Tony --
Right, so back to the original question: what eyepiece for Mars? If I remember right, you've got 5.7mm and 20mm eyepieces and a Barlow. (I assume the Barlow is a 2x?)
At f/4.5 (900mm focal length) that gives you:
5.7mm w/Barlow: 315x
5.7mm: 157x
20mm w/Barlow: 90x
20mm: 45x
I'd go ahead and try the 5.7 w/Barlow on Mars each night you're observing. Most nights you'll get a mushy view because that's too much power for the atmospheric conditions (known as "the seeing"), but on the more still nights (when the atmosphere is stable, aka the "seeing is good")you should get some really good views.
I'd also want to have an eyepiece (or eyepiece/Barlow combo) that delivered between 200x and 250x for use on most nights. The seeing will allow those kinds of powers much more often than 315x. Without the Barlow that's going to be a 4mm eyepeice, which will have pretty uncomfortable eye-relief (you'll have to have your eyeball nearly touching the glass). So I'd get something in the 7mm to 9mm range to use with the Barlow.
Cheers,
-- Jeff.
Nikon 18x70s / UA Millennium Colorado:
Solarscope SF70 / TV Pronto / AP400QMD Coronado SolarMax40 DS / Bogen 055+3130
APM MC1610 / Tak FC-125 / AP1200GTO Tak Mewlon 250 / AP600EGTO
