Impact of telescope height on the seeing

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christian viladrich
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Impact of telescope height on the seeing

Post by christian viladrich » Sun Sep 30, 2018 2:57 pm

Dear all,

Some time ago, I tried to measure the impact of the height of the telescope on the seeing. I compare the seeing at ground level and 1 m high.
Since I had only one SSM, the measurements at 0 m and 1 m where not simulaneous. Still some impacts are clearly visible:
Image

Last August, Karine Chevalier carried out measurements at St Veran using two SSM. The seeing was measured simultaneously at 1 m and 1.5 m high, which is typical of telescope aperture heigths.
The ground at St Véran is made of rocks with no grass. The mesurements were made over a period of time of more than one hour.

Karine, whom I thank for, made the measurements and the data reduction. Here are the results :

This graphic gives the % of time below a given value of seeing :

Image

The table is another way to present these results :

Image

In a nutshell, it means that going from 1 m to 1.5 m high improves the seeing by about 0.35 arcsec, which is ... quite significant.

In high resolution imaging, we typically keep less than 5% of the frames. We can see on the table that the quantile 5% goes from 0.7 to 0.3 arcsec when the aperture of the instrument rises from 1 m to 1.5 m high.

So now, we all have to use tall tripods :-)

PS : of course, all of this depends on wind speed, ground nature, hour in the day, etc.

Best regards

Christian
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Re: Impact of telescope height on the seeing

Post by krakatoa1883 » Sun Sep 30, 2018 3:12 pm

Very interesting Christian. I suppose height is relative to the ground on which the telescope rests, irrespective if the ground is at sea level or 2000 m a.s.l., am I right ? (anyway I would expect a generally better seeing at 2000 m...)
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Re: Impact of telescope height on the seeing

Post by christian viladrich » Sun Sep 30, 2018 3:22 pm

Yes indeed, this is the height relative to the ground.
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Re: Impact of telescope height on the seeing

Post by ffellah » Sun Sep 30, 2018 3:24 pm

Great to know, Christian. Thank you for sharing this with all of us.

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Re: Impact of telescope height on the seeing

Post by MalVeauX » Sun Sep 30, 2018 7:02 pm

Interesting stuff, I imagine it is different for different thickness optics and different types of ground (field of soil & grass, compared to desert or straight rocks, sedimentary soil versus hot quartz sand, etc).

I keep my instruments up on a pier, fairly high up, and my decking in the observatory is over the soil.

I wonder if it would matter for those using tripods, if they put a thermal barrier (like covers used to block solar) on the ground under the telescope's aperture/mirror?

Very best,

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Re: Impact of telescope height on the seeing

Post by christian viladrich » Sun Sep 30, 2018 7:19 pm

At home, I observe on a ground covered by grass, and I spray water on the grass during observation.
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Re: Impact of telescope height on the seeing

Post by bart1805 » Sun Sep 30, 2018 9:29 pm

Brilliant, thanks!

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Re: Impact of telescope height on the seeing

Post by eroel » Sun Sep 30, 2018 11:57 pm

Christian:
Very interesting information, my Mexico City home observatory is 2246 meters above mean sea level, the rural observatory is at 2265 meters above mean sea level.
So I don´t need a high pier. :roll: , but in the City the problem is air contamination, at the rural place I have a very even and clean seeing.
Best regards,
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Re: Impact of telescope height on the seeing

Post by Montana » Mon Oct 01, 2018 10:54 am

Very interesting! however I am barely tall enough to mount my telescope at 1m :lol:

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Re: Impact of telescope height on the seeing

Post by robert » Mon Oct 01, 2018 12:38 pm

Excellent clear results and presentation, thanks
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Re: Impact of telescope height on the seeing

Post by marktownley » Mon Oct 01, 2018 6:34 pm

Excellent and informative as always Christian!
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Re: Impact of telescope height on the seeing

Post by Valery » Tue Oct 02, 2018 1:44 am

Montana wrote:
Mon Oct 01, 2018 10:54 am
Very interesting! however I am barely tall enough to mount my telescope at 1m :lol:

Alexandra
We all know well about british grass lawns!

My SCT scopes are about 1,2 - 1,6m elevation (entrance aperture) above a grassy lawn level. :)
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Re: Impact of telescope height on the seeing

Post by Bruce G » Wed Oct 03, 2018 6:50 pm

Very interesting results and very well presented.

What do you feel might explain that much difference? Dust in the air layer just above the ground or perhaps some other form of scattering phenomenon? Thermal currents close to the ground?

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Re: Impact of telescope height on the seeing

Post by marktownley » Sun Oct 07, 2018 8:10 am

I'll give this a few more days here then move over to the library folks, I think is a useful thread for reference.
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Re: Impact of telescope height on the seeing

Post by Bruce G » Tue Oct 09, 2018 2:01 pm

What SSMs were used in these tests, should someone want to replicate your results?

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Re: Impact of telescope height on the seeing

Post by christian viladrich » Tue Oct 09, 2018 6:46 pm

Hello Bruce,
We used two AiryLab SSM.
We also started a session with three SSM running at 1, 1.5 and 2 m high, but this session was quickly clouded.
The inhomogeneities of the air in the boundery layer depens on the wind speed, type and temperature of the ground.
I am planning to make some tests at home in order to check whether there is a general pattern or not.
Christian Viladrich
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Re: Impact of telescope height on the seeing

Post by Bruce G » Sat Oct 13, 2018 4:51 pm

Thank you Christian
I look forward to seeing further results

Bruce G

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