Paolo has posted some astonishing images -- his gear clearly works very well.
I recall a fairly old exchange of ideas between Paolo and Wolfgang Rohr on optics measurements that might possibly be useful reading - should be able to find it with Google.
I would think that such an open structure might have dew issues, but have no direct experience to support that opinion.
edit: here is the Rohr correspondence - no idea if it still means anything.
http://astro-foren.de/index.php/Thre...%80%93-part-I/