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Why buy a DewBuster™ Controller when cheaper ones will keep dew off?
There's more to it than just preventing dew. As long as the lens is slightly warmer than the air dew can not form on it, but if it is any warmer then the excess heat will warm the air near the lens and cause destructive air currents in the optical path. You bought excellent optics to see sharp highly magnified images of objects in the night sky, yet blindly applying heat to your lens will blur those images and your telescope will never perform at its best. As humans we have difficulty sensing temperature and we can not rely on "feel" to adjust dew heaters, if it feels warm then it is definitely too hot. The only way to be sure we are not heating the lens any more than absolutely necessary is to accurately measure both the telescope temperature and the air temperature and then constantly fine tune the power going to the heater strip so that the telescope is kept just a little warmer than the air temperature. The DewBuster™ Controller does exactly that and it does it automatically leaving you free to enjoy your telescope
Why does dew form at night?
During the day, when warmed by the sun, the air is like a sponge absorbing any moisture it comes into contact with. After sunset the dropping air temperature is like a hand squeezing that sponge and most of the moisture absorbed during the day is deposited as dew. The air must rid itself of this moisture so dew will form, it is just a matter of what surfaces it will form on. Objects which radiate heat will cool faster than the air and dew up first, while objects which are warmer than the air will not dew up.
Why does dew form on my lens or corrector plate?
As described above, dew can form on any object which gets colder than the air temperature. Since the corrector plate is thin, it does not have much mass to store heat and since it is exposed to the night sky it quickly radiates that heat and cools below the air temperature. A dew shield will reduce the area of sky that is robbing heat from the corrector plate, but it will eventually cool below the air temperature and if the air is near the dew point temperature then dew will quickly form.
How can my telescope get colder than the air temperature?
When you first bring a warm telescope outside, the air around it immediately starts cooling it off because heat travels from warm objects (scope) to colder objects (air). The greater the temperature difference the faster it cools, so it may cool 3/4 of the way in the first 5 minutes, but take half an hour to cool the rest of the way. After that, radiant cooling by the night sky (cold object) will continue cooling the telescope (warm object) until a balance is reached where the air (warmer than scope) gives up heat to the telescope to replace what is being lost through radiant cooling by the night sky. The telescope tube will also drain heat away from the corrector plate and radiate it to space. A balance is point is reached with the telescope a few degrees cooler than the air temperature. The balance point is affected by wind (helps warm scope) and sky transparency (increases radiant heat loss).
Why not set the DewBuster™ Controller based upon the dew point?
Regardless of how low the dew point may get, your telescope performs best with the optics at the same temperature as the air around it. For this reason the minimum setting of the DewBuster™ Controller is zero degrees (telescope at same temperature as the outside air) so that the telescope will not get colder than the air. For humid conditions the temperature control knob can be set higher to maintain the telescope warmer than the outside air temperature. This is needed because some heat loss occurs between the telescope tube where the heater strip is located and the corrector plate. So unless the telescope tube is kept warmer than the air temperature, the corrector plate may fall below the air temperature.
Doesn't it waste power warming the corrector when the dew point is well below the air temperature?
A DewBuster™ Controller set a few degrees above 0 only applies a small amount of energy to keep the telescope tube at the air temperature, much less than the lowest setting of most controllers. The small amount of energy used is well worth it to keep the corrector from cooling below the air temperature and risking dew formation. In addition, keeping the telescope near ambient air temperature provides much better performance than waiting for the dew point to be reached and then applying large amounts of heat to warm it up. Also, it is not worth the risk of allowing dew to form because it takes much more energy to dry it up than to prevent it from forming in the first place.
My telescope does not have a metal tube, will the DewBuster™ Controller work with it?
Newer Celestron SCT's use Carbon fiber tubes. While not as thermally conductive as aluminum, they do conduct enough heat to use a DewBuster™ Controller in the normal fashion (heater just behind corrector casting). Plastic however will not work since it is an insulator. Some economy refractors and finderscopes use plastic lens mounts but the telescope tube is metal. In this case the heater strip should be place around the metal tube so that it warms the air within the tube. The lens will be kept warm by the warm air within the tube.
What temperature should the DewBuster™ Controller be set at for my telescope?
Every situation is different so I suggest starting at 1 degree per inch of aperture and try a lower the setting each night until you find the lowest temperature that never allows dew to form. So if you have an 8" telescope set the DewBuster™ Controller to 8 degrees. If you do not experience dew then try 7 degrees the next night, and 6 the next. The ideal setting is the one that works every time, so if 5 works all the time but 4 works most of the time then just use 5 so you never have to worry.
How can the DewBuster™ Controller apply heat without causing tube currents?
See Telescope Cooldown for a full discussion.
Where would be a good place to mount the DewBuster™ Controller?
One popular location on all SCT's is on the optical tube assembly. The advantage is that the heater wires don't have to flex as the scope moves, so the only cord you have to worry about is the DC power cord. The power cord is 6 feet long to allow flexibility in routing. On fork mounted SCT's a popular choice is to mount the DewBuster™ Controller on the fork arm. If you chose this location, run the heater cables through plastic cable wrap to keep them organized and out of the way of the moving parts of the telescope. Radio Shack 278-1654 Split Tubing works well.
