I've been reading observation reports by both experience and inexperienced observers. Some of which describe the terrible conditions they had to contend with in order to find that Deep Sky Object, or for some, to capture that image they hunger for. I see some people, usually the less experienced, often refer to the Astronomical Seeing and how terrible it was. But when reading many of these reports or Image capture descriptions I don't believe it is truly the Astronomical Seeing they are having problems with, but rather the "Transparency" of the sky. It seems to me some people have a tendency to blame the Seeing on the Transparency, especially this time of year in many areas where the humidities in the summer months are on the rise.

It's easy to see where the confusion might stem from because of the term "Seeing" itself is often thought to mean how well we can See with our eyes, like 20/20 vision.. When conditions make it difficult to see something with our telescopes some tend to automatically blame the Astronomical Seeing. This is IMHO not always the true case. There are many times when the Astronomical Seeing is stable but the skies are still preventing us from being able to see some of those faint fuzzy objects with our telescopes. Why? because it is the Transparency of the air which is preventing us from seeing those faint fuzzies, not poor Astronomical Seeing.

For those that might be unclear on the subjects let's take the time to clarify the differences between "Astronomical Seeing" and "Transparency".

First lets discuss what Astronomical Seeing is. >>> "Astronomical Seeing" is that twinkle or scintillation you see in the stars when the atmosphere is unstable. This instability or turbulence is caused by the motion of thermal currents of various temperatures mixing and riding the winds in varied directions and different levels throughout the atmosphere.

Those heat waves you see rising off the black top on a warm summer evening as the air above starts to cool; That distortion of heat mixing with cooler air is a good example of low level localized bad Seeing. ... The skies above might first appear to look completely clear but you can still have poor Astronomical Seeing.

Even on those nights when you look at stars as they rise above the horizon and their twinkle indicates what are poor Seeing conditions, stars more directly over head as you get closer to Zenith won't appear to twinkle because the Seeing is generally appears to be more stable more directly overhead, because your looking through less of the Earths atmosphere. When you try looking at stars closer to the horizon it's like looking at a coin in the bottom of a wishing well or pool with the wind blowing ripples in the water, the deeper the pool the more the effects of the wind on seeing that coin. Same principles apply and is why bright stars twinkle more when they are closer to the horizon. Additionally, the Astronomical Seeing can vary from bad to good to worse and back again throughout a single observing session. However! generally as the night progresses the seeing can improve significantly as the heat generated throughout the day escapes the atmosphere and the temperatures equalize thus becoming more stable. Or at a more localize level closer to the ground your standing on, as the winds calm and the temperatures of the surface and surrounding objects equalize with the air causing it to become more stable.

Some things that can effect Astronomical Seeing more localized to you might be Trees, Roof tops paved black top roads, anything that can store solar radiation throughout the day and release it back into the air at night. In the Winter months Heat escaping from homes will effect seeing if you aim your telescope over the house or building. The Heat Island effect of large cities. Hills and mountains have significant effects in how they effect the motion of the winds or thermal currents. Location to a large body of water can have a big effect on Seeing,(Sometimes for the better). Ground level winds can create poor Astronomical Seeing stirring up any radiant warmth released into the surrounding air. But there are times when you might want a little wind. When the winds calm completely you can start having problems with Dew build up on everything, This is a strong indicator there is a high level of moisture in the air which might be indicative of Poor Transparency.

When you look through the eyepiece at a star or the planets and they are dancing around going in and out of focus, one moment it will be nice and clear the next it will be blurred or distorted then this is a good indicator of Poor Astronomical Seeing.

"Transparency": exactly as it suggests, The Transparency we are referring to here is the total "Clarity" of the air we want to look through from the ground to the upper levels of our atmosphere. It is the total measure of moisture content in the air. e.g. Ground fog equals very poor transparency but doesn't necessarily have any effect on Astronomical Seeing, As a matter of fact ground fog accumulates when the winds are calm and the temperatures are more stable. Humid Hazy skies are poor Transparency, and sometimes it's a high thin layer of clouds that you might not be able to see with your eyes. These warm humid hazy skies of the summer months are often confused with poor Astronomical Seeing. When actually winds that cause poor Seeing can actually improve the transparency by moving that humid and hazy air dispersing it, where as when the winds are calm the moisture has the tendency to accumulate and remain confined to a specific area.

