Seeing and Transparency

[DarkSkyMan]

(was this beginners section created just for me?):biggrin:

how can I tell by looking at a weather forecast if the sky will be well suited to astronomy? I'm not sure what the difference between transparency and seeing is, but how can those things be predicted?

Thanks

Brett

[Mick]

Just for you Brett :biggrin:

http://7timer.y234.cn/V3/product.php?language=en&product_id=1&country=AS

[Radar]

All I do is check jetstream activity.

http://www.wunderground.com/global/Region/AU/JetStream.html

I know that the sky is supposed to be quite transparent after a bit of rain or stormy weather has passed through. There is more to predicting this type of stuff but thats about all I do. Now that I want to get into planetary I should learn more about it to be honest.

Cheers

Ray

[PeterP]

This link is effective to

http://squall.sfsu.edu/crws/jetstream.html

Pete

[BitsyLon]

I find these most useful:

http://www.skippysky.com.au/

http://cleardarksky.com/csk/

{{'Transparency' means just what astronomers mean by the word: the total transparency of the atmosphere from ground to space. It's calculated from the total amount of water vapor in the air. It is somewhat independent of the cloud cover forecast in that there can be isolated clouds in a transparent air mass, and poor transparency can occur when there is very little cloud.

Above average transparency is necessary for good observation of low contrast objects like galaxies and nebulae. However, open clusters and planetary nebulae are quite observable in below average transparency. Large globulars and planets can be observed in poor transparency.

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.}}

Cheers,

Lon

[Kokatha man]

Just for you Brett :biggrin:

http://7timer.y234.cn/V3/product.php?language=en&product_id=1&country=AS

Hmmmm....!:hmm::hmm::hmm: - interesting, it'd be a good exercise to log this info against actual planet images for the respective dates using a standardized optical setup.....it certainly has many localities for me here in South Oz - I might take the task on to see whether it gives any real appreciation of what to expect!

I'd heard of "skippy sky" but never visited the website.....can't quite understand the "seeing index" etc figures and couldn't find any explanatory texts (perhaps brain burn-out from getting my head around the 7timer codes etc:eek::confused::biggrin:)

For jetstreams I use this site:

http://wxmaps.org/pix/aus.jet.html

.....but in the end I go with a fair amount of "intuitive" reaction - the old checking of star "twinkle" (making sure you're using lesser magnitude stars at appropriate elevation) and other "less tangibles" - some of which could fall into the "gobbledygook" category except they seem to be indicators of sorts imho....!:Oh_No::Oh_No::Oh_No:

[Rumples Riot]

Transparency is well explained above.

Seeing is how much the air is moving at any given moment. Not only planetary imaging but hi res DSO imaging is affected by seeing.

Seeing is usually described by the Pickering scale and Damian Peach gives an excellent examples of each gradient to the scale.

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

Seeing is often in my experience more related to several issues. Fast seeing is most often related to the middle layer of air and the jet stream. Good seeing can often be seen even when the jet stream is above the location you are observing from. Using the jet stream maps is a good guide but not the be all.

A good guide to follow though is this. If you have a high pressure sitting directly over you location, with no surface wind, lowish sort of figures on the jet stream maps (please use these maps http://wxmaps.org/pix/aus.jet.html http://weather.unisys.com/gfs/12h/gfs_4panel_12h_aus.html ) without any cloud or high moisture level, then you are likely to have good seeing in your location. Star twinkle near the horizon is another indication. On nights of good seeing the star twinkle on the horizon is very slight. Looking at the stars over head is a bit deceptive, it can often look good but in the reality it will not be a reliable indicator.

Let me know if you need more explanations of Seeing. My site has many examples of good images taken at high resolution and therefore I am sure I know what I am on about.

[DarkSkyMan]

thanks guys for the answers.

