Hello Nick,
thinking about this again, I do remember a bit more condensation in the first year after installation of the observatory. I suspected that some moisture was coming out of the concrete base upon which the dome sits. I gave it several coats of concrete sealant paint which stopped that – I put another layer down every couple of years, just to be sure (and to give it a nice aesthetic finish!). I also made sure the join between the bottom fibreglass flange and the concrete base is properly sealed with exterior grade door sealant. I renew this periodically, again just to be sure.
All the best,
Jeremy