Seeing and Transparency
Published on Sep 9, 2007 at 2:11 pm.
5 Comments.
Filed under amateur astronomy, atmosphere.
When you want to have some idea how to dress to go to a ball game or a picnic, or even to walk around outdoors, you turn to the radio or television and listen to the weather forecast. If you want more detail, then there are web sites, such as the National Weather Service or Accuweather (both sites are mainly for here in the USA). But, often these sources of information don’t tell astronomers what we want to know. There have been plenty of times when the forecast was “clear” or “mostly clear,” yet the sky conditions were such that it was useless to even think about doing serious work (or sometimes even not-so-serious work). Obviously, the weather forecast version of “clear” for the general public isn’t what we mean by “clear” for astronomers. But, there are often few reliable sources for astronomical weather forecasts. As a result, most astronomers wind up learning a bit about meteorology in order to interpret satellite images and weather maps and make their own forecasts for observing conditions.
In my previous entry, I talked about twinkling stars. The stars (and sometimes even planets) twinkle due to turbulence and convective motions in the atmosphere. The same sort of thing that sometimes makes air travel bumpy also makes the stars twinkle, flash, and flicker. The more twinkling that is going on, the less detailed work that you can do. Astronomical images are generally time exposures. If a star is twinkling, though, then its light is dancing around, and the image is blurred. This means that the images of stars, instead of being pinpoints of light, are small blurs. When stars are close together, such as with close binary stars or in a star cluster image, the blurs overlap, making it impossible to distinguish one from another. This limits what sort of work you can with the telescope. For the general public, “clear” means that there is no precipitation and that if you look up you can see the Sun or Moon, and maybe some stars at night, if they are up. But, astronomers obviously need a better scale for rating the observing conditions of the sky besides just clear, partly clear, mostly clear, scattered clouds, etc. That is where a sky measure called seeing comes in.
Astronomical seeing, or usually just seeing, is a measure of the steadiness of the air. Unfortunately, there are a number of different ways to measure seeing. I won’t go into all of them, but I’ll mention at least three. The simplest measure of seeing is a qualitative rating scale. An example of this is on the American Association of Amateur Astronomers web site. Here, seeing is rated Level 1 through Level 5. In this scale, Level 5 is perfectly crisp images, in which the stars would be ideal tiny pinpoints of light. Level 1, on the other hand, is air so disturbed that planets themselves would be twinkling. I imagine that a Level 0 would be if the air is so disturbed that even the Moon appeared to be wavering to the naked eye (I’ve seen skies this bad before!).
Other measures of the seeing conditions would be a measure of the how spread out the light from a star is. One way of doing this is to make an analogy to the diffraction effects of a telescope. Light acts as a wave. When that light passes through an opening, such as the objective of a telescope, it spreads out some. This tends to blur the images. The smaller the opening, the more the spreading. So, large telescopes would have less of this effect than small telescopes. A measure of the spreading is resolution. The simplest (though not technically best) working definition of resolution is that it is a measure of how close together two identical stars could be and still be able to tell that there are two stars there. That would be the resolution limit of the telescope. A telescope with 11 arcseconds resolution would be able to distinguish binary stars 11 arcseconds apart. A telescope with 2.3 arcsecond resolution would be able to distinguish stars only 2.3 arcseconds apart. And, a telescope with 0.045 arcsecond resolution would be capable of distinguishing stars 0.045 arcseconds apart. In theory, that is. That assumes high quality optics and perfect collimation of the telescope’s optics. But, it also ignores the effect of the atmosphere. Typically, the atmosphere is what limits larger telescopes. Even on clear and steady nights, the atmosphere moves around enough that the resolution of a telescope is seldom more than about 2 or 3 arcseconds (better on the top of mountains, which is why many professional observatories are located there). But, often the air is moving around so much that even stars closer than 5 arcseconds apart can not be seen. Sometimes, the air is so unsteady that stars closer than 20 arcseconds apart appear as just a blob. In that case, most astronomers would just pack up. So, one of the ways that astronomers, particularly amateur astronomers, would judge seeing conditions would be in the closest pair of binary stars that could be seen. So, you might hear people talking about 4 arcsecond seeing, meaning that no matter how good the telescope, you still can’t see stars closer together than 4 arcseconds apart. Or, in the rare extremely good seeing conditions, then someone may say that the seeing is 0.5 arcseconds, indicating that a telescope whose resolution limit was at least 0.5 arcseconds could actually be used to see stars that close together under those sky conditions.