Brett

[Kokatha man]

I thought I'd post this here as it is already a thread.....as stated here by me before, I have some "very basic" appreciations of seeing etc conditions and over the last month or so have been capturing a number of hi-res images of Jupiter.....checking jetstream and weather charts for pressure systems etc (and of course the satellite images to see how much cloud is around - or coming!!!:Oh_No:)

Slowly, very slowly I am starting to build up a small appreciation of some of the factors involved to predict better opportunities and though a bit of a "log" with additional info when and where it seems appropriate might be usefull to others.....the charts are, of course, for South Oz but should be able to be extrapolated for elsewhere....!:smile::wink:

I'm indebted to the article "Beating the Seeing
by Alan M. MacRobert".....I've incorporated some parts thereof into the following.....I acknowledge his authorship of this article and have emboldened some salient points within it - as well as my own following short paragraph of preamble and the 2 attached images that this paragraph refers to.....the high pressure system is moving more centrally over Sth. Oz for tonight and the overhead jetstream velocity is going to be less than last night.....will this make for better seeing....? Clouds co-operating, I'll post some more tomorrow etc and see if my understanding is advanced in any way....!:eek::biggrin:


The attached images weather chart & Jetstream map were current when I attempted imaging on 7th Sept. 2009 – detail and focus were impeded by the disk of Jupiter being “fuzzy” or “hazy” which appears to conform to what is termed “fast seeing.” It should also be stated that there was no "pulsating" or expansion/contraction of the disk diameter of Jupiter and the image was really very "steady" but fuzzy re detail perception.....


Quote: "3. High Altitude effects.
Effects at this altitude are caused by fast moving “rivers” of air know as Jet streams. Wind shears at around the 200-300mb altitude level can cause images to appear stable, but very fuzzy, and devoid of fine detail. There isn’t anything the observer can do to prevent these effects, but forecasts are available, to help predict weather a Jet stream is present over your area.

Telescope users recognize two types of seeing: "slow" and "fast." Slow seeing makes stars and planets wiggle and wobble; fast seeing turns them into hazy balls that hardly move. You can look right through slow seeing to see sharp details as they dance around, because the eye does a wonderful job of following a slowly moving object. But fast seeing outraces the eye's response time.

An old piece of amateur folklore is that you can judge the seeing with the naked eye by checking how much stars twinkle. This often really does work. Most of the turbulence responsible for twinkling originates fairly near the ground, as does much poor seeing. But rapid, high-altitude seeing escapes this test. If the star is scintillating faster than your eye can follow (the eye's response time is about 1/10 of a second), the star will appear to shine steadily even if a telescope shows it as a hazy fuzzball.

Astronomers often talk of seeing cells — air-eddy lenses, millimeters to meters across, that swarm through the sky. These eddies originate wherever air masses rub past each other — either horizontally in winds, vertically by convection, or both. Sometimes, when watching an extended object like the Moon or a planet, you can focus the telescope on a horizontal layer of "shear turbulence" a few thousand feet high. The ripples sharpen up when you turn the focuser slightly to the outside of infinity focus (moving the eyepiece farther from the objective). This is the signature of an inversion layer, in which a mass of warm air flows across cooler air below. The actual temperature difference may be very slight.

Large or slow-moving eddies cause slow seeing, but they don't stay large forever. No matter what size the eddies are when they originate, they break up into smaller and smaller ones. When these finally become small enough to measure in millimeters, they die out and dissipate their energy as heat via the air's fluid friction (viscosity).

A light wave from a star is distorted on many size scales by the atmosphere. When the wavefront enters a telescope, its 'tilt' determine's the star's apparent position, while its 'roughness' determines how fuzzy the star looks. Generally a small telescope sees a relatively sharp star dancing around, while a large one sees a relatively steady but fuzzy star.

This complex situation belies an often-repeated piece of astronomer's lore: that seeing cells are 10 centimeters (4 inches) in size. In fact they come in all sizes. But cells in this middle range do have an important property: they affect a large telescope more seriously than a small one. If you have a 4-inch scope, cells 4 inches and larger passing through its line of sight will make an image move around while staying relatively intact. The same cells passing in front of a 12-inch aperture will superpose multiple images at once.

In Search of Steady Air
The seeing quality depends on the weather, but not by simple rules that apply everywhere. Poor seeing does seem more likely shortly before or after a change in the weather, in partial cloudiness, in wind, and in unseasonable cold. Any weather pattern that brings shearing air masses into your sky is bad news. Good seeing is most likely when a high-pressure system settles in to bring clear skies for several days running. Keep a seeing-versus-weather log for your locality, and you may discover correlations that will become your key to sharp viewing.

