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Scope Cool Down VS Atmosphere

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#1 Kent10

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Posted 19 August 2014 - 10:19 PM

Hi All,

 

Some have tried to explain this to me in the past but I am still not clear on it and I always wonder as I am viewing.  I have a Tec 180FL refractor that I usually let cool down before I start to use it.  But sometimes I just decide I want to view and I take the scope out and start viewing right away. 

 

If I view the moon, for example, and the view doesn't look great because it isn't settled, how can I tell if this is the scope still cooling or if it is the atmosphere not behaving or some of both.  Is there a difference to be seen in the eyepiece?

 

Thanks very much, Kent



#2 GlennLeDrew

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Posted 19 August 2014 - 10:45 PM

Overturning air within the tube has a quite slow 'boiling' aspect, compared to the usually much more rapid action induced by the atmosphere. And where the atmospheric seeing can have notable variability on time scales of order a second, air currents in an enclosed tube retain much the same aspect for minutes (when the tube is not moved about, which action would somewhat alter the direction of convection.)



#3 Asbytec

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Posted 20 August 2014 - 07:42 AM

Glenn, isn't there another way to tell? I've done it, but cannot remember exactly how. If you in-focus (or out focus, can't remember) a bit, you can actually observe and focus the atmospheric turbulence on the de-focused star or planetary image. I've done it with Jupiter, de-focused until I could see the atmosphere blowing over the de-focused image fairly clearly...and rapidly as you say. You can even tell the direction it is moving, or appears to move, anyway. 



#4 Kent10

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Posted 20 August 2014 - 10:44 AM

Thanks guys.  I am going to keep looking.  I have seen the turbulence on defocused stars.  It usually does move quite fast.  I am ultimately trying to determine if my scope is cooled down enough or if it can keep up with the falling temps.  When I start viewing immediately without cooling my scope down I have had some really good views of the moon and then things get worse.  So I think this must be the atmosphere but perhaps my scope is the same temp as the outdoors when I first bring it out and then as the temp falls it is having trouble keeping up.   Part of my problem is I have done all my viewing from my backyard in a neighborhood and so I must get lots of turbulence from the rooftops.  There are lots of pine trees in the area too and I have read they are not so great for seeing conditions. 



#5 Scott99

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Posted 20 August 2014 - 10:50 AM

I generally see pretty wild images for the first 15 minutes of cool-down.  After that the images are much better, I can see if the seeing is decent.  Complete cool down for best planetary, etc, takes longer if it's cold out, up to an hour in winter.

 

Never heard of a 7-inch apo having trouble keeping up with falling temps.  Have heard some stories of that problem in 10 or 12 inch apos.


Edited by Scott99, 20 August 2014 - 10:56 AM.


#6 DesertRat

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Posted 20 August 2014 - 12:55 PM

If you in-focus (or out focus, can't remember) a bit, you can actually observe and focus the atmospheric turbulence on the de-focused star or planetary image. I've done it with Jupiter, de-focused until I could see the atmosphere blowing over the de-focused image fairly clearly...and rapidly as you say. You can even tell the direction it is moving, or appears to move, anyway. 

 

By moving the eyepiece outwards you are focusing closer and hence on upper air turbulence.  Sometimes you can even focus on distinct layers.  Way out of focus may help seeing thermal currents which are finger and worm like features which move much more slowly.  See Texereau's "How to Make a Telescope"  for details.  You can also focus on a bright star and remove the eyepiece for a look.

 

Jupiter is too big to resolve well these wind driven features.

 

Nearby trees are troublesome.  They hold heat pretty well and any local air movement will drive the local seeing bonkers.  Nearby concrete, pavement and stone walls are problems as well.  Local seeing is from the scope out and around 100 meters or so.

 

Glenn



#7 sedmondson

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Posted 20 August 2014 - 02:57 PM

Hi All,

 

Some have tried to explain this to me in the past but I am still not clear on it and I always wonder as I am viewing.  I have a Tec 180FL refractor that I usually let cool down before I start to use it.  But sometimes I just decide I want to view and I take the scope out and start viewing right away. 

 

If I view the moon, for example, and the view doesn't look great because it isn't settled, how can I tell if this is the scope still cooling or if it is the atmosphere not behaving or some of both.  Is there a difference to be seen in the eyepiece?

 

Thanks very much, Kent

 

I don't know where you live, but in the summer, I have to let the scope heat up if I keep in the house. In fact, I can't take the cap off right away or the lens might fog up. Sometimes even my eye glasses fog up when I step outside.



#8 Kent10

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Posted 20 August 2014 - 03:02 PM

 

Hi All,

 

Some have tried to explain this to me in the past but I am still not clear on it and I always wonder as I am viewing.  I have a Tec 180FL refractor that I usually let cool down before I start to use it.  But sometimes I just decide I want to view and I take the scope out and start viewing right away. 

 

If I view the moon, for example, and the view doesn't look great because it isn't settled, how can I tell if this is the scope still cooling or if it is the atmosphere not behaving or some of both.  Is there a difference to be seen in the eyepiece?

 

Thanks very much, Kent

 

I don't know where you live, but in the summer, I have to let the scope heat up if I keep in the house. In fact, I can't take the cap off right away or the lens might fog up. Sometimes even my eye glasses fog up when I step outside.

 

 

That is completely different from my problem.  I live in the mountains at 7000 feet and so it is always relatively cool outside at night even in the summer and the temps drop quickly.



#9 Asbytec

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Posted 21 August 2014 - 08:51 AM

 

If you in-focus (or out focus, can't remember) a bit, you can actually observe and focus the atmospheric turbulence on the de-focused star or planetary image. I've done it with Jupiter, de-focused until I could see the atmosphere blowing over the de-focused image fairly clearly...and rapidly as you say. You can even tell the direction it is moving, or appears to move, anyway. 

 

By moving the eyepiece outwards you are focusing closer and hence on upper air turbulence.  Sometimes you can even focus on distinct layers.  Way out of focus may help seeing thermal currents which are finger and worm like features which move much more slowly.  See Texereau's "How to Make a Telescope"  for details.  You can also focus on a bright star and remove the eyepiece for a look.

 

Jupiter is too big to resolve well these wind driven features.

 

Nearby trees are troublesome.  They hold heat pretty well and any local air movement will drive the local seeing bonkers.  Nearby concrete, pavement and stone walls are problems as well.  Local seeing is from the scope out and around 100 meters or so.

 

Glenn

 

 Glenn, yes, outside focus. Thanks, that makes sense. Ah, yea, forgot about removing the eyepiece. 

 

Yea, Jupiter might be too large, but you can still see the turbulence. And unless some illusion is at work, maybe even infer the wind direction. A star may be better, as you say, but I observe Jupiter mostly and noticed it when coming to focus. 



#10 leviathan

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Posted 21 August 2014 - 08:58 AM

Seeing causes focus to shift. Another way to check it is by using Bahtinov mask - when the seeing is poor you can see noticable focus shift of >I< figure of mask.



#11 DesertRat

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Posted 21 August 2014 - 09:39 AM

Hi Norme,

 

No doubt you can see turbulence on Jupiter.  But the planet lacks sharp edges for reference.  Now the Moon does.  There after focusing and then coming out you may be able to discern the actual details of the disturbance.

 

Still a star is best.  There you may also use a knife edge, a card or a coarse ronchi ruling.  At null you can see lots of stuff, especially in smaller scopes.  Larger scopes need imaging help, like longer exposures or stacked video to average out the seeing to record not only optical errors but thermals too.

 

Glenn


Edited by DesertRat, 21 August 2014 - 09:45 AM.

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