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I want to say first off, I am a complete beginner, starting out my interest in astronomy (so go easy!).

I have been able to see some nice sights on some nights, however, I had a strange experience the other night when viewing Saturn.

I had been viewing it the night before, using my highest magnification eyepiece and got some really nice views. It can be noted my telescope has a focal length of 1000mm and aperture of 100mm and I was using a 5mm eyepiece, which was really pushing the limits of the useable magnification, as far as I am aware.

So anyway, the next night had clear skies again, if anything maybe clearer. I was excited to see Saturn again, but this time once I had found it, I put in my 5mm eyepiece and just couldn't get it into the same focus I had the previous night. I googled it and somewhere said that although the skies can be good, the seeing can be poor.

What is meant by this statement and how am I able to tell if there is going to be a night with poor seeing in advance so I know which eyepieces I can use for that particular session? The app I use to check the skies only tells me if it is going to be clear so I don;t know what to be looking out for.

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Seeing refers to turbulence in the atmosphere that distorts wavefronts passing through and results in the blurring of distant objects.

Unfortunately, there is no foolproof way of predicting what the seeing will be from night to night, though building up a database of local seeing and meteorological measurements might allow you to find some crude correlations using principal component analysis.

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  • $\begingroup$ Thank you for your answer. That's a shame it's something you can't predict easily. Just guess I'll keep going out on clear nights and hoping the seeing is good! $\endgroup$
    – MCG
    Commented Aug 22, 2017 at 6:21
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To expand on Rob J's answer:

Try for nights when the forecast is not only clear but no wind.

And almost invariably the best seeing is on the coldest winter nights, again in absence of wind.

Other things that affect seeing include: altitude (top of a mountain is already above the densest part of the atmosphere), and making sure your telescope is set up long enough to be in thermal equilibrium with the local environment, so that there's no updrafts right next to the telescope.

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  • $\begingroup$ This isn't always true. Some of the best seeing I've had was in the presence of steady wind forming a laminar airflow above me. I've also had plenty of bad seeing when there was no wind to speak of. Bad seeing is usually a function of different air temperatures (i.e. densities) mixing together higher in the atmosphere. Combine that with the jet stream, and you have a recipe for bad observing conditions even without local ground wind. $\endgroup$
    – AgmLauncher
    Commented Dec 16, 2021 at 19:19

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