Heat haze and what it can do to your views

People often ask me why the magnification on terrestrial telescopes is lower than on astronomical telescopes. It's because the atmosphere does horrible things to light rays.

Have you ever tried looking at something on the ground with a high-magnification telescope, especially on a warm afternoon in summer? You're unlikely to see much at all.

Here's a photograph of an aeroplane taking off. It was taken by my mate Paul, and I've shown some of his images before. Notice the trees in the background under the wing on the right? They're pretty clear.

However, the trees that should be visible to the left through the jet engine exhaust are very blurred.

Air isn't a smooth, consistent medium. It has warm and cold pockets, which act as lenses and distort light passing through. These move around chaotically, and the effect is what we call heat haze. When you magnify the image it becomes quite obvious, with the image moving about. The more magnification, the more obvious it is.

The same thing affects high-magnification astronomical telescopes, but it's not usually as bad. First, astronomical telescopes are normally pointed upwards. Most of the disturbed air is near the ground (where the warm stuff is), so you don't have to look through much chaotic air. Second, astronomy is normally done during the night, when heat haze is less.

But it is a factor in astronomy. You may have heard people discuss "the seeing". This refers to the quality of the viewing at the time. As you can see from this quick bit of video, on a warm night, seeing can be quite poor.

Another of Paul's photos shows a flock of Pink-eared Ducks. The ducks in the background aren't just out of focus, there's heat haze distortion there as well: it's not a nice smooth bokeh. Paul tells me that when shooting with a long lens, his day is over by 9am due to this problem.

The solution for terrestrial scopes is not to magnify too much. Sometimes magnification isn't your friend.

Credit both images: Paul Dodd