What is "seeing"?

[thegableguy]

Howdy

I see a lot of AP posts that mention the seeing being good or bad. How is this judged, is there a way to determine what it's like or will be like without actually spending an hour imaging? Is the standard method of imaging simply setting it up and hoping for the best, or can you look at a cloudless sky and somehow know there's not a lot of point in getting your rig set up? I've noticed that some cloudless nights the stars are amazingly clear and beautiful, other nights fairly dim - is seeing something you can easily judge by eye, or do you just have to spend the night shooting and see what you get?

I've been looking at SkippySky but I don't understand the differences between cloud cover, transparency and seeing...?

(http://www.skippysky.com.au/Australia/index.html)

[Merlin66]

This may assist with the Seeing question:
http://www.damianpeach.com/pickering.htm

[Camelopardalis]

+1 as above :)

You can estimate it with the eye...if stars are twinkling above, say, 40 degrees altitude, then the seeing generally isn't worth imaging in, unless your imaging resolution makes it irrelevant. The jetstream prediction is often a predictor of seeing, high velocity jetstream usually isn't good for seeing and vice versa, but there are other local factors that can come into play. Try looking at a reasonably bright star with a grab and go and increase the magnification slowly.

Transparency is how clear the air is...water vapour and particles in the air scatter rays of light resulting in reduced contrast. The same effect can happen when there's a thin veil of high cloud. By eye, calibrate yourself by looking at something like the Coalsack. On a night of poor transparency it can become practically invisible compared with a good night. Transparency is often linked to humidity, but not necessarily. Any nearby light pollution (street lights...) can often be reflected back when the transparency isn't good, making the sky glow unnaturally.

[Atmos]

In good seeing I image unbinned, in poor seeing I image 2x2 binned. I should probably have done a lot more 2x2 binning this year! It has been a shocker! I calculate what the seeing is like from plate solving a 5s luminance at high altitude. My eye sight doesn't allow me to eye ball star twinkling very well. I cannot see it at zenith until it is HORRIBLE!

[silv]

ah, thanks for the question - and the answers :)

[Wavytone]

Jump into a swimming pool, take a deep breath and swim down to the bottom, slowly (diving is too fast).

Once you get down there roll onto your back and look up at the surface.

What the water is doing to the way you see the surrounds above the water is very similar to what atmospheric turbulence does to light on the way down to your telescope, in "poor seeing".

[thegableguy]

Thanks for all the answers. That Pickering animation is great.

I guess I was just unclear about the difference between seeing, transparency and cloud cover. Someone kindly sent me a good summary via private message, along with a nicely detailed description of FWHM - something else I was unclear about.

Now I think I understand; you can have good transparency, but still have less than great seeing due to jetstreams etc. Cloud cover will ruin both transparency and seeing. Do I have that right?

Also, are you saying I should be able to see just by the naked eye whether the stars above about 40 degrees are twinkling, or through a telescope?

I'm trying imaging again tonight, it's clicking away right now, but I have to say the subs are looking a lot darker than I was hoping. Is there a time of night when the seeing is usually better or worse? Is it always better in the hours before sunrise, or is that just for meteor showers?

[Camelopardalis]

Yeah obviously any cloud cover ruins our chances of seeing something beyond, but the transparency and/or seeing could be otherwise decent. Indeed, I've had a couple of surprisingly good planetary imaging sessions with passing clouds.

But also the winds that blow the clouds along can wreak havoc with the seeing, and if there's wind up there it usually blows everything else away giving decent transparency. On the flip side, poor transparency may result in decent seeing. There's not just a single possibility!

As for twinkling, yeah just look with your eyes...if the stars are twinkling by eye, the seeing will be tumultuous. That doesn't mean that there won't be good moments. Seeing can change from good to bad in an instant. If the stars look steady to the eye, have a peek with a grab and go. With a reflector, if you defocus slightly to start revealing the rings they'll flare like they're on fire if the seeing is poor, whereas on a good night you'll see the airy disc with good focus. Ignore stars near the horizon as they are almost always twinkling.

With your image, you say it's darker than you were hoping...is it the background that is darker or is it the stars aren't as bright as you were expecting?

[andrew_d_cool]

Gable Guy,

Every page of SkippySky has a link to a Help file that offers some explanation.

Andrew
www.skippysky.com.au (http://www.skippysky.com.au)

[andrew_d_cool]

or, just feed your image into Skippy_Seeing along with your Focal Length and the pixel size of your sensor, and out pops the Seeing for that image!

http://www.cool.id.au/Skippy_Seeing/

Andrew
www.skippysky.com.au