how to read eyepiece ads

I want to buy a 2" eyepiece for my scope (first time for that for me). I don't really see anyone saying whether their eyepieces are 2" in the classifieds. Does one assume they are 1.25" unless otherwise stated?
Second question:
THis is for a 10-inch dob. Since a 2" gives a wider field of view, would you buy a more powerful 2" eyepiece, like a 9mm or 11mm, if you can only afford one 2" eyepiece right now???
Thanks.
Bob

Hi Bob,

You know, I never thought of it, but I suppose most people don't specify the barrel diameter in eyepiece ads because they assume that potential buyers are familiar enough with eyepiece types to sort of automatically know whether the barrel is 2" or not.

What you can do if you see an EP ad and can't tell what size the barrel is, is look up that EP on the Anacortes webite (or elsewhere if it isn't available there). Usually websites like that are more informative than the classified ads.

Also: more powerful eyepieces, meaning higher magnification / shorter focal lengths, simply don't require 2" barrels. It would be very unusual for a manufacturer to use a 2" barrel size if it weren't necessary. Only fairly low power EP's have any reason to use a 2" barrel.

In other words, the 2" barrel doesn't give the eyepiece a wider field of view, the design of the eyepiece does, and a given design may or may not require the 2" barrel. Does that make sense? If not let me know and I can possibly clarify.

Mike

The purpose of a 2 inch eyepice is to give a wide field of view, without vignetting from the 1.25 inch barrel. This usually means that 2'' becomes important around 25 to 30mm of focal length, depending on the apparent field of view. You are thus likely to want one or maybe two 2 inch eyepieces. Such as a 35mm Panoptic and a 20 mm Nagler (I happen to have a 22 Panoptic which is set up as a two inch, but need not be, and a 36 mm Siebert Observatory grade, which definitely needs to be 2 inch).

Next, the focal length of the eyepiece divided by the focal ratio of your telescope gives the exit pupil of the eyepiece. Exit pupils larger than 5 mm for most people begin to functionally reduce the acting aperture of the telescope. 7mm is considered the useful limit. (I have a 36 mm that give me an exit pupil of about 6.2 mm and it works just fine, especially for finding. It also gives me 2 degrees of true field which is very useful, and breathtaking on extended objects such as the Veil or Pelican Nebulae, or eight galaxies in one field frpm the Virgo cluster. But I wouldn't go above a 7mm exit pupil in a telescope (especially a reflector).