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Help me understand 'seeing'

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#26 Keith Rivich

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Posted 20 February 2024 - 10:51 PM

Seeing is atmospheric turbulence.


When the seeing is bad, you point at planets and they look as if they were "boiling". Few details are visible, they're just a blob.

When the seeing is OK, you get more details and those stay visible for a few seconds.

Once the seeing is superb, you get an amazing level of details and, even better, for longer periods.


Good seeing is critical for any target that requires a high resolution steady view.


Note that turbulence comes from three sources.


Above your telescope = seeing.

Inside your telescope = air flowing inside the tube, you have to wait for the telescope to reach thermal equilibrium (or the image will look as boiling, even if the seeing is superb)

Around your telescope = nearby heat sources. Don't mount your telescope on concrete, asphalt, sand or rock. Better choose lawn or wood. Don't observe above heat sources like a roof, chimney or trees.

I have to disagree with this last point. On paper this would seem to be intuitively correct but in reality it doesn't make much difference. I have a thermal image somewhere that shows my concrete pad at around 105° while the scope and air temps were around 90°. Jupiter was razor sharp in the eyepiece. Concrete releases its heat very slowly.


I agree, by experience, on roofs and heat sources like chimneys and a/c units. 


Edit: Here is the image:





Edited by Keith Rivich, 20 February 2024 - 10:55 PM.

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#27 Napp


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Posted 20 February 2024 - 11:03 PM

From what I'm gathering, I should take forecasted/actual seeing with less of a value than getting out and checking for myself. Clouds and haze are easy enough to see from radar but I have not gone to a darker site based on the bad seeing in the forecast. 

I really don't trust seeing forecasts.  It can change too quickly.   I do want to make sure that you understand transparency vs. seeing.  Transparency is basically how clear the air is.  It is good for observing objects like nebulae and galaxies.  These objects are typically faint and need clear skies to see well.  Seeing is air stability.  This is important for observing planets and the moon where you are tyring to see fine detail.  Seeing is not so important for observing nebulae and galaxies because you are observing amorphorus shapes without much detail.  Transparency is not so improtant when observing planets and the moon because they are so bright.  Some of the best seeing and planet views I have had have been through a thin, stagnant layer of cloud.  the best transparency here in north Florida occurs right after strong winter cold fronts.  The seeing at such tims is terrible due to to strong upper atmosphere winds accompaning the cold front.  Good transparency and good seeing at the same time are rare.  They are times to be treasured.

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#28 George N

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Posted 21 February 2024 - 01:46 PM

My experience:


"bad seeing" = point a 6-inch F/12 Astro Physics APO at Jupiter and see "fuzzy circle with two fuzzy dark lines - seen - at times"


"great seeing" = point a beat-up Coulter 10.1" Dob with 'not-quite 1-wave primary' at Jupiter and see HST-like views!

#29 Nerd1


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Posted 21 February 2024 - 02:00 PM

If you live in the central part of north America,  get used to really bad seeing.  The jet stream is horrendous quite often. You will have a lot more bad nights than good. 

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