Seeing conditions and the local environment

[jambouk]

We all talk about the importance of not looking over a neighbour's roof as the heat loss from the roof / chimney will disturb the seeing , but is there any research out there to prove or disprove this?

Thanks.

[Paz]

I think if I collated the details of location and seeing conditions from my notes it would show this. I quite often observe away from home at what I think of as cool sites or still sites, for this reason. Some of them are very close to home but the effort of a short journey can be worth it.

At my house I often pick where I observe so that my line of sight avoids being over heat sources if possible or passes as high as possible above them.

[F15Rules]

I don't know if there's any formal research on this, but in practical terms if you just look with your eyes the evidence is obvious.

We have a log burner in our lounge. If it's burning in cold weather and I look through a scope over the chimney, the image dances about like crazy. 

This also can happen in daytime when just looking over a building or garage for example.

But local seeing conditions are also affected by topography and local weather: I live in a village almost surround by low hills, and I definitely get interference with seeing when colder or warmer air rolls down into our village, causing temperature inversions etc.

I find that this type of seeing interference can often lessen later in the night after the atmospheric turbulence has eased.

Dave

[neil phillips]

The evidence is mostly anecdotal. But based on good science, often repeatable, and fairly consistent. I've seen it enough over the years to be convinced of it. Though the conditions for it, can vary quite a bit over the same area. From hardly noticeable to horrendous. The further the target is away from the problem the less its impact will have 

Agree with the other comments 100%

[Stu]

At my last house, I used to observe from the North facing garden, looking back over the house. It was a long garden (120ft) but even so, my best periods of seeing were when the target was over the gaps between our house and the neighbours.

Although my skies in Crewkerne are much better than Walton-on-Thames I observe over the town itself and the seeing is often not great. I need to try out somewhere south of the town, or even on the coast to see if it is better.

[Froeng]

All the factors above are important in my opinion! However, I also found it makes a difference to observe off-the-ground. My home observing is done from a mid-town 3rd floor balcony. You may think this sounds terrible, but I would say on average, the seeing seems to be better than my previous location where I observed from the garden. I read somewhere that 20m up (which I am not quite) is the transition zone between the air disturbed by the landscape’s features or relief and a more steady layer of atmosphere which typically should much improved seeing. As they say -YMMV

[GTom]

unfortunately it is just as obvious as wind will create waves on any larger open water surface. No additional taxpayers money is needed to be spent...

 

Scientific background: both air density AND refractive index change with temperature. Heat CONDUCTION in gases is very slow, at the same time MOBILITY in the gas phase is very easy, therefore boundary layers warmed AND expanded (higher temp, lower density) by a roof will instead of giving off its heat by conduction to neighbouring layers will simply move upwards creating random, continuously changing convection pattern with different sizes of appearing and disappearing eddies.

 

Long story short: an astronomer doesn't want to be anywhere near a large temperature gradient. I know, this is terrible news in the country of tiny-weeny "gardens" and horrible insulations.

Edited July 9, 2023 by GTom

[The60mmKid]

As others have stated, it's obvious. On a cool evening, hold your warm hand up in front of your telescope while looking through the eyepiece. You'll see the difference.

A friend of mine was testing a mirror for me once, and he prompted me to hold my hand in front of it while he had it on the bench. What showed up on the computer screen was a burst of frenzied thermal currents that took several seconds to dissipate after I moved my hand away.

This is why roofs, concrete, and other heatsinks pose problems for observation. It's also why planning to keep body heat away from one's objective/mirrors is a good idea. A steady atmosphere doesn't help much when the "poor seeing" is localized inches or feet from one's telescope.

Edited July 14, 2023 by The60mmKid

[AstralFields]

On 03/07/2023 at 08:38, jambouk said:

We all talk about the importance of not looking over a neighbour's roof as the heat loss from the roof / chimney will disturb the seeing , but is there any research out there to prove or disprove this?

Thanks.

I thought it could be all a bit overblown until I decided to test my new 4K camera from my balcony (which has a roof and two walls), imaging the Moon as it passed over the building in front of us. 

Let me just say, I thought there was definitely something wrong with that camera and that I had wasted my money on it.

The view through the eyepiece also felt like someone was literally boiling the whole Moon.

Few nights later, I went outside and everything was a lot better.

Since then, I am a believer 😅

Edited July 19, 2023 by AstralFields

[Franklin]

The refractive index of air changes with temperature and this is what causes wobbly seeing. The larger the aperture, the greater the resolution and because of this it is said that smaller apertures perform better in bad seeing because they cannot resolve the turbulence.

[Saganite]

On 19/07/2023 at 18:59, Franklin said:

The refractive index of air changes with temperature and this is what causes wobbly seeing. The larger the aperture, the greater the resolution and because of this it is said that smaller apertures perform better in bad seeing because they cannot resolve the turbulence.

I know this to be true.  I have my 6" refractor and 4" Vixen mounted side by side and earlier this year I was getting beautiful resolutions of double stars with the Vixen and disappointing ones with the 6" for several evenings.  So bad was it that I told F15 Dave that I was convinced I had bought a 'Lemon ', but  a week or so later the 6"was excelling.  This was over gardens and often in the early hours so nothing was contributing, it was just the smaller scope cutting through the less than perfect seeing and making  a better job of it.

Edited July 22, 2023 by Saganite