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Help me understand 'seeing'

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#1 HappyGalaxymore

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Posted 01 February 2023 - 11:52 AM

I searched the forums first so my apologies if I missed it somewhere.

 

I don't understand 'seeing'. Maybe it's me creeping into that age where it's more difficult to understand new things or maybe I haven't found an explanation that works for my mind but I don't get where seeing is figured in the forecasts. If it's clear skies, with good transparency, and little/no moonlight, how is seeing figured and what does it affect? What other variables should I be looking for in the forecasts to help me plan for times I want to get to a darker site to image and how much of a difference does seeing make to imaging? Is it something that can be compensated for with different techniques, like is there weight to the variables (75% cloud cover, 15% transparency, 10% seeing, etc.)?


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#2 dnayakan

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Posted 01 February 2023 - 11:58 AM

Seeing refers to the stability of the atmosphere. This is independent of transparency, light pollution, darkness etc. If there are lots of eddies and rising thermals etc., the seeing is poor. This will affect the resolution you can obtain because the shifting air shifts the focus point around and is experienced by us as smearing, speckling etc. Poor seeing typically affects the magnification that is tolerable on any given night. Matters a lot for lunar, planetary and double star work (things where you would use relatively high magnifications), typically less so for DSOs. 

Cheers, DJ 



#3 City Kid

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Posted 01 February 2023 - 11:59 AM

Seeing is the steadiness of the atmosphere. It's common for good transparency to be accompanied by bad seeing and poor transparency to be accompanied by good seeing.


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#4 cjarvis64

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Posted 01 February 2023 - 11:59 AM

From ClearDarkSky.com:

 

"The line, labeled Seeing, forecasts astronomical seeing. Excellent seeing means at high magnification you will see fine detail on planets. In bad seeing, planets might look like they are under a layer of rippling water and show little detail at any magnification, but the view of galaxies is probably undiminished. Bad seeing is caused by turbulence combined with temperature differences in the atmosphere. This forecast attempts to predict turbulence and temperature differences that affect seeing for all altitudes.

Bad seeing can occur during perfectly clear weather. Often good seeing occurs during poor transparency. It's because seeing is not very related to the water vapor content of the air.

The excellent-to-bad seeing scale is calibrated for instruments in the 11 to 14 inch range. There are some more details in CMC's seeing forecast page.

No computer model forecasts convective heating well, so consider the seeing forecasts for daytime hours to be less accurate. Seeing is forecast for 3-hour blocks, so triples of seeing blocks will show the same color. Those triplet boxes for the same forecast map are showen linked as a reminder. A white block on the seeing line means that there was too much cloud (>80% cover) to calculate it.

Note also that you may observe worse seeing though your telescope than what a perfect seeing forecast would predict. That is because tube currents and ground seeing mimic true atmospheric seeing. You may also observe better seeing than predicted here when observing with an instrument smaller than 11 inches."

 

To me, you would have more "twinkling" stars higher in the sky due to the rippling effect the atmosphere produces during poor seeing conditions. For DSO imaging, it's not too much to sweat over. For planetary, it may cause issues. I am not a planetary guy though.



#5 Chad7531

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Posted 01 February 2023 - 12:00 PM

Seeing is just basically atmospheric turbulence. It doesn’t make much difference for DSOs but it’s a main factor for planetary, moon, and solar. It’s the boiling effect you see on bright objects in bad seeing. (I see I was beat to it.)

Edited by Chad7531, 01 February 2023 - 12:01 PM.


