Posted 14 July 2013 - 01:22 AM
To prevent future occurrences, you need to think in terms of moisture content (absolute humidity) in the air in the various environments the scope finds itself in, like indoors, garage, shed, outside, etc. And how the temperature changes will then affect relative humidity.
One good example. Taking a chilled scope indoors when the house dew point is higher than the scope's temperature results in instant condensation if the scope is not protected.
If the scope is put back in its case, but that case has air within which has a high dew point, same problem. The case should be first brought outside so that its air can be exchanged.
Furthermore, be most careful about moist air inside the scope when taken out on cooler nights. For then condensation on the inside is quite possible! Remember, it's near impossible to fully seal a scope, and diffusion will always be striving to equalize the partial pressures of the gasses of the atmospheres inside and out. Over time, moister summer conditions can lead to a higher water vapour content inside. If this is a concern, occasionally 'flush' the air with that known to be drier. This is easily done outside before observing on a cooler evening when the dew point is lower than it has been for a while. Just open up the back end for a half hour or so; this will probably not result in a complete exchange, but it's a good start. You could expedite things with some means of blowing air in. (Naturally, for SCTs employing corrective lenses inside the rear baffle, this is not possible. But if fans are installed in the rear cell, they should achieve the same thing.)
There are other aspects to be aware of, but this covers the important stuff. Always be aware of moisture content, and understand the concepts of relative and absolute humidity. (Hint: Warm air at low relative humidity can contain more water vapour than cold air at high relative humidity.)