Time of night also plays a role, but again there are few universal rules. Right after sunset the seeing is apt to be excellent, so start your planetary observing as soon as you can find a planet in twilight. The seeing is apt to deteriorate before dusk fades out. Some observers find that their seeing improves after midnight; others say it goes to pieces. This depends largely on local topography; observers in valleys might get worse seeing as the night goes on and cold air flows down to pool in the valley. Just before sunrise may be another excellent time.

Geography is critical. Smooth, laminar airflow is the ideal sought by observatory-siting committees worldwide. The best sites on Earth are mountaintops facing into prevailing winds that have crossed thousands of miles of flat, cool ocean. You don't want to be downwind of a mountain; the airstream breaks up into turbulent swirls after crossing the peak. Nor do you want to be downwind of varied terrain that absorbs solar heat differently from one spot to the next. Flat, uniform plains or gently rolling hills extending far upwind can be almost as good as an ocean for providing laminar airflow. You may learn to predict which wind direction brings the best seeing to your observing site.

Mostly, though, beating the seeing is just a matter of patience. Just keep watching, and intermittent good moments may surprise you. One reason why experienced observers see more detail on the planets than beginners do is that they simply watch longer, ignoring all but the steadiest moments. Moreover, the seeing can change as radically from minute to minute as it does from second to second. When that perfect minute comes along, the dedicated observer is the one most likely to be there at the eyepiece to catch it." End Quote

[Radar]

Thanks Darryl,

I think I am going to compile the links and information from this thread and make a tutorial out of it on the homepage (authors credited). I think with planetary work becoming much more popular these days it would be very handy for people to have this information on tap.

Ray

[Kokatha man]

Sounds good to me Ray.....my own postings here are a "work in progress" - part of a larger tome with capture and processing but I think you're right to create some sort of compendium.....I'm trying to tie together the fine-tuned collimating of scopes with the artificial star I constructed, observations on seeing, a capture tute with screenshots and some on processing in Registax and AstraImage and CS3.....but it's a tall order, and to be able to put it into an "over-view" framework would be great....!

(tonight, before the clouds came, we had slow seeing" as opposed to last night's "fast seeing".....took some screenshots of the Jupiter image in the IC capture program showing settings etc.....and I'll continue taking screenshots of the weather patterns therein) - I'm here hoping the clouds will lift before it gets too late.....:hmm::pipethinker:

[azizajalal]

I know that the sky is supposed to be quite transparent after a bit of rain or stormy weather has passed through. There is more to predicting this type of stuff but thats about all I do. Now that I want to get into planetary I should learn more about it to be honest.

[davidwillson]

I thought I'd post this here as it is already a thread.....as stated here by me before, I have some "very basic" appreciations of seeing etc conditions and over the last month or so have been capturing a number of hi-res images of Jupiter.....checking jetstream and weather charts for pressure systems etc (and of course the satellite images to see how much cloud is around - or coming!!!:Oh_No:)

Slowly, very slowly I am starting to build up a small appreciation of some of the factors involved to predict better opportunities and though a bit of a "log" with additional info when and where it seems appropriate might be usefull to others.....the charts are, of course, for South Oz but should be able to be extrapolated for elsewhere....!:smile::wink:

I'm indebted to the article "Beating the Seeing
by Alan M. MacRobert".....I've incorporated some parts thereof into the following.....I acknowledge his authorship of this article and have emboldened some salient points within it - as well as my own following short paragraph of preamble and the 2 attached images that this paragraph refers to.....the high pressure system is moving more centrally over Sth. Oz for tonight and the overhead jetstream velocity is going to be less than last night.....will this make for better seeing....? Clouds co-operating, I'll post some more tomorrow etc and see if my understanding is advanced in any way....!:eek::biggrin:


The attached images weather chart & Jetstream map were current when I attempted imaging on 7th Sept. 2009 – detail and focus were impeded by the disk of Jupiter being “fuzzy” or “hazy” which appears to conform to what is termed “fast seeing.” It should also be stated that there was no "pulsating" or expansion/contraction of the disk diameter of Jupiter and the image was really very "steady" but fuzzy re detail perception.....


Quote: "3. High Altitude effects.
Effects at this altitude are caused by fast moving “rivers” of air know as Jet streams. Wind shears at around the 200-300mb altitude level can cause images to appear stable, but very fuzzy, and devoid of fine detail. There isn’t anything the observer can do to prevent these effects, but forecasts are available, to help predict weather a Jet stream is present over your area.

