Altitude influence on seeing

[CH]

Clearly altitude has an influence on seeing, or else they wouldn't build professional observatories on the top of mountains (unless it is done to be nearer the stars!).

Can anyone tell me where altitude begins to affect seeing ? i.e is there a notable difference between say sea level and say 500 feet or 500 feet and 1500 feet etc.

The reason I'm asking is that I'm looking at various plots of land around here on which to build a home / observatory / star motel. I would be interested to hear any comments you all have

[Kaptain Klevtsov]

Anything above the cloud tops would do fine CH.

Captain Chaos

[MartinB]

:)

The improvement in seeing with altitude is progressive don't know any more than that. I would guess that for high powered planetary viewing 1000' will be significantly better than at see level all other things being equal. However, if you are thinking about an observatory there are other factors which are probably more important.

Very local factors can have more influence on seeing than events in the upper atmosphere or altitude. The old fashioned enclosed observatories suffered very poor seeing because there wasn't a through air flow. Local convection currents would run across the scope creating havoc. People now go to great lengths to achieve some ventilation and to avoid convection currents developing - including wrapping everything in silver foil! The large modern observatories have a much more open structure.

Obviously, as far as possible, you should avoid observing above buildings. However you want a site which has a smooth air flow - avoid lots of surrounding trees, valleys are a no no. The very top of hills can cause problems as the air breaks away from the land. Apparently the best position is near the top of a hill on the side of the prevailing wind - wind is good for seeing!

Raising the observatory up a few feet apparently has a major benefit by lifting the observatory clear of the more disturbed air at ground level. Just a few feet helps.

http://www.rc-astro.com/equipment/observatory/index.htm

[CH]

CC thanks for that but I don't think clouds will be a problem http://wwwa.accuweather.com/world-index-forecast.asp?submit=Go&zipcode=Datca%2C+Turkey

Martin, some useful stuff there cheers, so I guess you are saying equipment in the open air is generally better than couped up in a dome ?

Why is observing above a building not a good idea (assuming there are no major LP problems) ?

[MartinB]

Because the building will be very slow to cool at night it will cause convection currents especially if there is no breeze.

[CELESCOPE]

Domes are fine as long as you have very good air flow through then , they will get hot of course so treat it like a scope get it cooled down as soon as possible .

Rog

[MartinB]

I agree with Rog, best to keep your dome outside! :)

[CH]

Ok, I now understand about the convection currents inside domes etc but back to the original question if I may.

At which point does altitude have a notable difference on seeing.

[Kaptain Klevtsov]

If I may add, a good southern horizon will be a big factor, especially if you can get one that looks over water. Water is good for seeing as it doesn't heat the air for hours after the sun goes down so no wobbly air to look through. They don't build hotels with loads of nasty lights on water either. Below mountain type altitude (thousands of feet) the difference will be minimal, all other things being equal.

The small scale turbulence that we detect as the seeing only occurs relatively close to the ground so if you can get still air close to the 'scope the rest will be OK. Above 200 feet the air turbulence is much bigger in scale so the larger changes in refractive index don't happen. It's all to do with air flow caused by temperature differences and the refractive index of the air changing with temperature.

If you can see a chimney in front of say, a mountain, you can see the serious wobble effect looking through the exhaust close to the chimney and the way the wobbliness diminishes as the plume rises.

Captain Chaos

[Astroman]

Some good answers in there, CH. To sum up, below ~5000 feet above sealevel, local effects will affect seeing more. Topography, air flow and ventilation inside the dome itself are the majot factors. From personal experience, there is a lot of difference between 5000' and 8000', but not so much from 9000' to 14,000'. Air flow across the optics is more important at very high elevations as CC said and thermals rising from local valleys matter more below 5000' as MartinB said.

Rogers advice of keeping the dome outside is an important factor, too.

I nearly forgot. I coated my dome with what we call Kool Seal. It's an elastomeric, (plastic) coating used for roofing here in the SW. It reflects enormous amounts of heat and keeps the interior of my dome at ambient temps even in summer when it's 120ºF whereas other surfaces, (like my pickup truck) are much higher.

HTH

[Jeff RV]

CH, Maybe you should start thinking of buying a lighthouse

Jeff.

[CH]

Cheers fot the useful and amusing replies, unfortunately the only really high places on the peninsula already have military listening post on them so 5000 ft is out of the question.

There are plenty of other areas available that will be suitable though. Thanks again