Thanks for the thoughtful reply Patrick...
Yes, it's a 1200mm focal length dob..and maybe I should clarify a bit - when I said DSO I meant globs, open clusters, nebulae etc as well as galaxies....
It's a very good point about the TFOV, and now I'm wondering exactly how wide a TFOV I need....I was thinking the 24 Panoptic would cover all the wide field needs.
I chose the 16mm T5 because I read in a post (from 2005!) by Dave47Tuc (#15
http://www.iceinspace.com.au/forum/s...ead.php?t=1209) that 2-2.5mm exit pupil is desirable for best contrast and brightness in DSO. The 16mm T5 would give me an exit pupil of 2.7mm...But your point about eye relief is something that I had considered as well, and maybe I should think a bit more about this one....The 17mm does sound very good....
Another point I was considering was that with my 2x telextender (1.25") the 16 becomes an 8, but this might be a bit unnecessary with the Pentax 10mm.....
hmmm, food for thought....