A related, but different, seeing metric was developed by the American astronomer Edward Pickering. To use the pickering scale, a star’s image is studied at maximum magnification. Due to diffraction (the spreading of light as it passes through an opening) star images actually look like little disks surrounded by concentric rings of light (assuming perfect optics in the telescope). But, often even with perfect optics, the twinkling (or scintillation) of the starlight blurs the diffraction pattern. The Pickering scale is a ten point scale that measures this. Damian Peach has produced a wonderful set of animations showing the Pickering seeing scale.
The other measure of sky conditions is astronomical sky transparency, or usually just transparency. Seeing is a measure of how steady the sky appears. Transparency is a measure of the clarity of the sky. At first glance, these seem to be the same thing, but they are not. Transparency is a a sky condition that is affected by two factors: darkness and extinction. The two properties are closely related, but perhaps not in the way that you might think. First of all, darkness is a measure of how dark the sky is. The brighter the sky, the harder it is to see dim objects. If the sky is too bright, then it is brighter than the objects trying to be observed, and so seeing them with the even brighter sky interposed between the object and the observer is impossible. Sky brightness is affected by several things. One thing affecting the sky brightness is the amount of ambient light. The more light that there is shining around, the more light that can be scattered by the air (or things in the air), and so the brighter the sky will be. When the Sun is up, there is so much light that only the brightest objects can be seen in the sky. When the Moon is up, then its light, too, can be scattered enough to overwhelm the dimmer objects. But, man-made light is also a problem. Light pollution can make the sky appear as bright as if the Moon were full (or even brighter), overwhelming dimmer objects.
But, ambient light is not all that affects transparency. You see, it doesn’t matter how much light there is shining around. What really matters is how much light shines into the telescope (or your eye). This is determined by the amount of scattering that goes on in the air. The air molecules themselves scatter light. That is why the sky appears blue during the day. This same effect happens at night. Blue light from all of the stars and the Moon and anything else in the sky is scattered. The other colors of light are also scattered, of course, but the blue gets scattered more. Normally, there is simply not so much light available to scatter as to seriously interfere with observations. But, the Full Moon is bright enough to have such an effect. Even a lot of man-made light pollution can have a strong effect. But, normally far more important that scattering from air molecules is scattering from water droplets, dust, and aerosols in the atmosphere. So, the more polluted the air, the worse the transparency if there is any light whatsoever to scatter. Volcanoes, wildfires, smoke stakes, etc. all pump things into the atmosphere. This makes for pretty sunsets, but lousy observing. But, another factor that is normally an even bigger effect is water in the air. The more water in the air, particularly water droplets, the more light is scattered. So, a slight haze can make the sky much brighter by scattering moonlight. Worse,the haze not only scatters, but also reflects, ground based light pollution. So, the more haze, the brighter the sky. Often this haze is very light, and most people from the general public don’t even notice it during the day. But, astronomers notice it (and curse it) at night. It gets in the way. It makes the sky brighter. It hides the dimmer objects. The less that is in the atmosphere, the darker it will be and the so the better (higher) the transparency. This is another reason to put observatories on mountain tops. The higher the altitude, the more likely you are to be above many of the aerosols and cloud droplets, and so the darker the sky will be.