When I read a observation report or an image post where the person indicates the Seeing was poor and they had dew accumulating on everything as the night progressed making things difficult for them, my guess is they actually meant the Transparency was the culprit giving them the difficulties they encountered. And unlike Poor Seeing where magnification can make the effects more pronounced, It won't matter how much magnification you use with Poor Transparency, and, if you have any amount of light pollution in your area, the sky glow will scatter more and increase the nasty effects of poor Transparency. Especially common on these warm humid hazy nights. The Moon Glow will also scatter more making it more likely to wash out any possibility to observe many faint objects even if they are not near the Moon.

When you look through the eyepiece at a star or the planets and there is an unusual glow around them and they appear a little washed, or you try to find a small faint galaxy but can't because it seems everything is awash, this is a good indicator of Bad Transparency.

Which is worse? Well, for astrophotography purposes both are not good. If I had a choice however, I would rather contend with poor Seeing than bad Transparency. At least with a DSLR or purpose built CCD with a large chip you can use a short focal length telescope and get some nice wide field images without significant effects from seeing. But bad transparency can wash it all out, moisture starts to accumulate and your lens or mirrors start fogging making it impossible to capture any good images. Same goes for observing, I would rather observe on a night of poor seeing than a night of bad transparency. Poor Seeing will usually limit you to the amount of magnification you're able to utilize before it gets too annoying, but, there are still a bunch of DSO's you can observe at lower magnifications and still enjoy doing it. Now if you like to observe double stars then of course that would be a different story. If you want to observe the planets then you want Stable Seeing so you can push the magnification. But! on nights when the transparency is poor you might not be able to use any amount of magnification to observe anything. especially if there is any amount of Light pollution or the Moon casting a sky glow. Not to mention you'll probably be fighting to keep your equipment and your optics dry and clear.

Great write-up, Tim.  It looks like you've had some time on your hands lately.  Seriously, though you are dead on about the summer weather.  On the few occassions I've been able to get out the nights have been still and very muggy causing me serious problems with the transparency, but the seeing has been relatively good.  A good example is my session in late June when I couldn't keep the dew from forming on my eyepieces.  I had to end that night because even M13 was a smeared blob even at low power in the 18" scope.  On that same night, through that same nasty transparency, Jupiter looked great because the air was very steady, and the planet was bright enough to cut through the haze.

Very well thought out post Tim.  Since most of my time is spent under skies with poor transparency the seeing is what usally gets mentioned as it is more variable.  Like most of us summer's humidity can cause a great deal of transparency issues while winter's weather systems offer crystal clear skies but seeing issues.

Good issue to expound on Tim,

L

leo731:

Very well thought out post Tim.  Since most of my time is spent under skies with poor transparency the seeing is what usally gets mentioned as it is more variable.  Like most of us summer's humidity can cause a great deal of transparency issues while winter's weather systems offer crystal clear skies but seeing issues.

Good issue to expound on Tim,

L

Today was probably one of the most humid days so far this summer. I was out with my camera taking picture of the scenery and large birds. When looking across the river it was so hazy it was like the green forest on the opposite side had been white washed. The air was so thick you could cut it with a butter knife, Just sitting in the shade I was drenched in sweat. Today is a good example of poor transparency during the daylight. Coupled with the heat, and strong winds you could even see the turbulence in the moving air over the land. Now mind you this is what I can see in the middle of the afternoon, Imagine what the night sky would look like through a telescope in these conditions.

The camera I use for both daytime and night time is a Canon EOS 50D. Most of the time during the daytime I like to shoot pictures of the Osprey, Hawks, Great Egrets and other large birds. Since it is difficult to get close enough to these birds to get a nice picture I use for these shots is a EF 100-400mm L IS USM lens, usually at 400mm. On days like today I am finding that just like with a telescope this lens is effected by astronomical seeing. Whether you see it or not it is still present day or night. And a day like today when you have High Temps, High Humidity and High Winds taking a good clear picture was proving to be quite the challenge.

