You should try keeping a flip top dry! The moisture condenses on the inside of the open roof and can even rain down on the gear when you close it up. I run a dehumidifier 24/7 (with a tubular heater when it is below freezing), though apart from after I close up, it never seems to be running when I go back to it. The observatory volume is very small though!
http://www.threehillsobservatory.co.uk/astro/observatory/observatory_C11_LHIRES_feb_2010.jpg
