2" Eyepiece size range

[MarkVIIIMarc]

Looking about I noticed the range for 2" eyepieces seems to be 19mm to about 80mm.   Is there a point to having a 2" eyepiece smaller than 19mm or is any field of view advantage given by the 2" size negated by the lens size?

Thanks

[Brasspoodle]

Barrel sizes, as far as I understand it, are related to the focal length and the Apparent Field of View of the eyepiece. You need a larger sized lens (or grouping of lenses) to have a wide field once you hit a certain focal length (for 82° FoVs its around the 18mm mark you mentioned). Below that you don't need a 2" barrel. Explore Scientific offer 2" Barrels all the way down to 5.5mm in their 100° eyepieces but not, as I understand it, for any visual benefit. Rather this means you don't have to use a 1.25" adapter using them and they have some extra stability, which is useful given their size and weight. 

[Dave In Vermont]

Seems to me to be the wide-field experience that draws folks to 2" EP's. There's a guy in Arizona I've corrosponded with who custom-builds eyepieces who knows a great deal about such things. Lives out in the middle of nowhere - lucky devil! He offers an 18mm Konig in 2." Here's a link:

http://stores.ebay.com/Russell-Optics

Enjoy -

Dave

[Louis D]

It all has to do with the maximum field stop you can fit inside, or just above, a 1.25" barrel.  It's right about 27mm inside the barrel, slightly more but vignetted if above (as in the 35mm Celestron Ultrascopic).  This translates to a 32mm 50 degree eyepiece, a 24mm 68 degree eyepiece, an 18mm 82 degree eyepiece, a 13mm/14mm 100 degree eyepiece, and a 140 degree 9mm eyepiece.  Thus, to go wider than each of these in a similar focal length requires moving up to a 2 inch barrel.  Sometimes, the particular design requires a larger barrel to accommodate the large field lens as in the 17mm Nagler T4 or the 14m ES-100.  Sometimes, it's the sheer weight and bulk of an eyepiece that necessitates the larger barrel, as in the case of the 9mm ES-120.  My 140 degree data point comes from reports from the owners of the first 10 pieces of that eyepiece that were missing the field stop.  They reported that the true field of view was just about equal to the 13mm Ethos which is longest 100 degree eyepiece in a 1.25" barrel.

[MarkVIIIMarc]

1 hour ago, Dave In Vermont said:

Seems to me to be the wide-field experience that draws folks to 2" EP's. There's a guy in Arizona I've corrosponded with who custom-builds eyepieces who knows a great deal about such things. Lives out in the middle of nowhere - lucky devil! He offers an 18mm Konig in 2." Here's a link:

http://stores.ebay.com/Russell-Optics

Enjoy -

Dave

I saw that on ebay just a bit ago!  He had a few offerings.  Arizona is a good place to see the stars if you can get away from the city lights.

Even with my limited experience I will say my Z10's factory 2" 30mm eyepiece is very comfortable to use.