Is a dew shield important?
A dew heater is a supplement to a dew shield, not a replacement. A dew shield is an important part of any telescope, especially a
Schmidt-Cassegrain. It not only reduces heat loss to the night sky, but also
blocks stray light and improves image contrast. The dew shield should extend at
least one lens diameter in front of the lens to be effective, but even a 1/2
diameter dew shield is better than none . Most refractors have built in dew
shield but if it does not extend at least one lens diameter past the objective
then an extension is recommended. A dew shield does not have to be expensive
either, a home made
Reflectix™ dew shield is easy to make yet very effective. Many dew
shields have insulating qualities and when fitted over the dew heater can
reduce heat loss so that more heat travels to the telescope where it is needed.
What are tube currents and telescope cool down?
When inside of your home, your telescope is at thermal
equilibrium, meaning all of its parts are at the same temperature. When brought outside, the parts begin to "cool down" to the outside air temperature.
Until the telescope reaches thermal equilibrium again, it will not perform at
its best. This happens for several reasons including distortions of the optical
components from uneven temperatures, and air movements (tube currents) within the optical tube
assembly (OTA) due to warm air rising and cold air sinking. Pictured below is a
Schmidt-Cassegrain telescope undergoing "cool down". Tube currents within the
OTA are shown (red for warm and blue for cool air, the more arrows the greater
volume of air movement). This drawing shows "worse case" conditions such
as when the telescope is first brought from inside a warm house to the cold
outside air.
The metal telescope tube rapidly cools down to the outside air temperature because it has a large surface area and conducts heat easily. Air inside the OTA is cooled by the inside surfaces of the telescope tube, and becomes heavier and sinks to the bottom of the tube (blue arrows). The primary mirror is now surrounded by this cold air and gives up heat, warming the air and making it lighter. The warm air rises (red arrows) and warms the corrector plate and prevents dew formation. These "tube currents" help cool the primary mirror and heat the corrector plate, but they also blur the highly magnified images in the telescope.
As the primary mirror's temperature approaches that of the air inside the OTA, the tube currents are reduced (fewer arrows) and the telescope begins to perform well (almost no air currents). Unfortunately the telescope tube does not remain at this ideal temperature but cools even further due to radiation cooling. This causes the air inside the OTA to get colder than the outside air, robbing heat from the corrector plate and increasing the likelihood that dew will form.
What is Radiation Cooling?
The laws of thermodynamics tell us that heat flows naturally from hot to
cold objects. This means that the telescope (warm) gives up its heat to outer space
(cold). A dew shield helps because it reduces the area of night sky that is
robbing heat from the corrector plate. The telescope tube is also affected by
radiation cooling because it has a
large surface area exposed to the night sky and metal is very efficient at radiating
heat (this is why metal objects dew up quickly). While beneficial during
telescope cooldown, it now works against us by cooling the air inside the
telescope to below the temperature of the corrector plate. So the corrector
plate now loses heat to the air inside the telescope as well as to the night
sky. If the corrector plate temperature drops below the dew point then dew rapidly forms.
Why do conventional dew heaters cause problems?
Normal dew heater systems are adjusted by guesswork. Experience
soon teaches what power setting prevents dew under any conditions, but
this "worst case" setting applies too much heat 90% of the time. The drawing below
illustrates this situation.
In the above drawing a heater strip and dew shield have been installed, both good for dew prevention. The problem is that too much heat is being applied which causes image degrading air currents in front of the corrector plate. The telescope will perform poorly and the owner will blame it on telescope cooldown or poor seeing conditions. Since the dew shield is open to the air, the excess heat escapes as rising warm air, causing air currents and degrading telescope performance. To correct this problem, the heat must be controlled so that the corrector plate is only slightly warmer than the air.
Radiation cooling by the night sky further complicates the above situation. The telescope tube is usually dark-colored and has a large surface area exposed to the sky, which promotes heat loss and the tube gets colder than the outside air. Furthermore, the thin metal tube is very efficient at removing heat from the air inside the telescope which cools the air within the telescope to below the temperature of the corrector plate. This means that not only does the corrector plate lose heat to the sky through radiation cooling, but it also loses heat to the air inside the telescope.
Another problem is that the heater strip is placed around the corrector plate casting. While this would seem to be the best location, in actuality most of the heat is wasted warming the air inside the dew shield which then escapes into the atmosphere (warm air rises) only to be replaced by an endless supply of cold air. Some heat does enter the corrector plate, but since it is made of thin glass (an insulator) and has a large surface area, most of the heat never conducts to the center of the corrector plate but instead escapes upward warming the air in the dew shield. Another problem with this heater location is that none of the heat warms the air inside the telescope tube to counteract radiation cooling, so the corrector plate has cold air on both sides of it and the center of corrector gets cold and dews up first. To clear the dew, the telescope owner generally cranks up the heat creating more air currents (lots of red and blue arrows in the drawing above). The result is dew free optics at the expense of telescope performance.
The above situation can be improved by moving the dew heater back so that it is around the telescope tube (see photo below), and reducing heat to the minimum needed to keep the corrector plate slightly warmer than the air.
How does the DewBuster™ controller prevent tube currents and overheating?
In the drawing below, we have a telescope fitted with a properly positioned heater strip being thermostatically controlled by a DewBuster™ controller. The DewBuster™ controller is not heating up the telescope, it applies just enough heat energy to replace what is lost to the night sky by radiation cooling and keep the telescope at the air temperature.
The DewBuster™ controller measures the telescope and air temperatures and applies only enough heat energy to maintain the telescope tube warmer than the outside air temperature by a user selectable amount (0 to 20 degrees F.). The telescope tube then gives up heat energy to the air within the telescope tube keeping it warmer than the outside air yet cooler than the telescope tube. The large surface area of the corrector plate allows it to effectively gain heat from the air inside the tube and thus counteracts radiation cooling by the night sky. Compared to the previous drawings, there are fewer arrows because there are smaller temperature differences and thus much less air movement. The warmest part of the telescope will be the telescope tube underneath the heater strip (red arrows) but the heater strip will not even feel warm because it is temperature controlled. The violet arrows are heat energy conducting down the telescope tube to prevent radiation cooling from chilling the tube colder than the outside air. This helps stabilizes the air temperature within the tube so that the primary mirror can reach temperature equilibrium (violet arrow from primary indicates very little heat coming off primary).
The above photo shows the heater strip correctly placed around the telescope tube, not the corrector plate casting. The Telescope Temperature Sensor must be placed under the heater strip and in good contact with the tube in order to accurately measure the temperature. The Air Temperature Sensor may be routed into the dew shield or left outside, whichever is most convenient. Efficiency can also be improved by wrapping an insulating material around the outside of the heater strip to reduce heat loss and force more of the heat into the telescope. The dew shield is not shown in this photo, but it is very important to use a dew shield. A home made Reflectix™ dew shield is very easy to make but very effective.
