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DewBuster™ Controller Technical Bulletin
Moisture Forms Inside Telescope
Any closed-tube telescope will have air trapped inside, and all air has some amount of moisture in it and hence a dewpoint. The air trapped inside will cool as the telescope cools during the night, and when it cools to its dewpoint temperature, moisture will begin to condense onto the interior optical surfaces. The problem is at its worst on cold winter nights because the night air temperature gets much colder than the dewpoint of the air inside the OTA.
How does moisture get inside your telescope? As a general rule, warm air absorbs a great deal of moisture. Removing the caps from your telescope indoors or during the day allows warm air to enter which will "dry up" the interior moisture, but the moisture is still inside, it has simply changed from liquid into water vapor. In the winter, chances are that the daytime air's dewpoint is well above the coldest temperature that will be reached that night. This means that if we uncap the telescope until it is "dry", during the night the air inside will cool to its dewpoint and moisture will begin forming on the interior optical surfaces again.
So how do we stop moisture from forming inside the telescope on those cold winter nights? Instead of trying to dry the telescope during the daytime when the dewpoint is at its highest, do it at night when the dewpoint is at its lowest. If moisture forms on the inside of the OTA at night, remove the eyepiece/diagonal and point a hair dryer toward the rear opening of the telescope so that it blows warm (not hot) air into the telescope. Do not press the hair dryer directly to the eyepiece opening, keep it an inch or two away so that no pressure builds up (a hair dryer will overheat if there is no air flow). On an SCT there are openings around the corrector plate so air will escape there providing air flow. Since the hair dryer warms the air, this will help it to absorb moisture from the inside of the OTA.
For a more elegant solution than a hair dryer, the Lymax SCT Cooler circulates air through the telescope so it can be used early in the evening to cool the scope and late at night to remove any damp air. If you prefer the build-it-yourself approach, take a look at Tom's Scope Cooler or Scott's SCT Cooler .
How do we keep moisture from getting back into the telescope? The best way is to keep the low humidity air trapped inside the telescope by capping up the eyepiece end of the telescope before bringing it in from the cold night air. It is OK to uncap the objective lens or corrector plate, but do not uncap the eyepiece end as you do not want to let the dry air escape from the interior of the telescope.