Atmospheric Limitations

If there’s one thing I find I end up explaining to people time and time again at public outreach events, it’s the concept of atmospheric seeing conditions and how they affect the view through a telescope.  Whenever our group gets together, there is always a handful of different telescope designs and sizes on hand for the public to look through.  I am often asked why the view through some of the smaller telescopes appears to be better than that through the larger telescopes at the event.  It surprises most people when I tell them that most of the time here in the northeast a large telescope doesn’t always equate to having a better view. The problem is as simple as the air we have to look through.  Regardless of how big or how good your optics are, the thing that has the most influence on how well your optics perform is the atmospheric conditions you are looking through in the first place.

A good analogy I always like to use goes like this.  It is the middle of the summer on a hot muggy day and you are looking at a mountain far off in the distance.  You notice that the top of the mountain is much clearer than the bottom, and a mirage prevents you from clearly seeing the surface of the road leading into the distance. You also notice that the air just above the road seems to shimmer.  This all happens for a few simple reasons.  Light gets refracted, or “bent”, differently as it passes through air at different temperatures.  As the sun’s energy heats the surface of the road, the air directly above it heats up and starts rising.  As it rises, cooler air has to rush in to fill the void.  This causes the air to be very turbulent, and light passing through it refracts, or “bends”, differently depending on the temperature of the air it is passing through.  This causes the shimmering effect seen near the road.  On a larger scale, it is the reason why the top of the mountain appears clearer than the bottom.  Looking higher and higher above the horizon, you are effectively looking through less air that is close to the surface of Earth so you are looking through less air that is turbulent.

The difference in the refracting of light through differing air temperatures is best seen as the mirage appearing near the surface of the road.  Light refracts more the higher the air temperature is, so light incident from the sky gradually gets “bent” away from the road the closer it gets to the hot pavement surface until finally the light is traveling “back” at the observer. Of course light refracting to this greater extent only happens really close to the surface of the Earth and even then the light being refracted is coming in from a very shallow angle, originating quite close to the horizon itself.  Otherwise everything we would look at on a hot summer day would be distorted!

Looking through a telescope is very much the same as looking at that mountain far off in the distance.  Astronomers use the term atmospheric seeing to describe how steady the atmosphere is.  The less turbulence there is in the atmosphere, the better the seeing conditions are, and hence the better the views through a telescope.  The atmospheric seeing conditions directly influence the smallest point a telescope will be able to resolve.  Think of it this way.  If you are looking at the moon through a telescope on a night with excellent seeing conditions, you are able to resolve many more of the very small craters on its surface than in poor seeing conditions where everything seems to blur.  In fact the seeing conditions vary continuously and while looking at the moon at higher magnification, it becomes evident that the conditions change pretty fast sometimes.  You’ll be looking at the moon and see finer detail come in and out of view as though the focus is changing back and forth!

It is for this reason that using a larger telescope doesn’t always equate to better views.  The atmosphere plays the roll of the final lens element in any telescope design, and the effect of atmospheric seeing conditions  are greater for large telescopes because there is a larger column of air the telescope has to “look” through.  Most of the time when I take the telescope out, I use my smaller telescope for this reason.  Only when the seeing conditions are excellent do I venture to bring out my larger telescopes.  Unfortunately, in this area of the world, this means only being able to use my 11 inch SCT a handful of times every year!