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Seeing and Atmospheric Transparency
Seeing and Atmospheric Transparency

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

The movement of the atmosphere can affect your ability to observe stars and planets to a surprising extent

The weather is generally considered to be the biggest hindrance to astronomy. What's the betting that the night you decide to head out for the night that spell of fine weather changes for the worse? So you'd have thought that when the skies finally clear, your problems would be over. Surprisingly, though, even a clear night may not be the best time to go out and observe.

The issue is the 'seeing'. In astronomy, this doesn't mean how you look at something. It's a term that describes how much the view you see through your telescope is disturbed by what's going on in the atmosphere above you.

At times of good seeing, you'll get sharp, steady views through your telescope. But bad seeing produces turbulent, unstable telescope views of the Moon and shuddering, shaky images of stars. On the other hand, deep-sky objects like galaxies and nebulae aren't as badly affected by bad seeing.

This is thanks to the layers of moving air between you and the object you're looking at, the effects of which are magnified by your telescope. In the atmosphere, air at different temperatures is always moving around and mixing together. Light travels through hot and cold air at different speeds, so it is continually bent this way and that before it finally arrives at your telescope all shaken and stirred.

Sometimes there are very few moments of clarity. One of the best ways to see this distortion is to watch the Sun setting on a clear horizon. It will have a jagged appearance, thanks to the sunlight moving through layers of turbulent air.

The other factor that affects observing conditions is the transparency of the night — just how clear the sky is. After it's been raining, the sky is transparent because the rain clears away particles of dust and smog from the air. However, when it's been raining it also tends to be windy, which means that the seeing is bad. You'll notice that the stars are twinkling because of this. Transparent conditions are, however, good for large, faint objects like nebulae and galaxies, which really benefit from the better contrast. Poor transparency generally means the air is steady with good seeing, but dust and particles are sitting in the still atmosphere. These conditions are good for looking at the Moon and stars.

A good way to think of seeing and transparency is to imagine a swimming pool with a coin resting on the bottom. The water represents our atmosphere and the coin the starry object you're looking at. Through completely still water with no currents, the coin looks still, crisp and clear. In this case the seeing is perfect and so is the transparency. If the water is made to move — causing ripples — the coin's image will shake around; the transparency is still good but the seeing is bad. And if some milk is spilt in the pool so you can't see the coin very clearly, the transparency will be reduced.

It goes to show that you're at the mercy of the atmosphere ... and that moments of clarity are a wonderful thing.

Clear and present

You can't do anything about 'high-level seeing' — the air currents far above you — but you can influence the 'low-level seeing' to create steadier air conditions immediately around you and your scope. Here's how:

1. Leave your scope outside to cool to the ambient temperature, eliminating any air currents in the tube.

2. Observe on grass rather than concrete. Concrete absorbs more heat from the Sun and radiates it out to the air above it for longer.

3. Air currents tend to stay low to the ground, so it can be a good idea to raise up your scope on a platform.

4. If you build an observatory, make it using thin materials such as wood that can cool quickly.

5. The geography of your observing site affects how air behaves. Being near the sea gives you calmer air than if you're near a range of hills, where air is forced upwards, causing turbulence.

Using the Antoniadi Scale

It's very useful to note down what the seeing is when you're observing. Many astronomers use the Antoniadi scale as a measure of what the atmosphere is up to. It's a five-point scale using Roman numerals. I indicates the best conditions, while V describes the worst.

I. Perfect seeing, without any quiver of turbulence whatsoever.

II. Slight shimmers; moments of stillness last several seconds.

III. Average seeing; larger air tremors blur the view.

IV. Poor views, with constant troublesome undulations of the image.

V. Bad views with severe undulations; so unstable that even quick sketches are out of the question.

Copyright Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Date Taken: 12/21/2016
Author: BBC Sky at Night Magazine
Category: Astronomy

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