Dave7a

Thanks Michael, Yes my planned scope does only have a 1.25" focuser, so it all makes sense now.

First of all, thanks for a fantastic post! Lots of answers to questions I was planning to ask and many I hadn't thought of.

I'm just a little confused about the difference in suggested EPS for f/8 and f/5. Why is the lowest power EP for an f/5 FIVE times the focal ratio yet only THREE times the focal ratio for an f/8?

I'm planning on buying a 150mm f/8 Dob for my 11 yr old son who is showing a great interest in all things astronomical from the moon and planets to galaxies and nebulae. I had planned on buying some extra EPS in addition to the 10mm and 24mm that are supplied in order to cover all possible viewing magnifications. I worked out that a 6mm for high power and a 40mm for low power (with 5mm exit pupil for me) would do nicely, but it would seem I am missing something??

Can anyone explain why I can't (or maybe shouldn't?) go to 40mm with this f/8?