#6 dnayakan

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Posted 01 February 2023 - 12:01 PM

Nothing much you can do compensate other than wait for a better night or move to an area that enjoys steady seeing conditions (and even that can be poor on some nights). Typically, those of us who live under the jetstream are likely to experience poor seeing conditions in some seasons. Some places enjoy very still air or very clean, laminar airflow which results in steady seeing conditions. DJ


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#7 havasman

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Posted 01 February 2023 - 12:06 PM

When you look down into a clear shallow stream running over rocks you cannot see the bottom clearly because of how the moving water distorts the light as it tumbles and swirls. You can see the pebbles on the bottom but it's hard to focus on one and see it very well. When the river of air overhead is turbulent it does the same thing and we call that poor seeing. Sometimes the air will flow smoothly in many locations and that's called a laminar flow. Those conditions favor higher magnification and detailed observations. We call that good seeing.


Edited by havasman, 01 February 2023 - 12:07 PM.

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#8 vtornado

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Posted 01 February 2023 - 12:16 PM

Here are some metaphors.  

 

Picture a swimming pool, and something painted on the bottom of the pool, like a school name or something like that.  The letters look crisp and easy to see when the pool is calm (good seeing).  Now imagine the swim team doing "cannon balls" now it is hard to see the letters.

 

Seeing is also like viewing terrestrial objects close to the ground in the day time.

For example look at a road sign hundreds of feet away with your scope.

If the sun is out, it heats the ground and the view will soften.  If the clouds roll in

and the ground cools down the same view will be sharper.

 

The air has different refractive indexes depending upon temperature and pressure.

When different air masses are mixed high vs low pressure, warm vs cold, the air

has a blend of refractive indexes. 

 

In the upper midwest good seeing occurse when there is a large high or low pressure

system anchored to us.  Barometric and Tempertures are stable for more than 24 hours.

Changes in temperature occur after sundown too.  The best seeing for me occurs

in the summer when the extra humidity does not allow the air to cool rapidly.

 

Don't discount local seeing.  I used to setup on concrete because it was easy.

One day I picked up my rig and moved to a place where I was looking over 100 feet

of grass instead of 100 feet of pavement.  The view immediately sharpened up.


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#9 deepwoods1

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Posted 01 February 2023 - 12:23 PM

Local seeing can be a disaster. Houses, roads and the lay of the land can all effect low level, local seeing. My front yard looks over a road towards the east, however there are few houses in that direction. I notice seeing diminishes every time a car passes, but overall it has better seeing than looking south, directly over a neighborhood. Always awash with turbulence. 



#10 bobzeq25

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Posted 01 February 2023 - 12:33 PM

Note that.

 

Forecasting seeing is difficult and uncertain.

 

A night can start out with bad seeing, and it can get better in minutes.  Or vice versa.  It can happen multiple times in the same night.

 

This is one reason why big observatories tend to be at high altitudes surrounded by either oceans or deserts.  The air is both more transparent and steadier.  On average.



#11 jimhoward999

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Posted 01 February 2023 - 12:38 PM

One may not be help to help turbulence in the upper atmosphere, but for most observers the majority of the atmospheric turbulence is not happening high in the sky where you can't do anything about it. but very close to the telescope.

 

Bring a telescope out of a warm house, set it up on concrete driveway or patio, look at a planet low in the sky over top of neighbors roof tops and seeing is guaranteed to be terrible.

 

Do the opposite:  let the telescope stabilize, set it up on grass, and look at the zenith and seeing will be much better.

 

Also, a counter intuitive fact is that a well placed fan improves turbulence be breaking up thermals and hence improves seeing.  One might think that a fan moving the air would make turbulence worse, but it doesn't. (a gentle breeze not hurricane force).

 

So if one has poor seeing, one should not automatically assume it is in the upper atmosphere and you are helpless.  It may not be.

 

I know this is all old hat to most,  but I saw comments suggesting nothing could be done about seeing, so I though it might be a good reminder.



#12 HappyGalaxymore

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Posted 01 February 2023 - 12:50 PM

All of these make more sense to me so thank you! I really grasp the water analogies best because I can visualize that. When looking/imaging DSOs, is it less impactful because you're essentially zooming through them? 

 

Really good to know about the local seeing as I've been limited to basically high images from trees on the East side and my house on the West so that would have been new to me when I have more sky to view. I also would have never predicted that setting up on concrete vs grass would affect my view so that's huge!

