an Amici prism or pentaprism, in your refractor or Cassegrain, then you get a “correct image” that is neither rotated nor mirror-reversed (but there may be disadvantages that I won’t get into here).

One thing that stays the same regardless of your telescope type is that without any tracking motor engaged, the stars will always drift to the west (or, if you like, enter the field of view from the east). That’s because the Earth is rotating toward the east, and your telescope is fixed to the Earth. So you can always start with an easy reference point by noting the direction toward which the stars are drifting- that’s west. From there, you apply the correct diagram below and you are good to go!

If you’re interested in more information on how your equipment affects image orientation, see this article from the British Astronomical Association.

Choose the diagram that applies to your telescope

In all telescopes, stars and other objects will always drift to the WEST if the scope does not have a tracking motor operating. Know your telescope and directions in the eyepiece. 

The diagrams below show an example of how position angle (PA) is used to indicate the direction from a primary star to its secondary companion (PA 225 in this example) in reflectors and refractors/Cassegrains (with diagonal). You can also give any directions in the sky using PA or compass direction (270 or west, for example), as in many of these Space Walks. This view would be facing south.

This is for a NEWTONIAN REFLECTOR, such as a Dobsonian, and also for a straight-through finderscope. These show the image rotated 180 degrees from what you would see just looking up or in binoculars. North is COUNTER-CLOCKWISE from West:

 

Tip: In Sky Safari Pro, tap the field of view measurement in the upper right and select "Flip: Both" so the chart will match your view in the eyepiece. Note that it might still be rotated somewhat because of your eye's orientation to the eyepiece.

REFRACTORS and CASSEGRAIN telescopes, typically used WITH A MIRROR DIAGONAL, will show the image correct side up but mirror reversed from what you would see just looking up or in binoculars. North is CLOCKWISE from West:

Tip: In Sky Safari Pro, tap the field of view measurement in the upper right and select "Flip: Horz" so the chart will match your view in the eyepiece.