Sky Orientation through a TelescopeLooking through an eyepiece, the sky orientation will take on a different appearance depending on which type of telescope you use.
This article is part of a series of articles. Please use the links below to navigate between the articles.
- A Beginner's Guide To Observing The Night Sky - Stargazing!
- Tips for Improving Your Dark Eye Adaptation in Low Light Conditions
- Light Pollution and Dark Skies - Causes and Solutions
- How to Use Star Charts, Planispheres and Star Hopping
- Top Tips for Binocular Astronomy to See The Night Sky
- The Ultimate Guide to Moon Watching and Observing the Moon
- Tips for Buying Your First Telescope - What Type? How Big?
- What to Expect From Your First Night With Your First Telescope
- Sky Orientation through a Telescope
- Polar Alignment of an Equatorial Telescope Mount
- Everything You Wanted To Know About Telescope Eyepieces
- Which Astronomy Filters To Use For Astrophotography and Observation
- How to Photograph Constellations and Starry Nights
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 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.
Even though 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, but 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 by each telescope design.
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 from left to right. A simple refracting telescope produces an upside-down view.
Changing face of the Moon
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
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.
