5 or 7,5 mm?

I'm considering to buy an Ultima ep but I don't know which one to choose. I have a 10 inch f4,8 reflector. I want to be able to see details on Mars and Jupiter. Right now my highest power ep yields 120x.

What would you recommend as a first planetary ep, the 5 (240x) or the 7,5mm (160x)?

Is the 5mm useful for DSOs other than PNs?

What is the minimum useful magnification for seeing detail beyond the red spot and shadow transits on Jupiter?

I don't know much about the seeing conditions in my area as I'm not very expirienced yet but at least one night I have estimated the seeing to around 7 (could have been 6) on the pickering scale according to this link that also has been posted elsewhere.

http://uk.geocities.com/dpeach_78/pickering.htm

Any help will be appreciated.

Nicolas.

Provided your 10mm ep is of good quality, why not get a quality barlow (TV 2x, Ultima 2x) to get you up to 240x with better eye relief than the 5mm ultima. If you don't go the barlow route you would probably get more use out of the 7.5mm, but at only 160x you will want to be in the 220x-250x range for those upclose looks.
Good luck,
Jason

I don't know your budget, but I'd get a good 15mm eyepiece and a good 2x barlow. Then you'll have both sets of magnification you mention (the 15mm and your 10mm will give you the equivalent of 7.5mm and 5mm eyepieces), without suffering from the short eye relief of sub-10mm eyepieces.

What, you thought this hobby was cheap?

If eye relief is not an issue for you (or budget is), and you want just one of those eyepieces, I'd go for the 5mm for primarily planetary use, the 7.5 if you want something a little more general purpose. Image scale at 160x for planets is pretty good, but the 240x should be pretty darn perfect for planetary in a 10-inch (barring poor local seeing conditions).

Re DSO's and 240x: The 5mm might be good for smaller globulars as well as planetaries; also tight double stars.

I concur with the "get thee a Barlow" (&/or Powermate) advice.

The best 5mm, 4mm, and ~3.3mm eyepieces I've ever used all were "born" from a single 10mm Clave. Sure, I probably have an exceptional older one, but the fact remains -- whether used alone or coupled with my Ultima Barlow &/or my 2.5X Powermate, the Clave never ceases to make me grin.

A very good 10-to-15mm Abbe, Masuyama, Monocentric, or Plossl (or even a UO Konig), when coupled with a very good magnifier, will do an excellent job in the ~3-to-7.5mm range.

Best wishes and luck.
-Dan

I would echo previous sentiments - a barlowed 15mm and your current 10mm will give you greater versatility. However . . .

If you do choose to go the separate eyepiece route, I highly recommend the 7.5mm Ultima. It is an exceptional planetary performer and one of my favorites. If you want to add the 5mm, skip the Ultima. It's eye relief is extremely tight and I have experienced ghosting on it. Instead, I would recommend the Orion and Antares 5mm versions that have the built-in barlow. A 5mm with 10mm eye relief, and the images are just as stunning as in the rest of the Ultima line.

Either way, invest a little more and you won't be sorry.

- Peter