Things that happen this time of year

Roland Christen

Hello Astronuts,

I want to alert and make you all aware of things that you may encounter as the weather gets cold. And that is cool-down issues. People will take their scopes out from warm rooms, set them up and start taking images and freak out! What's happening??

The issue is almost always related to cool-down of optical components. It is not limited to the main telescope optics, but can include everything downstream - the diagonal, eyepieces, cameras etc. that are attached to the scope. What tends to happen is that warm components inserted into the focuser sends warm air up the focuser drawtube and distorts the star shapes in the image. Inside of focus you will see a flat on the out-of-focus star, outside of focus you will see a plume. It is always in the direction of vertical with respect to the scope, regardless of which way you are pointing. Only straight up at the zenith, the plume may disappear, to be replaced by a slowly swirling pattern, causing the stars to do a circular dance.

Many a customer has freaked out, sent their optics in for evaluation and not realized what may be really going on. How long does it take for things to settle down? If you take a scope out from a warm room and set it up, it might take 2 hours or more to get round stars (maybe longer for big scopes). Then if you switch the eyepiece with another one from your pocket, the tube currents will start all over again.

So, please be aware of this possibility if you don't get pristine stars right away on these cold nights. Even quartz optics are not immune to this problem. Even though the quartz itself is almost totally immune to dropping temperatures, it's really the tube currents inside the enclosed scope structure that causes the problem.


Roland Christen

Greg McCall

This time of year, it's actually starting to warm up. It was about 28 deg C today. And it will get warmer as we get into Summer.
Unfortunately, no stars lately as the east coast of Australia continues to get lots of clouds and rain.
(just one of those wet years you sometimes get with changing temperatures in the Pacific Ocean)

Andrew J

@Roland. Thanks for the “tips and tricks”. Useful.