What do you think?

Going to make a decision between 17t4 or 20t5 for 10" f5 dob low end ep. With my eyes and backyard light pollution 27 Pan was a little to bright so I want to stay around 4mm for exit pupil size. Once tried 13t6 and found the er a bit short, but perhaps the 20t5 with an 18mm concave eye lens maybe Ok, but don't know. Cost and weight are not an issue. If some of the "been there, done that" people would commit it would be appreciated.
Thanks

I would go with the 17mm for the ER and the comfort that comes with it. Another good reason is that it puts you right in the wheelhouse on a good mid power EP. Good luck with whatever you decide.