Posts Made By: Edward Tuinstra

If the XO-2.5 is anything like the XO-5, it's your best choice given your stated criteria. I have the XO-5 and have corresponded with three other owners. All of us were impressed by how sharp this little gem is in comparison to some of the best 4-6mm planetary eyepieces available.

Hi Kent,

This was sold by GOTO Optical Mfg of japan:
http://www.goto.co.jp/index-e.html

They mostly do Planetariums now but they had a full line of scopes, mounts and eyepieces.

"Do people still use this size?"

Sure. There are a lot of opticly excellent eyepieces in this size and its no problem with 1.25" adapters. Like other eyepieces and filters, if they're junk you can't give them away and if they're excellent they will command a high price. Some Zeiss and Pentax eyepieces are worth hundreds of dollars each. There are also valuable Takahashi and Carton .965" eyepieces, among others.

You might also want to consider the Vixen Custom-D. Theyre no longer made but they come up for sale here from time to time.

I think the investment in equipment will be largely for testing equipment, not product to be reviewed. The parties involved are committed to bringing a new level of objectivity to product review. You can't do that on a shoe-string budget.

Hmmm...I'd need a 2", maximum field: Pentax XW 40mm. High power planetary: Pentax XO 5mm. Mid power: Nagler 13mm T6.

Search the UncensoredTakGroup on Yahoo, there is a huge archive about these issues.

It would certainly be interesting! I think there's enough traffic to keep one going...Herb? 8O

Kinda makes those used T6's at 10% off look a little weak, huh?

"A couple of my points of indecision come from not knowing where the best advantages of the 3/4" gain show up. Field of view? Brightness?"

Field of view. As you go from short focal length eyepieces to longer focal lengths you see more and more sky in your eyepiece until finally the amount of sky the eyepiece can deliver to your eye is limited by the barrel diameter of the eyepiece. With 1.25" plossl this begins to happen at an eyepiece focal length around 32mm and higher. You'll notice that 35-40mm plossls with a 1.25" barrel have a smaller apparent field of view for this reason (same amount of sky shown at a lower magnification). Also, the wider the apparent field of view of an eyepiece design, the lower the eyepiece focal length where this begins to happen.

The 2" barrel will let you see more sky if you choose a focal length/field-size combination that would be barrel limited in a 1.25" barrel. In practice, this means plossls over 32mm, wide fields over ~25mm and Naglers over 16mm. I'd go for the gold and get an eyepiece that shows the widest patch of sky possible in a 2" eyepiece, like a 55mm plossl or a 40mm University Optics MK-70. Its really going to help you view extended objects in such a long focal length scope. Try out one of the eyepiece calculators online and see for yourself.