Sky Orientation through a Telescope

Through some telescopes, night sky objects will appear to be upside down, back to front or both at the same time. Add the often confusing equatorial mount movement and tilt, and you'd be forgiven for thinking something was wrong.

Each telescope design has a different optical configuration and that these different systems produce different views of the skies. With time you get used to the way your telescope's optics work just like you get used to your reflection in the mirror, or using a rear-view mirror in the car. In these cases, our brains reverse the images without a second thought, and a telescope is no different.

Despite the fact that there is no "up" in astronomy, after all, what is up in the northern hemisphere, is down in the southern hemisphere, it is often comforting to correct things to what you are accustomed to. Not only does it bring objects back to their familiar orientation, it also makes reading star charts and star hopping easier - when you are trying to star hop to a faint target under the glow of a red torch, revered images can be trying.

The difference in orientation is a consequence of how the light is brought to focus be each telescope design.

Newtonian Telescope Cutaway showing the focal length

In general, if your telescope has an even number of elements - such as a Newtonian reflector your target will appear upside down. If it has an odd number, such as a Nasmyth-Coude with its three-mirror configuration, the image is reversed left to right. A simple refracting telescope produces an upside-down view.

Changing face of the Moon

Moon Correctly Orientated

Moon up side down in Newtonian

Moon upright and mirrored

Moon upside down and mirrored

Correcting Sky Orientation

Fortunately, there are a few accessories which use prism diagonals to solve this problem. They provide a correctly oriented view. Porro prisms (classical erecting prisms) provide correct images while allowing viewing straight through the scope.

Star Diagonals

45° Erecting Prism

Star diagonals work by adding a mirror angled at 45° into the light path. This bounces the light through 90° from the direction it entered the telescope which has the effect of orientating the image so they are the right way up, but back to front.

Erecting Eyepieces

These also reflect light through 90°, but they use prisms. They are sometimes sold with telescopes but are much more useful for terrestrial observation as the extra glass they contain dims the view of faint starlight.

Remember that when adding extra equipment between your telescope and your eye or camera, the more degraded the image will become. You will lose light as optics will scatter some away, and you may introduce aberrations to the image.

Tutorial Series

This post is part of the series Observational Astronomy. Use the links below to advance to the next tutorial in the couse, or go back and see the previous in the tutorial series.

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Your First Night With Your First Telescope

Polar Alignment of an Equatorial Telescope Mount

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