 

At what altitudes does seeing start/stop taking an effect or is it from the front of the scope all the way to the edge of the atmosphere?

 

Again, thank you all for entertaining my 21 question session. 


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#13 UnityLover

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Posted 01 February 2023 - 12:58 PM

Imagine a hot summer day. In one direction, the air is easy to see through. In the other, its blurry. Seeing is like that. it can be judged bu the twinkling of the stars.


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#14 jimhoward999

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Posted 01 February 2023 - 02:07 PM

All of these make more sense to me so thank you! I really grasp the water analogies best because I can visualize that. When looking/imaging DSOs, is it less impactful because you're essentially zooming through them? 

 

Really good to know about the local seeing as I've been limited to basically high images from trees on the East side and my house on the West so that would have been new to me when I have more sky to view. I also would have never predicted that setting up on concrete vs grass would affect my view so that's huge!

 

At what altitudes does seeing start/stop taking an effect or is it from the front of the scope all the way to the edge of the atmosphere?

 

Again, thank you all for entertaining my 21 question session. 

Most local seeing effects are close to the telescope.....within a foot or 2, and sometime a lot closer. 

 

When you get into the atmospheric seeing effect on planets vs DSOs it gets a little more complicated.  You have fast seeing and slow seeing.  If the atmosphere distorts slowly, then the image of large objects like planets will boil degrading resolution, whereas a small point-like object will simply move slowly and you can track it. .  That is the seeing degradation that is most noticeable.  But there is also faster higher frequency turbulence which jitters every point blurring it out.  High frequency jitter affects all objects.  But the amplitude is inversely related to the frequency.   Visual observers wont notice the high frequencies on planets because the low frequency is more distracting, but will see distant objects as fuzzy.


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#15 HappyGalaxymore

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Posted 01 February 2023 - 02:23 PM

Most local seeing effects are close to the telescope.....within a foot or 2, and sometime a lot closer. 

 

When you get into the atmospheric seeing effect on planets vs DSOs it gets a little more complicated.  You have fast seeing and slow seeing.  If the atmosphere distorts slowly, then the image of large objects like planets will boil degrading resolution, whereas a small point-like object will simply move slowly and you can track it. .  That is the seeing degradation that is most noticeable.  But there is also faster higher frequency turbulence which jitters every point blurring it out.  High frequency jitter affects all objects.  But the amplitude is inversely related to the frequency.   Visual observers wont notice the high frequencies on planets because the low frequency is more distracting, but will see distant objects as fuzzy.

That's super super helpful and it feels like it all clicks a lot better in my brain now. 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. 



#16 rcooley

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Posted 02 February 2023 - 02:34 PM

Not to be flippant, but you know it when you "see" it.  There are just some nights when it's so crystal clear that you realize the stars haven't been this sharp in ages.  I know that clear sky chart has color coded designations, and those surely help, but frankly I don't check it every time I take the scope out, (which frankly isn't often this time of year).  But if I do go outside at night for some reason other than to so some astronomy, and the night sky is exceptional, and I don't want to be bothered setting up, I'll just drag out my Galileoscope (which lives on a tripod in our office space) and take a quick peak at a few of the usual suspects. Never disappoints.


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#17 monolithic

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Posted 03 February 2023 - 08:34 PM

Imagine you're looking at something under a body of clear water.
Quality of the image you see depends on how much the water moves. If the water is moving (turbulent), the image will be wobbly and fine detail is harder to see. If the water is still, you can almost peer through it like it's not there and observe fine detail to your heart's content.
Atmospheric seeing isn't too far off from this. If the night is windy and/or you have a jet stream overhead, the turbulence of the air will smother fine detail. A calm, still night unlocks subtle features you normally don't see on planets.
Seeing is not as critical for DSOs unless you like to use upwards of 300x on clusters, galaxies, planetary nebulae, etc.



