How to determine how good seeing is?

[Bried]

Hello there,

I'm sure I read somewhere there is a technique that is used to determine how good seeing conditions are by looking at a star. I've been searching around trying to find something on the subject but can't find anything. I'm sure it was a particular star in question that it is analysed to determine seeing conditions but for the life of me I can't remember. Can anyone shed some light on this technique or am I making things up in my mind?

Thanks

Brian

[kniclander]

close double stars are a good test - if you can split them at relatively low mags then the seeing must be good. epsilon lyrae is the classic summer test. not sure what good ones would be in winter - maybe castor? also, look at jupiter - some nights it seems to "boil" - others (rare) it's rock steady. You'll notice the difference after a few sessions.

[Freff]

close double stars are a good test - if you can split them at relatively low mags then the seeing must be good. epsilon lyrae is the classic summer test. not sure what good ones would be in winter - maybe castor? also, look at jupiter - some nights it seems to "boil" - others (rare) it's rock steady. You'll notice the difference after a few sessions.

Good synopsis.

[John]

Kniclander's suggestion is good - I use double stars as a test of seeing conditions. Polaris has a 9th magnitude companion that's worth a look to see how things are.

There are also tests with your eyes like counting the number of stars you can see in the great square of Pegasus and the ability to see the 4 stars in the square of Ursa Minor.

[Telrad]

I don't know what techniques are employed to determine good seeing conditions, I guess not too much turbulence, thermal equilibrium and transparency with no clouds (obviously), using double stars seems like a good test; but here are my rules:

1) If the night is perfectly transparent, it is a low power night, but not necessarily a high power night;

2) If the night is perfectly steady, it is a high power night, but not necessarily a low power night;

3) If the night is transparent AND steady at the same time, it is a PERFECT night, both for low- and high-power viewing;

4) If the night is cloudy, it's SGL time...

[perrin6]

All good advice but can we tell the difference between bad seeing and an un-aclimatised (ie warmer than the atmosphere) mirror ??

[stolenfeather]

4) If the night is cloudy, it's SGL time...

Ha! Ha! It sure is!

Isabelle

[kniclander]

There are also tests with your eyes like counting the number of stars you can see in the great square of Pegasus and the ability to see the 4 stars in the square of Ursa Minor.

imho these are more a test of sky darkness than seeing.

ps - make sure your scope's cooled down first;)

[John]

All good advice but can we tell the difference between bad seeing and an un-aclimatised (ie warmer than the atmosphere) mirror ??

Look at an out of focus star disk. If there is still warm air in the tube you will see them against the disk. Sometimes you can see spikes and plumes of heated air as well. This images shows them:

http://file.vustv.com/ARxKqkEEV2Ztw.jpg

[Mr Q]

A quick test is to look at a fairly bright star that's nearly overhead. If it's "twinkling" or scintilating, the "seeing is poor and poorer closer to the horizon. If the star appears "steady", the seeing is fair to good.

"Seeing" is a measurement of the amount of turbulents in the atmosphere that effect pin points of light while "transparency" is a measurement of how clear the atmosphere is from pollutants that can scatter light form ground sources and brighten the sky - washing out faint objects.

Once you determine the seeing conditions near the zenith with the unaided eye, a more accurate test can be made by being able to "split" (seperate) close double stars near the zenith. Both seeing and transparency suffer most near the horizon and least overhead, where we are looking through a smaller amount of atmosphere.

[FraserClarke]

For a quick judgement, I use the twinkling of stars like Mr Q. My rule of thumb, which I've spent a few years 'calibrating' with DIMMS at telescopes around the world is;

+ No twinkling at 15 degrees: <0.7 arcseconds

+ Twinkling at 20 degrees and below: ~1 arcsecond

+ Twinkling at 45 degrees and below: ~1.5 arcseconds

+ Twinkling at 60 degrees and below: ~2 arcseconds

+ Twinkling at Zenith: >3 arcseconds

The UK is usually somewhere in the middle three.

The classical definition of 'seeing' (how steady the sky is) has nothing to do with 'transparency' (how clear the sky is). Quite often good seeing happens when you have a thin hazy layer of cirrus around (presumably a stable upper atmosphere is condusive to both...).

[Bried]

Thanks everyone. Very helpful and now I've got my head around the difference between seeing and transparency too.

[kniclander]

I would say that tonight the seeing was good (jupiter was pretty steady) but the transparency was very poor (some stars I can usually see, I couldn't see).

[pvaz]

4) If the night is cloudy, it's SGL time...

LOL! I been getting too many of those lately.

For steadiness I just look at some stars, naked eye, and see how much they apear to twinkle. Avoid the very bright ones, something at mag 5 up to 3 should do.

For transparency I just let my eyes adapt to the dark and see how well defined is the milky way.

If you check it every time before using the scope, then do your thing, after a while you can simply look at the sky and have a pretty decent guess of what you'll be able to see, if you take the scope out.

I know it's not very scientific but it just works nice and easy.