I'm a little late to this party, but have some useful information. The scope that I'm working with is a Meade LX-90. Dew. 1. Above freezing, dew will form on the scope as water, it'll be misty, then droplets will form is allowed. 2. Below freezing, Frost will form, if left this will become a layer of ice. If you want to experiment and see the effects, just look at car windscreens. This is exactly the same thing that happens. How to clear it. There are bascially 3 ways. 1. Hair dryer - I'm not kidding, use warm air to gently evaporate the moister. pro's Can get rid of dew once formed. con's Mains hairdryer = 240v in a outdoors! Heat could damage the optics. 12v hair dryer = pulls lots of power, lower temperature, so less chance of heat damage. Will leave hotspots all over your optics, so they'll need to cool again to get a stable image. (not good for photography) 2. Dew shield Pro's No power required Con's cannot sort out dew if already formed on the optics. They slow the onset of dew forming, but a long time. This alone can be enough for some observing sessions. 3. Dew heater. Pro's actively heats the options, or the air in front to prevent the heat radiating away con's needs a power supply I made my own dew heater several years ago, here's version 1 http://astronomy.cjdawson.com/projects/DewHeater.html and here's version 2 http://astronomy.cjdawson.com/projects/PowerBox.html#NewHeatingElement When I first made my heater, I did the maths needed and found that "at full power" the heating element could draw 2.67amps at 12v. Whilst this sounds like alot of power, this happens when the heater is running at 100% power output. This is actually enough to clear frost from the optics. That's alot of power available. In reality I have found that I run the heater between 10% and 20%, This works out that at 20% power 0.534 Amps will be drawn, or 534mA. No one has said that you have to only have one of these options. There's no problem with having all three options available to you - this is what I do. Bascially I put a dew shield on my scope for normal use. Then have the heater, running at 10% to keep the dew away, and finally, can keep a 12v hair dryer in my kit for that one time when the heater is set too low, and didn't prevent the due.