A guide to seeing and atmospheric transparency
What Moving air in the atmosphere can spoil our views of the stars, making them shimmer and dance in the eyepiece
How to Rate the stillness of the atmosphere and your view of the stars
Where Find observing locations with the stillest views
Tricks Techniques to create placid air around your scope
The weather is generally considered to be the biggest hindrance to astronomy. What’s the betting that the night you decide to use your Christmas telescope is 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 moments 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. 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. On the other hand, deep-sky objects like galaxies and nebulae aren’t that affected by bad seeing.
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 scope 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 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 a quiver of turbulence at all.
II Slight shimmers; moments of stillness last several seconds.
III Average seeing; larger air tremors blur the view.
IV Poor views with a constant and disturbing swell.
V Bad views with severe undulations; so unstable that even quick sketches are out of the question.
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 completely 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 the bad seeing.
How faint can you see?
Atmospheric conditions have an impact on the faintness of the stars you can observe. Use the chart below to check the faintest stars you can see by looking at Ursa Minor on a very clear night to work out your limiting magnitude. This is the faintest star magnitude, or brightness, that you can see from your location – higher numbers mean fainter stars.
Transparent conditions, though, are good for larger, fainter 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 atmosphere because the air is still. These conditions are good for looking at the Moon and stars.
A good way to think of it is to imagine a swimming pool with a penny coin on the bottom. The water represents our atmosphere and the coin the starry object you’re looking at. Through completely still water (ie no air 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 moments of clarity are a wonderful thing.
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, getting rid of 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 like 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 near a range of hills, where air is forced up, causing turbulence.