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Sky Orientation through a Telescope

By on in Astronomy

467 words, estimated reading time 3 minutes.

Observational Astronomy Series
  1. Tips for Getting Started in Astronomy
  2. Dark Eye Adaption - How We See In the Dark
  3. Light Pollution
  4. Using Star Charts and Measuring Distance
  5. Constellation Guide
  6. Binocular Astronomy
  7. Moon Watching - How to Observe the Moon
  8. Buying Your First Telescope
  9. Your First Night With Your First Telescope
  10. Sky Orientation through a Telescope
  11. Polar Alignment of an Equatorial Telescope Mount
  12. Useful Astronomy Filters for Astrophotography

Looking through an eyepiece, the sky will take on a different orientation depending on which telescope you use. This is due to the telescope optics reflecting and refracting the light in different ways.

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 shows the path of light through the optical tube assembly and how the sky orientation changes
Newtonian telescope cutaway shows the path of light through the optical tube assembly and how the sky orientation changes

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 Correctly Orientated
Moon up side down in Newtonian
Moon up side down in Newtonian
Moon upright and mirrored
Moon upright and mirrored
Moon upside down 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
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.

Last updated on: Monday 15th January 2018



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