Seeing/Transparency

[Geoff Barnes]

I was working today at one of my clents gardens high up on the mountain which has fantastic views stretching  some 40 miles to a headland at the bottom end of the bay that Melbourne surrounds.

Although skies were clear and sunny there was a lot of haze in the air and the peninsula was barely visible.

There are other days with seemingly similar conditions when the peninsula is so crystal clear you feel  you could reach out and touch it.

This got me thinking with my astronomer hat on, is there any correlation between the quality of the horizontal seeing to the horizon and that of the views of the night sky?

I think I will do my own survey when conditions allow to see if this bears out. Tonight should be cloud free so I hope to make a start, horizontal seeing is poor today, lets see what the astro seeing is like.

Has anyone else done such a study?

I shall follow up this post as I continue my study.

Edited January 12, 2021 by Geoff Barnes

[Pixies]

Is this not 'transparency' rather than seeing? 

There may be some correlation, but if you live near the coast, I find, you can have haze at different levels. 

When I'm out walking the dog late evening, there are 2 distant lighthouses I can see. Fidra (4 flashes every 30 seconds) and Isle of May (2 flashes every 15 seconds). Fidra is much closer and always visible unless it's very misty/foggy. The Isle of May, though, is a good guide to horizontal visibility. I reckon  can see it only 60% of the time. However, there are nights when I'm not able to see Isle of May but the sky transparency was good - and vice-versa.

And there's the dreaded haar. At times you can't see the end of the street, but look up and see the sky!

[Waddensky]

I agree with Pixies, it's not seeing but transparency you're talking about. The extinction effects of the atmosphere are caused by dust, smoke and other aerosols (absorption) and Rayleigh scattering. Here's an intersting article on the subject.

Edit: and yes, both horizontal transparency and night time transparency are caused by the same effects, although things can change significantly after the Sun sets, just like clouds.

Edited January 12, 2021 by Waddensky

[Geoff Barnes]

Thanks @Pixies and @Waddensky, I did contemplate whether to call this thread "Transparency" when I wrote it, so I've added it to the title.  

Edited January 12, 2021 by Geoff Barnes

[chiltonstar]

Here at least (Oxford), seeing and transparency are often mutually exclusive, ie seeing is best on warm hazy evenings with poor transparency, and transparency is often best after rain - usually nights with poor seeing. 

Must be somewhere in the world with 300 nights + of excellent seeing and transparency?

Chris

[Geoff Barnes]

Just a quick brief follow up, I had a look at Mars and Rigel yesterday evening in clear skies but with a still hazy horizon and the views were terrible, impossible to achieve sharp focus over 100x magnification. I found the 20mm setting with the Baader zoom giving 75x was the best I could achieve. 

Will do more investigating in different conditions.

 

[Marki]

1 hour ago, Geoff Barnes said:

Just a quick brief follow up, I had a look at Mars and Rigel yesterday evening in clear skies but with a still hazy horizon and the views were terrible, impossible to achieve sharp focus over 100x magnification. I found the 20mm setting with the Baader zoom giving 75x was the best I could achieve. 

Will do more investigating in different conditions.

 

I seem to have had too many nights ike that recently :(.

 

[Geoff Barnes]

My second chance this evening to compare the quality of views. The horizon today has been all but invisible in the haze despite clear blue skies. Just had the 12 inch Dob out to have a look at the Moon, Mars and even Uranus which is currently very close to the red planet. Very poor views, impossible to get a steady image in the eyepiece of any of the targets, certainly Mars and Uranus were very poor, the Moon was shimmering constantly at 175x, but much better at 75x as one would expect.

Looks like a good settled warm spell of weather coming up for us in the next few days so I hope to have a chance to compare views with greater transparency both vertically and horizontally.  

[Geoff Barnes]

After quite a hot and humid day with reasonably good but not great visibility and a moderate breeze I was not expecting the best viewing conditions last night, how wrong I was!

It soon became obvious that it was one of those rare nights when everything I looked at was razor sharp and steady as a rock.

I aligned the 12 inch Dob with the Baader zoon on Rigel and Sirius and with no effort at all at 175x saw both B stars.

Rigel B is always easy but Sirius Pup almost always takes a bit of effort, not this time, even with the fierce glare from Sirius  it was immediately there.

Time to try a few more stiff tests, in with the 6 5mm Morpheus and onto the nearby Trapezium to see E and F. Oh wow, too easy, both easily visible!

Alnitak next, quite a challenge to get a clean split with this one, but again as clean and clear as I've ever seen with a definite black line between the pair.

This was going so well I suddenly realised the half crescent moon would be perfect for a go at the elusive Alpine Rille. The lunar terninator was breathtaking in its detail but as I homed in on the target the floor of the Alpine Valley was  in complete shade, just a fraction too near the terminator, such a shame.

So I'm still none the wiser as to whether there is a definite link between horizontal and vertical transparency.

Oh well, mustn't grumble, it had been a rare and memorable session, albeit cut short by my worrying loss of blood to the infuriating mosquitoes who were out in force, time to beat a hasty retreat! 😁

 

 

 

Edited February 20, 2021 by Geoff Barnes

[Stu]

Mosquitoes aside, that it sounds like you had some excellent seeing there Geoff. As Chris said, seeing and transparency are two very different effects and are often mutually exclusive ie a night of excellent seeing has high haze/poor transparency, and nights of excellent transparency often have poor seeing. By definition, that makes some nights great for planets and doubles, and others better suited for galaxies and nebulae. Quite often the best transparency occurs after heavy rain, when all the pollutants have been washed out of the atmosphere. Then it builds up again over time until the next rain showers. The Jetstream heavily influences the seeing conditions; when bad it’s like viewing under flowing water!

[Geoff Barnes]

Yes, that's what surprised me the most Stu. The atmosphere was really humid so must have been full of water vapour even though we're in a bit of a dry spell (for a change) but the seeing and transparency were both as good as it gets here.

At 37 degrees south we are a bit too near the equator to be influenced by the jetstream much here.

You never can tell until you stick your eye up to the eyepiece. 😁

[SiriusB]

On 20/02/2021 at 08:35, Stu said:

Mosquitoes aside, that it sounds like you had some excellent seeing there Geoff. As Chris said, seeing and transparency are two very different effects and are often mutually exclusive ie a night of excellent seeing has high haze/poor transparency, and nights of excellent transparency often have poor seeing. By definition, that makes some nights great for planets and doubles, and others better suited for galaxies and nebulae. Quite often the best transparency occurs after heavy rain, when all the pollutants have been washed out of the atmosphere. Then it builds up again over time until the next rain showers. The Jetstream heavily influences the seeing conditions; when bad it’s like viewing under flowing water!

That about sums it up Stu.

Sirius is dancing & you may have good transparency but poor seeing.

an Autumn light mist & you may have excellent seeing but poor transparency.

Very rare in UK to get both.

I do remember one memorable night when i had both, 250x mag(scope limit) & Saturn was without a shimmer, but also, dso's were like i had a larger aperture in play.

I stopped out very,very late, in fact till dawn light started to intrude. I never wanted that night to end.