A yard fogger type of insecticide used around the perimeter of the building will be effective for a while. (We use that on our patio out here, and it's pretty effective.)

Another option is something like this - a distance away from your observatory:

Stinger ultra bug zapper

But honestly the most effective thing I've tried, other than coating myself in DEET is Thermacell. It works just exceedingly well.

Links from the equipment forum:

How 'bout them thar Thermacell bug ejectors? (Apologies to Uncle Rod)

Thermacell Question

Thermacell and observatories

Quick Report on Thermacell