[Wavytone]

1. What’s the scope ? SCT/refractor/newtonian ?
2. Where did you set up ?

Things to note:

A. Dew should not be an issue if there is a breeze so no need for dewcaps etc if there is one.

B. The local microclimate around you has a lot to do with dew - setting up on lawns, ovals or anything else with grass and black soil that has been watered is going to have dew. Conversely light sandy soils don’t hold water and even better sandstone outcrops... won’t have dew on these. Ditto tarred or concrete pavements.

Simply picking the scope up and moving a few metres to a different surface can make a significant difference.

C. Colour of the OTA affects the rate of heat loss - black is the worst.

D. If the scope has a closed tube (SCT or maksutov) consider applying an insulated dewcap that extends back over the OTA and/or insulation around the OTA to retain whatever heat it has acquired earlier. This has proven very effective against both dew and the internal tube current these scopes often develop, despite being contrary to the mantra of “must cool the scope”. OTOH if the scope is an open-tube type (newtonian or Cassegrain) this won’t have any effect and you’re stuck with using fans.

E. Install a small fan to blow air across the surface you want to keep dry. It works.

F. Add a heater strap around the OTA, particularly effective if this is underneath some insulation (eg EVA foam or yoga mat). Which brings us to...

G. Don’t cool the scope using forced ventilation (fans).

H. Use a handheld warm hair dryer to evaporate the dew and warm up the optics a tad. There are portable ones that work off 12VDC.

I. Any combination of the above.

Basically don’t let the scope cool in the first place, insulate the OTA and use a dewcap. If however dew does form a blower is probably the best way to remove it.