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). These fans are sold at electronic supply shops. You can easily suspend a computer muffin fan behind the mirror on rubber bands from hooks in the tube walls. Installing a fan behind a reflector's mirror has become a popular way to speed cooling and blow out mixed-temperature air. Note that they're slightly offset to the rear of the tube to help ensure that the flowing air 'scrubs' the mirror before leaving. Right: Opposite the fan, Adler put exhaust holes that allow warm air to exit the tube. 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. 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. Amateurs today agree that any open-ended tube should be ventilated as well as possible. Reflectors are notorious for tube currents, but closed-tube Schmidt-Cassegrains and refractors can get them too. "Tube currents" of warm and cool air in a telescope are real performance killers. In this case gentle heat not only prevents dew but also keeps the scope closer to the air temperature - thus sharpening its resolution. Whenever a telescope begins to collect dew or frost, you know that it has grown colder than the air, thanks to radiational cooling. Usually the telescope is too warm, especially if it is stored indoors. The full cool-down time for a large, heavy instrument may be much longer. Amateurs soon learn that the view sharpens within about a half hour after bringing a telescope outdoors. 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. 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. ![]() 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.Ītmospheric seeing problems often are at their worst a fraction of an inch from your telescope's objective lens or mirror. ![]() Much of the "atmospheric seeing" problem, however, arises surprisingly close to the telescope, where you can take steps to reduce it. ![]() 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. 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. 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. The air's light-bending power, or refractive index, depends on its density and therefore its temperature. Such a change results from a temperature difference of just 0.2° Celsius.Īdd 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. But if the refractive power of the air down one part of the telescope tube differs from the rest by more than just one part in 1,600, the ¼-wave tolerance will be breached. In an ideal world the air would affect every part of a light wave equally. Clearly the air is an important optical element. But that same light wave, in traversing just three feet of air inside a telescope tube, is retarded by about 400 wavelengths compared to where it would be if the telescope contained a vacuum. 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. Alan Adler took these pictures during two minutes with his 8-inch Newtonian reflector. 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.
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