But, there is another factor affecting transparency, and that is extinction. Atmospheric extinction is the dimming of light as it passes through the air. In some cases, the light is scattered, but in other cases it is absorbed. Ultraviolet light, for example, is absorbed by ozone in the stratosphere, and little makes it to the ground. But, if you are only considering visual light (as most amateur astronomers would), then you worry about dust, aerosols, and water droplets. These things will absorb or reflect light from the stars, preventing it from reaching the observer. This is much of what causes extinction. Interestingly, it is much of the same stuff that causes extinction that also scatters light. So, more things in the atmosphere causes more scattering (brighter sky) and more extinction (making the objects being observed dimmer). Since the bright sky makes dim objects harder to see, this is a double effect!
Amateur astronomers typically rate sky transparency by how dim of a star that they can see. The same page from the American Association of Amateur Astronomers that I referenced earlier has a transparency scale based upon how many stars in the Little Dipper can be seen. Forecasting transparency is seldom done by radio and television weather forecasters. The Canadian Weather Office, though, has a nice graphical forecast of sky transparency that is useful for observers in North America.
You might think that transparency and seeing are closely related. In a sense they are. Both are important for astronomers. However, they are also somewhat independent. The ideal case is for perfectly transparent skies that are perfectly steady. This does happen on occasion, but is is very rare. Many times, though, you can get good transparency and poor seeing, or poor transparency and good seeing. For example, on very calm days, the air is steady and seeing is very good. However, those same calm days permit layers of thin clouds to form. The winds that blow away those clouds to make clear skies also create turbulence that makes the seeing poor. It has been my observation here in Texas that in the summertime, good seeing and transparent skies seldom come together. However, the reverse is true in the winter. The absolute best seeing around here occurs a couple days after a massive cold front comes through. The atmosphere is slowly sinking (high pressure), and it does so steadily, without much turbulence. The meteorological conditions do not favor clouds or haze. So, you wind up with crystal clear skies (very high transparency) and very steady skies (very good seeing). Of course, it is also very cold, so you just have to dress accordingly.
-Astroprof
Justin Roberti on September 10, 2007 at 10:30 am: 1
Hi there — thanks for the mention, but have you taken a look at AccuWeather.com’s new Astronomy Center - http://www.accuweather.com/astronomy? It offers nightime viewing conditions, sunset/sunrise times, visibility, wind, and humidity. It also offers an interactive star chart from Imaginova, a video piece, hobbyist corner, and blog with tips.
Justin Roberti on September 10, 2007 at 10:30 am: 2
Hi there — thanks for the mention, but have you taken a look at AccuWeather.com’s new Astronomy Center - http://www.accuweather.com/astronomy? It offers nightime viewing conditions, sunset/sunrise times, visibility, wind, and humidity. It also offers an interactive star chart from Imaginova, a video piece, hobbyist corner, and blog with tips.
Astroprof on September 10, 2007 at 5:17 pm: 3
No, I hadn’t noticed the new Astronomy Center. Cool.
A Ler…-- Rastos de Luz on September 12, 2007 at 9:04 am: 4
[…] Seeing and Transparency no Astroprof’s Page […]
Astroprof’s Page » Clear Sky Clock on October 16, 2007 at 5:58 pm: 5
[…] That is where another tool comes in that I have found. It is called the Clear Sky Clock. At the top of this post, I have placed a graphic produced by the Clear Sky Clock for Fort Worth. It is not as easy to look at and read quickly as the data at Accuweather, but it is a bit more informative. Cloud cover is rated not by simply partly cloudy or partly clear, but by a forecast of the percentage of cloud cover. Now, I am not really convinced that such a forecast can be so exact, so I don’t know that this is any improvement over the Accuweather hourly forecast, and it is tougher to read. But, more importantly for astronomy, it has a forecast for transparency and seeing. Both are rated on five point scales, and that is the detailed forecast that you really want for astronomy. There is also a forecast of the sky darkness. This accounts for moonlight, twilight, known light pollution, etc. The Clear Sky Clock also has a graphic for relative humidity and wind speed, both of which can be found on Accuweather’s hourly forecast. Unfortunately, the Clear Sky Clock does not have Accuweather’s RealFeel, which clues you in on how best to dress for the observing session. Also, the Clear Sky Clock at present only covers North America, so observes elsewhere will have to use other resources. […]