Hi Nick,
For many years I used to run two small 60w tubular heaters located close to the mount and telescope which was covered with a large cotton sheet “tent”. The heaters where controlled by a time switch that put them on for a few hours generally before dawn when the equipment was at its coldest. This worked well in most situations. However in the late autumn when weather conditions can change rapidly between near freezing temperatures to a mild south westerly maritime airstream I still occasionally suffered condensation problems.
Last summer I reduced the ventilation in the dome and installed a desiccant type dehumidifier which I run manually for a couple of hours on occasions when I think the conditions are likely to produce condensation or after I’ve shut the observatory up following an observing run.
Since installing the dehumidifier I’ve not noticed any condensation problems even during weather conditions that would have previously been an issue.
It’s important to purchase a desiccant based dehumidifier as the refrigerant types do not work at cold temperatures. The dehumidifier need not be large and can easily be purchased from most domestic appliance suppliers.
Peter