The difference between 1.25" and 2" EPs

[kljucd1]

Hi,

I went to bintel/myastroshop websites to look at eyepieces. I have often looked at the stats for the 2" eyepieces, but, have never known what the difference is between them and the 1.25".

I looked at wiki (http://en.wikipedia.org/wiki/Eyepiece#Barrel_diameter) and they say that it mostly has to do with the focal length of the eyepiece and the FOV. It it suggested that 32mm is about the limit for 1.25", but I have seen longer. For the 2" they suggest that 50mm is about the limit, but once again I have seen longer.

Given the price difference, and practically speaking, what is the difference between the 2" and 1.25" eyepieces?

Thanks for any info :)

Daniel...

[astronut]

Hi Daniel,
The reasons given by Wiki are correct.
I have gone over to 2" e/p's for my 12" Lightbridge.
Eye relief is a major factor, with the smaller barrel size you are able to have large FOV, but at the expense of eye relief.
E/r is the diameter of the lens that you look through.
As you go to smaller focal lengths the e/r gets smaller.
The 2" e/p's don't suffer from this because of their physical size.
All of my Siebert e/p's are 70DFOV and all have 20mm e/r, right down to the 7mm.:thumbsup:

[kljucd1]

Hi,

Thanks astonut.

So, sometimes when I am looking through an eyepiece it seems like my eye is being strained a little, this happens almost exclusively on the 6/9mm eyepieces. Is this as a result of the eye relief?

Daniel...

[astronut]

Correct!!
Most 1 1/4" (not all) 9mm and less in Fl have very small lenses to look through.
This causes the problems you mention.
Now that doesn't mean that you can't get good e/r in 1 1/4 "
The trade off is that they are expensive.