Seeing is essentially the amount of disturbance caused by the atmosphere at a given time. Sometimes you may notice stars twinkle more on one night than another. Twinkly stars occur during times of bad seeing. When looking at the Moon through a telescope you may see it shaking or shuddering as the atmosphere is turbulent. When the seeing is considered to be good the Moon and stars will be still and objects will have higher resolution. Typically the seeing worsens as the light from the sky traverses more of the atmosphere. This is the reason why objects appearing on the horizon don't show up as well in a telescope or binoculars.
Planets are notoriously affected by bad seeing. Astrophotographers have an interesting way around this problem. A high frame rate video camera is used to capture approximately 30 seconds of footage of a planet through a telescope. The frames with good seeing during that 30 second period are kept, while the bad frames are filtered out. This produces amazing images of planets taken from places as simple as your backyard. When the weather gets better I will try out my 100 fps camera on Venus and Jupiter to test this method out.
There are some things you can do to improve your local seeing.
- Leave your telescope outside so that it cools down to the ambient temperature. Temperature differentials cause air flow which, in turn, leads to poor seeing.
- Observe on grass. Concrete tends to absorb and radiate more heat causing additional temperature differentials.
- Observe on a platform to avoid low to the ground air currents.
The other important thing to note before hauling out the telescope is transparency. Transparency is simply how clear the night sky is. Water vapor(clouds), dust particles, smog and smoke all negatively affect the transparency. Some of these scatter the light making the background lighter and the objective object dimmer. In bad conditions the sky may appear completely opaque disallowing any chance for visual astronomy.
Consider an object underwater at a lake. The amount of ripples relates to the seeing and the amount of dirt and dust in the way relates to the transparency.
Unfortunately for astronomy a lot of the time good transparency may mean bad seeing and vice-versa. After a storm the rain has cleared away a lot of the pollution from the skies giving good transparency. However, the wind currents in the vicinity will provide poor seeing. Thus two simple ideas must be shared.
- Good seeing, poor transparency - Planets and the Moon make great targets
- Poor seeing, good transparency - Deep-sky objects make great targets
An example of comparative seeing is shown below in an image taken by the well-known astrophotographer Damian Peach.
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| Comparative seeing of Jupiter |
As always, it is always good to check the forecast before heading out observing. A great site for Calgary is linked below. It provides the cloud cover, transparency, seeing, darkness, wind, temperature and humidity.
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