All backyard astronomers have been a victim of ‘bad seeing’ at some point in their stargazing career. Have you?
It’s a cloudless night, and you’re looking forward to a wonderful stargazing session you’ve been planning for days…
You point your scope toward the skies - and instead of a clear, crisp star, you see this.
Does this seem familiar? Are you curious why this happens?
And most importantly, do you want to know how to minimize this problem so your well-planned sessions are not wasted?
If your answer is yes to these questions, consider your problem solved!
We here at Love the Night Sky have created this article to help you know why, when, and how this happens. In this article, you’ll discover:
- What is ‘seeing’, anyways?
- How is Seeing Measured
- Causes of Poor Seeing
- How to Maximize your Chances of Good Seeing
- Astronomy you can do in Poor Seeing
Here we go!
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What is 'Seeing', Anyways?
Seeing is a term commonly used among astronomers. It refers to the steadiness of the atmosphere. Good seeing has little to do with cloud cover, instead, it is the absence of thermal turbulence.
What is thermal turbulence, you ask?
Thermal turbulence is the wobbliness, or shimmering seen while viewing objects in certain air conditions.
Examples include the heat haze from an asphalt road on a hot afternoon or the air behind a vehicle’s exhaust.
Anything that allows light to pass through it, like Earth’s atmosphere, has a refractive index. Changes in the refractive index cause a bend in the path of light travelling through it. If these changes happen quickly enough, you get a wobbly image.
Okay, good. But how is this related to good seeing?
When you view the stars, you are seeing them through a 100 km thick atmosphere. Different layers of the atmosphere have different temperatures, hence different densities. When these layers move, the total refractive index of the air between you and the stars changes and you see a little wobble in the stars.
If this is within a limit, it causes a pleasing twinkle in the stars. But, high winds or big temperature differences between layers of atmosphere turn this twinkle into poor seeing. Which ruins telescope detail and is no good for taking photos of stars in the night sky.
How is Seeing Measured?
There is no perfect way of measuring seeing, since it is, in part, your comprehension of 'wobbliness'. Its perception varies from person to person.
This hasn’t stopped the development of scales to measure seeing and put a number to it, though. There are dozens of scales for measuring seeing, each having their pros and cons.
Below is the most basic scale used to measure seeing:
- 1 - Very poor images, impossible to see details or to sketch
- Almost continuous distortion with occasional brief good moments
- 4-6 - More continuous distortions with short intervals of good seeing
- 7-8 - Intervals of perfect seeing with fine scale distortions between
- 9-10 - Perfect seeing with steady images at high magnification
(source - where you can also find comprehensive details on other systems used for measuring seeing conditions.)
Poor seeing has a variety of causes and is present in different levels, as we’ll discuss below.
Causes and Types of Poor Seeing
Broadly speaking, poor seeing can be classified into three types:
1. Tube Currents
Temperature differences between the air in your telescope tube and the outside world cause air currents to form. These 'tube currents' cause poor seeing in larger scopes, but are not a big problem in smaller optical tubes.
Some modern telescopes come equipped with a fan which expels the air inside the tube of the telescopes, which does add to their cost.
You don’t need to go to such expense, though.
To minimise tube currents you need the air temperature inside your telescope tube to match the outside air. Leaving your telescope outside for a couple of hours before you use it will have the desired effect.
The only exception is in winter when temperatures can fall rapidly at sunset. Even if you have left your scope outside, it can take more time for your telescope tube to cool than the outside air.
2. Ground Seeing
Local effects, such as hot air rising from a driveway or hot gases rising from a nearby factory cause poor ground seeing.
The best way to get rid of this is - you guessed it - go to different place.
Try to find somewhere nearby which is dark and away from built-up areas. It will naturally have less of these thermal disturbances. Also, avoid standing your telescope on concrete or asphalt pads as they can radiate heat for hours after the sun sets. Try setting your mount on firm grass instead which cools down more rapidly.
If you have tried the remedies above and seeing is still poor, it’s most probable that you're dealing with our last culprit: poor atmospheric seeing.
3. Atmospheric Seeing
This has the biggest impact on astronomer's seeing quality. Caused by air currents high up in the atmosphere, such as the jet stream, it is the cause of poor seeing you have the least control over.
Sadly, there isn’t much we can do about this. But all is not lost!
To maximize your chances of having a good stargazing session, read on!
(For a more detailed description of types of poor seeing conditions, this article is a good read.)
How to Maximize Your Chances of Good Seeing
We can't control or change poor atmospheric seeing, but that doesn’t mean that you have to depend on luck for a good stargazing session.
Here are a few things to keep in mind to decrease your chances of having to deal with poor atmospheric seeing.
Firstly, it is vital to understand that seeing conditions are, to a large extent, dependent on the time of day. This is because convection currents (which are the main cause of poor seeing) are absent at certain times of the day.
Annoyingly, evenings are usually a bad time to stargaze.
This is because the land releases the heat it absorbed in the afternoon back into the atmosphere. This radiation causes high amounts of convection.
The best time to plan for stargazing sessions with good atmospheric seeing is between midnight and dawn.
Secondly, good seeing is most likely when a high-pressure system settles in and brings clear skies for several days running.
This site details the five weather conditions that induce good seeing:
- Oceanic winds with high pressure
- Thermal inversion
- The passage of anticyclonic ridges
- Dawn and twilight
- Before a cold front
Track this for yourself by keeping a record of seeing against local weather conditions. You should learn which weather gives the best and worst seeing in your neighborhood.
Astronomy you can do in Poor Seeing Conditions
Okay, so your well planned skywatching session turns out to be a bad seeing time. Never mind, there are still a few things that you can do, depending on the amount of turbulence.
One of the best things to do in poor seeing conditions is to give up precision observations! Keep the telescope to one side, and look out for those places in the sky which don’t normally command your attention.
For instance, you probably don't get the time to learn the sky as a whole. As backyard astronomers, we're happier gazing through a scope.
But, learning constellations is best done with only your eyes. Once you know them by heart, backyard astronomy becomes more enjoyable.
When conditions only moderately poor focus on observing larger objects, like the moon. Use the poorer nights to see what new features you can find.
Visibility of meteor showers is not badly affected by poor seeing conditions, and you may enjoy binary star spotting too.
There are many meteorological websites that predict seeing conditions. If you live in North America, this chart from the clear dark sky website can help you plan your skywatching sessions with ease.
Now you know that seeing is a measure of atmospheric turbulence.
You've seen how we, as backyard astronomers, can measure it. You've also learned how we can get the better of it.
Sadly though, there will always be (too many) nights when mother nature defeats us. But, for those times, there is always the Cloudy Nights forum to turn to.
Written by Adish War