Colder nights = longer cool-down times and better seeing? Or not?

[blusky]

What do people feel?

[swamp thing]

Hi

I'm afraid it's not quite so simple.

By seeing do you mean transparency or steadiness (seeing)?

Transparency is often down to the moisture content of the atmosphere. Seeing is often down to steadiness of the air.

Therefore; nights of great transparency can provide great deep sky views but shocking planetary images, and nights of good seeing can provide great planetary images and shocking deep sky views.

It's very rare to get both good seeing and clear transparency.

It's enough to drive you mad.

Regards Steve

[umadog]

I think a lot depends on where you are situated. Around here, seeing is best around August and worst around December. Transparency, on the other hand, is more uniformly distributed through the year, although it is slightly worse in the peak of summer. You can see those numbers plotted here: Custer Institute Clear Sky Chart History

[faulksy]

Hi

I'm afraid it's not quite so simple.

By seeing do you mean transparency or steadiness (seeing)?

Transparency is often down to the moisture content of the atmosphere. Seeing is often down to steadiness of the air.

Therefore; nights of great transparency can provide great deep sky views but shocking planetary images, and nights of good seeing can provide great planetary images and shocking deep sky views.

It's very rare to get both good seeing and clear transparency.

It's enough to drive you mad.

Regards Steve

wow didnt know that

[JonnyP]

So I'm guessing that the terrible 'visibility' I had last night was actually poor transparency because of the moisture in the air, not poor seeing as I said on another thread? (Last night I could barely make out the main stars in Leo, for example.)

[mrphildog]

Hi

I'm afraid it's not quite so simple.

By seeing do you mean transparency or steadiness (seeing)?

Transparency is often down to the moisture content of the atmosphere. Seeing is often down to steadiness of the air.

Therefore; nights of great transparency can provide great deep sky views but shocking planetary images, and nights of good seeing can provide great planetary images and shocking deep sky views.

It's very rare to get both good seeing and clear transparency.

It's enough to drive you mad.

Regards Steve

Excellent. That's very informative!

I had recently been looking at astro forecasts on 7Timer and I was puzzled why it would show a day/time with good transparency, zero cloud but poor seeing. Now I know. Thanks

[spaceboy]

Contrary to Umadog I have always found summer to give the best transparency and to be honest despite the light nights there is no other time I'd rather have it. Come July the milky way can be directly overhead and on a night of good transparency you can lie outside in reasonable comfortable admiring the faint wash of the milky way even over my light polluted suburb. Don't get me wrong it's nothing like admiring it under dark skies but the fact I see it at all under my dismal skies gives me a warm glow inside.

[Nibor]

In my garden I have a weather station transmitter which transmits both temperature and humidity to the monitor indoors.

Is there a list somewhere linking relative humidity to transparency?? On those odd occasions during January when I was out observing the humidity was in the 70's and 80's.

When I did the Campaign to Protect Rural England's light pollution survey I was able to count only 12 stars (30+ can be seen from dark sites) in Orion's square (as per instructions). - I must take more notice of the humidity next time Orion is visible to see the effect on the star count related to transparency.

Star Count 2012 | Campaign to Protect Rural England

An interesting thread - covering fundamental observing issues!!!

[Moonshane]

as hinted at, it's not as simple as even Steve's explanation.

local problems such as heat plumes from chimneys, central heating, badly insulated roof voids and the like all make observing in urban areas very tricky. and that's ignoring the issue of dazzling bedroom lights etc.

one issue not addressed so far is that of optical cooling. yes, if going from a centrally heated house to a cold garden (as I do) means that mirrors etc take a lot longer to get to equlilibrium but of course you can always observe at lower powers almost immediately. furthermore, if the temperature drops rapidly outside, a fan may be needed to maintain the position.

the main issue for me is how much time I can observe for. in the winter it's generally more so it suits me!

[iwatkins]

And you must also take into account the temperature and humidity at various levels in the air in the column of air above your head (assuming you are looking up). And also how fast it is moving a different levels (jet stream especially).

Might be perfect air, low humidity, aerosol free, adiabatic lapse rate, well mixed etc. Then all you need is a 180 knot jet stream aloft over where you are looking to destroy seeing.

There is a good reason they put land based telescopes high up. The less air you have to look through, the less problems you have. Even so, the pros are using adaptive optics these days.

Or better still put your telescope in orbit.

Cheers

Ian