This is an example of the dewing behaviour of my 10" LX200 at the onset of dew formation. I use a resistive heating tape to control dew, it is held in place by a soft elastic band, and in this case has come loose on the low edge of the OTA - possibly part of the reason it dewed up. Sometimes the conditions allow a small amount of dew to form around the central obstruction. I took this picture to capture the striking interference colours that the thin film of very small water droplets had formed. It was taken about an hour before dawn and the grass was dripping wet.
I suspect heat loss from the central obstruction may be a factor. Chris Heapy suggested that the transfer of heat from the dew heater to the front corrector plate may be mainly due to heating the air inside the OTA and the warm air heating it from behind. This is because the front plate is isolated from the metal cell by a combination of plastic and cork spacers/shock absorbers. The pattern of dewing suggests he is probably right since it is least pronounced on the upper side of the OTA where the warmest air inside the tube will be located. Looking anew at this picture I will definitely try an aluminium foil radiation shield on the central obstruction to see if it cuts down the amount of current needed to keep the dew from forming.
