From: "Tom Krajci" 
To: 
Reply-To: "Tom Krajci" 


>From: Goran Hosinsky 

>I am planning for my observation hut and in the meantime I have set up
a
>fixed pier in the garden.
>I am living on the slope of a volcanic island and the area consists of
>small ridges going down slope,
>perhaps 50’ high and 300’ away from the next one. My pier is in the
>valley between two ridges,
>near one of them. At night there is a marked temperature variation
>between the ridges and the
>valleys with the valleys being notable colder and sometimes with cool
>air moving downslope.

This flow of cold air you mention is similar to what I have repeatedly
suffered while at the Okie-Tex Star Party.  See: http://www.okie-tex.com/

It spoiled seeing because we were at the mouth of a box canyon...and the
flood of downstream cold air rolled right over the observing site. Pretty
by day...dark, transparent skies...lousy seeing.  It was great for low
power and binocular observing.

>what would be the best place for the observation hut. Up
>on the ridge or down
>in the valley?

Valley is often pretty bad.  Ridge tops are usually better.

>With the telescope being near ground the cool air moving
>down the slope might
>blow away the local heating from the ground?

Perhaps in the early evening this is true, but as night continues, the
flood of cold air will probably get worse.  This is how it is at Okie-Tex. 
I've also experienced something similar in Chimgan, Uzbekistan...near the
foot of a 10,000 ridge.  I need to find an observing site a few kilometers
farther from this ridge to avoid the cold 'avalance' of air from the
mountaintops.

>I would like to make comparative tests of the seeing before deciding
>where to put my hut but
>I do not see how I can do that. Any suggestions?

Use a temperature probe/thermometer to measure air temperature at various
heights above ground at various sites, on various nights.  Keep and analyze
the data.  Look at the rate of temperature drop versus height above ground.
 If you are serious about good seeing you may find your data implies that
if you put your scope 8 feet, or 12 feet above ground...you'll avoid the
lousy seeing from rapid temperature change near the ground.

Clyde Tombaugh, discoverer of Pluto...his 16 inch telescope was elevated
above the ground for this reason.  How high?  I don't recall...maybe 8 feet
or so.

If you are a devotee of the Stellafane bathing suit contest approach to
telescope making...an elevated observing platform is not for you.  If you
are serious about good seeing...you may need an elevated platform.

Tom Krajci
Tashkent, Uzbekistan


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