[AstralTraveller (David)]

A mate has asked me to help him chose a top-notch set of eps to go with his Obsession 18" f/4.2 (ultra compact) with servocat. Not having much experience with large scopes I'm looking for a bit of advice.

I think I have the low-power end of the DSO eps covered. I'm suggesting a 31 nagler and 21 and 13 ethos, though I notice that some people would leave out the 21. The 13 gives 147x and 41' fov which I'm sure will get a lot of use but I'm wondering whether a higher power would get much use for DSOs? The 10E (191x and 31') is probably a bit close to the 13 so it might be the 8E (238x and 25') but how much use would it get? How often will the seeing allow this sort of power?

The other consideration is planetary eps. I love my orthos and my 10mm Zeiss Jena ortho is better on-axis than my 9mm T6. I assume the orthos would also be better than the ethos on-axis but I like to hear from anyone who has made the comparison. I'd also like to know what is the highest power it is likely to be practical to use. The rule-of-thumb of 50x per inch gives 900x but of course the seeing will never support that. What would be a reasonable upper limit?

Of course the orthos biggest drawback is the short eye relief. Both my mate and I are old-school (for a good reason) but even I find my 5mm ortho difficult to use. If eye relief becomes an issue what would be a good alternative to the ortho, a TMB perhaps?

Thanks for any comments!