Star trails - Any solutions for keeping a lens warm?

Dear willo78

Warm air holds more moisture than cooler air. Dewing occurs when the the nocturnal fall in temperature makes the relative humidity of the air reach 100%, so that for a given weather/climate related amount of moisture in the air, there is a temperature (known as the "dew point") below which moisture will start to condense from the air onto any object whose own temperature is less than the air temperature.

Since metal is an efficent radiator of heat, it can actually happen that your camera/telescope gets colder than the surrounding atmosphere, and becomes a "dew magnet" as it were (this is one reason why car roofs are among the first things to dew up at night). The most efficient line of defence against dew for a long shoot such as star trailing, is to find a way of slightly warming the camera/lens so that it remains a few degrees above the temperature of the surrounding air. I know that there are such warmers made for telescopes, but I don't know much about them, never having used one.

Amateur astronomers with small telescopes and limited means have been fighting dewy nights for a very long time (I am one of them). Short of obtainiing or devising some warming apparatus, the simplest line of defence is a "dewshield" which is a cylindrical tube a bit wider than the lens and as long as it can be made without vignetting (shadowing) the lens's field of view. For a 10 cm diameter lens, the dewshield might be about 12 cm wide and 20 cm deep (the general rule is the deeper, the better). For a wide angle lens such as yours, a slightly conical dewshield might be the way to go. Telescope dewshields are usually made of metal, but I have seen plastic dewshields used on larger telescopes. They seem to work by "drawing off" some of the atmospheric moisture that would settle on the lens, and possibly also by slowing down the radiation of heat from the surface of the lens itself. In my experience using a 10 cm telescope, the dewshield lengthens the time that elapses before the lens starts to dew, but in very dewy conditions the lens will eventually start to dew up anyway. I have also found that viewing from underneath a roof such as an open-fronted shed (provided you are not trying to photgraph towards the zenith) can also lessen dew, probably again because the shed roof draws off some of the condensation and prevents heat radiation from ground to sky. Telescope observatories serve this purpose, as well as sheltering from wind.

P.S. Bringing a cold (but still dry) camera indoors after a night shoot, it is a good idea to seal the camera in a dry polythene bag before entering the house, and letting it warm up to room temperature inside the bag. This prevents the camera getting a dose of "indoor dew".

I hope these comments may be of some help - others may give you details on methods of warming the camera/lens.