One of the most important factors to amateur astronomers is seeing, which is defined as the steadiness of the atmosphere at your observing site. You can tell how good (or bad) the seeing is by viewing a reasonably bright (2nd magnitude) star near the zenith. If it’s twinkling wildly, the seeing is bad. But if the image is relatively steady, your seeing is good. And while the overall seeing might be good or even great, you may still see its effects when you look through your telescope. Could it be tube currents?
How to tell
The degradation of seeing due to tube currents (also called thermals) is a widely recognized problem. But how do you know if it’s your problem? Point your telescope at a fairly bright star and use your focuser to defocus its image. This will allow you to actually see if any air currents are present. If the image is reasonably steady, your problem isn’t tube currents.
If, however, you see lots of circular motions moving around inside the image, you have severe tube currents. My late observing buddy, Jeff Medkeff, once told me, “If your out-of-focus star image has ‘hair’ or a ‘ring of fire’ around the outer edges, and these patterns change subtly over a period of 30 seconds to five minutes, then you have image-damaging tube currents — even though no swirling air is visible in the star itself.”
The solution
This problem is easy to fix, but you have to have the right piece of equipment. The solution is to use a small, low-flow fan to blow the warmer air out of the tube. Your goal is to get your mirror to the same temperature as the outside air. Once you’ve done this, recheck the out-of-focus star image and you should see noticeable improvement.