When I got home and took a look at the pictures on my PC I can see heat distortions and blurring in most of them. Compared to most of the pictures I took, the ones taken over the water were the most clear of them all. This is because the air over the water is closer to the temperature of the water than the air over land near the water. Therefore the air over the water is usually pretty calm, but the air over the land where the cooled air from the water meets the rising air from the heated land is very turbulent. (Convection Currents) People living near the coast might know this as the sea breeze boundary.

tkerr:

Today is a good example of poor transparency during the daylight. Coupled with the heat, and strong winds you could even see the turbulence in the moving air over the land. Now mind you this is what I can see in the middle of the afternoon, Imagine what the night sky would look like through a telescope in these conditions.

That sounds mighty uncomfortable!  It is interesting that you were able to see the effects of the moving air from your own perch.  Even though we breathe it in & out and regularly I think there are times when we quite forget just how much air and its movement affects seeing and telephoto photography.

tkerr:

Therefore the air over the water is usually pretty calm, but the air over the land where the cooled air from the water meets the rising air from the heated land is very turbulent. (Convection Currents) People living near the coast might know this as the sea breeze boundary.

Yes, I can see that some here too living fairly close to the Pacific Ocean.  We also have the geographical effect of living in a basin with the ocean on one side and mountains encirling the other three.  It often makes for some strong inversion layers which trap the haze and smog under an inversion layer which makes the air very still but very stale with poor transparency.  Thus bright objects and lunar and planetary detail can be easier to come by but fainter objects like nebulae can remain hidden.  Of course the light pollution scattering its photons across the humidity and airborne particulate matter doesn't help either!

It is what, six weeks till September? Hope you can remain cooler till then,

L

 

 

I have a similar thing where I live as you do Leo,.,I'm in a bowl with mountians to the north .,and a lake 3.5mi across,.and over 100ft. deep,.,. to the south,.,and in the summer and winter this effects the seeing.., as the air moves up from the lake and is funneled right over me.,.,I have looked at the moon and seen ripples just like blacktop on a summers day,,.in spring and fall.,,when the air temp changes less between night and day.,.,the seeing is usually more stable,.,.still I'm not complaining,.,except about the clouds that have made themselves at home here for the last month.,.,,O+O

 Where I live in northern Colorado, you can predict the night's transparency (when the weather is not rapidly changing) by comparing the color difference between the sky at the horizon and at the zenith.

• When the zenith is pale blue and the horizon is white, transparency will be poor. Smoke and/or humidity are the usual suspects. Maybe tonight you have something else to do.
  
• When the zenith is rich blue and the horizon is pale blue, transparency will be decent. A good night for clusters, doubles, and brighter nebulae & galaxies.
  
• When the zenith is deep blue and the horizon is a rich blue, ah! That's the night for hunting the faint fuzzies and eking out the last dim stars of rich clusters.

I don't know how this stacks up against other regions, but in the typically dry air at my 5000+ ft. elevation, the above rules generally tell you what to expect.

-- Tim
   Colorado

I'm a beginner, and made some observations that I put in the cloudy nights forum.

 To some it up, what I've learned when it comes to DSO in urban skies is that transparency can make or break a night of viewing. Two weekends ago we had a cool front move in after some rain. Seeing was horrid, but the transparency was as good as it was during the winter. I bagged more Messier objects with my new binoculars than I had in 10 nights with my telescope. What I learned was that transparency not only effects the "clarity" of the sky, but it also amplifies light polution.

Where I live, there is a bit of sky glow coming from the south. When I used my binoculars to look at Sagittarius and Scorpious during two nights of this cool front, I could easily make out the butterfly cluster, the larger cluster below it, M24 sky cloud, M22, and a host of other brighter M objects.

 Two days later at the same time with the binoculars, I could barely make out the constellation sagittarius against the LP(amplified by haze). I could barely see M22, and the star cloud was not visible at all.

 Dj