Understanding Seeing

One of the more misunderstood astronomical definitions that is critical to our enjoyment of our hobby, whether you’re observing or imaging is understanding astronomical seeing.  Its important to understand this concept because it will directly affect your ability to enjoy the views of the objects you choose to look at as well as making sure you choose the right equipment to match the average atmospheric conditions you encounter at the viewing sites you regularly have available to you.

The “What’s Up” webcast put out a great explainer on this topic a couple of years ago and you can find it here.  I’m going to summarize some key points of this webcast, but I encourage you to view the full 50+ minutes.

First, atmospheric seeing conditions are always variable; they will constantly change throughout the night (or the day if you do solar observing or imaging).  You are resigned to chasing good seeing conditions over the entire time you are observing or imaging.  This is not just night to night (or day to day) but can be hour by hour or even minute by minute.

There are 2 factors you need to consider with regard to how well the atmospheric seeing conditions are for any particular site; the first is Transparency and the second is Stability.

Transparency is defined as the clarity of the sky.  It will define the contrast that can be seen with any object you observe.  Transparency is affected by anything that will scatter light, so problems with Transparency will primarily come from:

1. High Clouds in the atmosphere
2. Smoke such as from wildfires
3. Dust

For week sky observing and imaging, the general rule is that the dryer the air and the higher the altitude, the better the Transparency will be.

Stability is defined as how calm the atmosphere is which will translate directly to how sharp the view or image you can get.  Stability is affected by:

1. Wind, especially if the jet stream is directly above in the high atmosphere
2. Heat
3. Moisture content of the air or humidity

Just to note, high humidity of the air can actually be good in the case of planetary or lunar imaging.  When you have a lot of humidity, the air will be denser and will not move around much.

Good seeing will allow you to achieve higher magnifications with sharp views or images.  However, you need to be careful that for imaging, the image scale of your equipment matches the average seeing conditions at the site you’re working from.  If this isn’t the case, you are going to run into problems.  The seeing will dictate what kind of equipment (optical tube, imaging camera, mount) will be effective.