Good seeing

[figgylion]

A phrase I 've come across a lot on these forums is "the seeing was good" or poor or anything in between.
Now I understand what is meant by this in visual terms but I was wondering if one of you helpful people could explain what determines good/bad seeing?
What I mean is what makes the seeing good or bad. Obviously cloud cover, light pollution, dust & smoke make for bad seeing. What conditions would make for good seeing? High/low temperature, dry/humid, atmospheric pressure,winds or lack of, time of night or even year?
Are there any weather patterns to look out for(good or bad)?
As I've only been serious with Astronomy for about six months, I haven't been out on a clear crisp winters night yet,would the seeing be any better than in the warm Summer months?(note to self, get a nice warm beanie to cover my bald head:lol:).
I hope my question isn't too stoopid but if I don't ask I won't find out

Cheers
Steve

[ngcles]

Hi Steve,



I've never been serious with astronomy, its not a prison sentence ...;)

Obviously cloud cover, light pollution, dust & smoke make for bad seeing.

No, not really. In a nutshell (so far as I'm able ...)

The list you have made here relates to transparency in other words how see-through the atmosphere is. Good transparency means the sky is, well, transparent and dark. Some poeple count sky-darkness as a separate thing to transparency but I think this just over-complicates things.

Seeing is how steady the atmosphere is which results in fine detail being easy to see and smaller star images in the telescope. Bad seeing will make the star images look bloated and wobbly and or like tennis-balls. Equally, planetary images look washed-out, fuzzy or wobbly.

In good seeing, the star images even at high magnification look small, intense and sharp. Planetary detail is more abundant and easier to see. Much higher magnifications can be used without the aesthetics of the image deteriorating.

There are basically two different types of bad seeing that have different causes. fast seeing is caused by high-velocity winds high in the atmosphere that make star images look like a tennis ball. Take a look at the jet-stream maps on the wether sites. A jet-stream passing overhead is bad news and will make bad fast seeing.

slow seeing is generally caused by the air much lower or even the movement of air inside the 'scope. It generally, or in effect causes an otherwise reasonable or good image to slowly ripple and wobble.

Without writing a whole essay, that's the summary. And noooo, it isn't a stoopid question at all.

BTW, like the sign-off quote.

To complete ... "You might think its a long way down to the chemist but that's just peanuts to space. Listen ..."


Best,

Les D

[Dennis]

Here is a very visual webpage showing the Pickering Seeing scale with some nice animations.

http://uk.geocities.com/dpeach_78/pickering.htm

Cheers

Dennis

[Paul Haese]

I just want to add a little and make a small correction to what Les has said here.

Seeing can still be good even when a jet stream is fast moving over head. Fast seeing is really where the interaction between the layers is either at cross angles or diving or rising rapidly into each layer. Careful studies via observation has shown both myself and Anthony convincing evidence that seeing is more likely to be affected by the interaction between the jet stream level and the level of atmosphere directly underneath it. It all levels are moving at the same speed and not interacting the seeing can still be great. My images of 10 May Jupiter (http://paulhaese.net/Jupiter%2010%20may.html)are evidence of this. The jet stream was moving over the top on that night, yet the seeing was probably some of the best I have seen at my Blackwood home.

Typically though the presence of a high pressure system over the top of your location and the absence of either the Jet or localised winds will likely result in good seeing to varying degress. As Les stated it is the stillness of the atmosphere which leads to statements of good seeing.

Hope that helps.

[figgylion]

Cheers fella's, very helpful and informative :thumbsup:

[Geoff45]

Sometimes very good seeing comes when the atmosphere is hazy. The presence of the haze can mean that the air is very still. The steadiness will make things good for looking at planets and the moon, but the haze will wipe out faint objects.

[Kal]

I'm not sure if you are aware of it, but if you click on the 'Resources' link in the left hand tab of the IIS webpage, then click on 'weather resources', you will get to a few links which show the jetstream. This can be handy at times if you are planning which nights to get some good planetary viewing in.