What phenomena are working here? Light has two kinds of properties of electromagnetic waves and particles. The phenomena we have just mentioned result from diffraction, which is one of light's characteristics as a wave.
Diffraction occurs when light hits and bends around an object. In a bathroom, light hits steam particles, which cause it to bend around and fade. Diffraction occurs when light hits an obstacle, and then bends around the edge of the object. The light bends as shown in Figure 1. Figure 2 shows another light diffraction example through a slit in the board. Light is also a wave, so if it goes through a narrow slit or small hole, it changes from a plane wave to a spherical one.
This transformation is the same as with waves of water. Diffraction describes that change in light's direction. Picture and Sky Q menus are too big or small for the TV screen. My picture is dull or flickers after watching in HDR. I've got a picture problem when watching recordings. In addition, the artist uses a clearly bluish tint on some of the distant peaks. In many ways these effects mimic those seen in the photograph of the Grand Canyon above.
The atmosphere scatters light. That is what you have probably heard causes the sky to be blue. Well, it is true, but the fact that the atmosphere scatters light also plays a role in our depth perception. Heat waves seem to shimmer across the Moon. Close double stars that your telescope ought to resolve look single. These photos show the double star Zeta Aquarii which has a separation of 2 arcseconds being messed up by atmospheric seeing, which varies from moment to moment.
Alan Adler took these pictures during two minutes with his 8-inch Newtonian reflector. It's not hard to understand why. The usual definition of an optically "good" telescope is one that keeps all parts of a light wave entering it nicely squared up to within quarter-wavelength accuracy by the time the wave comes to focus.
But that same light wave, in traversing just three feet of air inside a telescope tube, is retarded by about wavelengths compared to where it would be if the telescope contained a vacuum. Clearly the air is an important optical element. In an ideal world the air would affect every part of a light wave equally. Such a change results from a temperature difference of just 0.
Add the miles of air that the light wave traverses before it even gets to the telescope, and it's a wonder that we can see any detail at all on objects above our atmosphere. The air's light-bending power, or refractive index , depends on its density and therefore its temperature. Wherever air masses with different temperatures meet, the boundary layer between them breaks up into swirling ripples and eddies that act as weak, irregular lenses.
You can see this where hot air from a fire or a sunbaked road mixes with cooler air above; those ordinary heat waves are astronomers' poor seeing writ large. Our windy, weather-ridden atmosphere is almost always full of slight temperature irregularities, and when you look through a telescope you see their effect magnified. Much of the "atmospheric seeing" problem, however, arises surprisingly close to the telescope, where you can take steps to reduce it.
In the past, tube currents warm air rising up the length of a telescope tube were thought to be the principal thermal problem in reflectors, but it now seems clear that the 'boundary layer' of warm air directly in front of the primary mirror is the chief culprit. Atmospheric seeing problems often are at their worst a fraction of an inch from your telescope's objective lens or mirror.
If the objective is not at air temperature, it will surround itself with a wavy, irregular, slowly shifting envelope of air slightly warmer or cooler than the ambient night. So will every other telescope part. Therefore, one of the most important ways to "beat the atmospheric seeing" is to give your telescope time to come to equilibrium with its surroundings.
Amateurs soon learn that the view sharpens within about a half hour after bringing a telescope outdoors. The full cool-down time for a large, heavy instrument may be much longer. It pays to set up early. Usually the telescope is too warm, especially if it is stored indoors.
But sometimes the opposite happens. Whenever a telescope begins to collect dew or frost, you know that it has grown colder than the air, thanks to radiational cooling. In this case gentle heat not only prevents dew but also keeps the scope closer to the air temperature — thus sharpening its resolution. Reflectors are notorious for tube currents, but closed-tube Schmidt-Cassegrains and refractors can get them too. Amateurs today agree that any open-ended tube should be ventilated as well as possible.
This means designing lots of open space around a reflector's mirror cell, keeping cell itself light and airy, and keeping the tube walls at least an inch away from the optical path. Left: Cooling fans are traditionally mounted behind a reflector's primary mirror, but inventor Alan Adler has shown that you can break up heat waves better by placing the fan in the tube's side so it blows across the mirror's face. Right: Opposite the fan, Adler put exhaust holes that allow warm air to exit the tube.
Note that they're slightly offset to the rear of the tube to help ensure that the flowing air 'scrubs' the mirror before leaving.
Installing a fan behind a reflector's mirror has become a popular way to speed cooling and blow out mixed-temperature air. You can easily suspend a computer muffin fan behind the mirror on rubber bands from hooks in the tube walls. The bigger the fan the better. These fans are sold at electronic supply shops. Some owners of Newtonian reflectors have improved image quality with fans built into the side of the tube just in front of the primary mirror as shown above. Mounted this way, a fan can blow air across the mirror's front surface directly.
For decades, the conventional wisdom in the amateur world was that reflectors cannot be as optically excellent as refractors. Many books say an 8-inch reflector equals a 5-inch refractor, at least for image sharpness.
The discovery of the wonders of fans in reflectors has gone a long way to ending this disparity! Do tube currents trouble your images? It's easy to check. Turn a very bright star far out of focus until it's a big, uniform disk of light. Tube currents will show themselves as thin lines of light and shadow slowly looping and curling across the bright disk.
Turn on a fan; the lines quickly swirl, break up, and almost disappear.
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