#18 Tony Flanders

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Posted 04 February 2023 - 05:59 AM

Most local seeing effects are close to the telescope.....within a foot or 2, and sometime a lot closer.


I consider those to be in a different category entirely -- I would never call that bad seeing. It's true that the very worst problems typically occur inside the telescope itself, as when bringing a warm telescope out into the cold air. And it's also true that your body (and hand) can cause serious problems if you're upwind of the telescope.

But I think of local seeing as stuff that happens within a few hundred feet of the telescope. For instance, observing through the plume of your house's furnace will really mess up the image quality big time! And even ignoring any artificial heat sources, the windward side of a hilltop typically has much better seeing than the lee side.

#19 Dave Mitsky

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Posted 04 February 2023 - 02:21 PM

The articles at the following links discuss seeing and how it affects astronomical observing:

 

http://www.damianpeach.com/seeing1.htm

 

http://www.damianpea...m/pickering.htm

 

https://skyandtelesc...ing-the-seeing/

 

https://www.skyatnig...nomical-seeing/

 

https://weather.gc.c...o/seeing_e.html



#20 jimhoward999

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Posted 04 February 2023 - 02:39 PM

I consider those to be in a different category entirely -- I would never call that bad seeing. It's true that the very worst problems typically occur inside the telescope itself, as when bringing a warm telescope out into the cold air. And it's also true that your body (and hand) can cause serious problems if you're upwind of the telescope.

But I think of local seeing as stuff that happens within a few hundred feet of the telescope. For instance, observing through the plume of your house's furnace will really mess up the image quality big time! And even ignoring any artificial heat sources, the windward side of a hilltop typically has much better seeing than the lee side.

Putting the local thermal effects in a different category is fine and lumping it in is no doubt  an imprecise use of the terms.  But, I think that the atmosphere, your neighbors roof, and your driveway and your hot scope all deserve mention in seeing discussion, even if they are all different phenomena.



#21 tcifani

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Posted 20 February 2024 - 05:25 PM

I searched the forums first so my apologies if I missed it somewhere.

 

I don't understand 'seeing'....

 

Here's an informative video by Astronomy and Nature TV that delves into the topic a bit more:

 

https://www.youtube....h?v=4XW4FBJMpkc



#22 Sebastian_Sajaroff

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Posted 20 February 2024 - 06:40 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.



#23 mac57

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Posted 20 February 2024 - 06:43 PM

It's like looking across an asphalt parking lot in the summer,  Imagine the shimmering,  We are all slaves to our own local skies,  Perhaps the jetstream has come down on you for a while, or the temp differences can make dew or atmospheric moisture come and go.  Welcome to the backyard.  Mark



#24 PKDfan

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Posted 20 February 2024 - 10:35 PM



I think of seeing as a Sine wave overlaid us with slow undulating lows or rapidly pitching highs like a voice with chaotic modulation (very bad seeing) or a gentle swell or a placid lakes surface the very best.


Edmonton has both the slow very good seeing and chaotic poor, the twinkles, which frustratingly usually has very good transparencies.

The last three nights and nearly 12 hours observing lunar at its highest culmination i could have pushed past 500X eaaily but stayed below 170X limited by aperture choice.

My 100mm would have done that exremè power easily.


Frustratingly for us with the sinewave analogy their is usually more then one fundamental tone over us and some pulses of major instabilities can masquerade as fair seeing with clear skies but is barely adequate for binocular sweeping.




Clearest Stable Skies

#25 daveb2022

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Posted 20 February 2024 - 10:43 PM

I kinda use the Pickering scale for my logs to judge turbulence. I note seeing and transparency. Last year I had several days of above average seeing. This year has had a lot of days with stable seeing, but the poor transparency has been an issue for months. (edit) My skies average P-7 and feel lucky.

 

https://www.damianpe...m/pickering.htm


Edited by daveb2022, 20 February 2024 - 11:02 PM.



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