Telescope users recognize two types of seeing: "slow" and "fast." Slow seeing makes stars and planets wiggle and wobble; fast seeing turns them into hazy balls that hardly move. You can look right through slow seeing to see sharp details as they dance around, because the eye does a wonderful job of following a slowly moving object. But fast seeing outraces the eye's response time.

An old piece of amateur folklore is that you can judge the seeing with the naked eye by checking how much stars twinkle. This often really does work. Most of the turbulence responsible for twinkling originates fairly near the ground, as does much poor seeing. But rapid, high-altitude seeing escapes this test. If the star is scintillating faster than your eye can follow (the eye's response time is about 1/10 of a second), the star will appear to shine steadily even if a telescope shows it as a hazy fuzzball.

Astronomers often talk of seeing cells — air-eddy lenses, millimeters to meters across, that swarm through the sky. These eddies originate wherever air masses rub past each other — either horizontally in winds, vertically by convection, or both. Sometimes, when watching an extended object like the Moon or a planet, you can focus the telescope on a horizontal layer of "shear turbulence" a few thousand feet high. The ripples sharpen up when you turn the focuser slightly to the outside of infinity focus (moving the eyepiece farther from the objective). This is the signature of an inversion layer, in which a mass of warm air flows across cooler air below. The actual temperature difference may be very slight.

Large or slow-moving eddies cause slow seeing, but they don't stay large forever. No matter what size the eddies are when they originate, they break up into smaller and smaller ones. When these finally become small enough to measure in millimeters, they die out and dissipate their energy as heat via the air's fluid friction (viscosity).

A light wave from a star is distorted on many size scales by the atmosphere. When the wavefront enters a telescope, its 'tilt' determine's the star's apparent position, while its 'roughness' determines how fuzzy the star looks. Generally a small telescope sees a relatively sharp star dancing around, while a large one sees a relatively steady but fuzzy star.

This complex situation belies an often-repeated piece of astronomer's lore: that seeing cells are 10 centimeters (4 inches) in size. In fact they come in all sizes. But cells in this middle range do have an important property: they affect a large telescope more seriously than a small one. If you have a 4-inch scope, cells 4 inches and larger passing through its line of sight will make an image move around while staying relatively intact. The same cells passing in front of a 12-inch aperture will superpose multiple images at once.

In Search of Steady Air
The seeing quality depends on the weather, but not by simple rules that apply everywhere. Poor seeing does seem more likely shortly before or after a change in the weather, in partial cloudiness, in wind, and in unseasonable cold. Any weather pattern that brings shearing air masses into your sky is bad news. Good seeing is most likely when a high-pressure system settles in to bring clear skies for several days running. Keep a seeing-versus-weather log for your locality, and you may discover correlations that will become your key to sharp viewing.

Time of night also plays a role, but again there are few universal rules. Right after sunset the seeing is apt to be excellent, so start your planetary observing as soon as you can find a planet in twilight. The seeing is apt to deteriorate before dusk fades out. Some observers find that their seeing improves after midnight; others say it goes to pieces. This depends largely on local topography; observers in valleys might get worse seeing as the night goes on and cold air flows down to pool in the valley. Just before sunrise may be another excellent time.

Geography is critical. Smooth, laminar airflow is the ideal sought by observatory-siting committees worldwide. The best sites on Earth are mountaintops facing into prevailing winds that have crossed thousands of miles of flat, cool ocean. You don't want to be downwind of a mountain; the airstream breaks up into turbulent swirls after crossing the peak. Nor do you want to be downwind of varied terrain that absorbs solar heat differently from one spot to the next. Flat, uniform plains or gently rolling hills extending far upwind can be almost as good as an ocean for providing laminar airflow. You may learn to predict which wind direction brings the best seeing to your observing site.

Mostly, though, beating the seeing is just a matter of patience. Just keep watching, and intermittent good moments may surprise you. One reason why experienced observers see more detail on the planets than beginners do is that they simply watch longer, ignoring all but the steadiest moments. Moreover, the seeing can change as radically from minute to minute as it does from second to second. When that perfect minute comes along, the dedicated observer is the one most likely to be there at the eyepiece to catch it." End Quote


I think I am going to compile the links and information from this thread and make a tutorial out of it on the homepage (authors credited). I think with planetary work becoming much more popular these days it would be very handy for people to have this information on tap.