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1.25" vs 2" eyepieces

Started by JMAgey, 09/24/2011 10:00AM
Posted 09/24/2011 10:00AM Opening Post
I am new to astronomy and recently got an excelent deal on a Celestron 11" telescope. It came with a few eyepieces but was looking to buy something better for viewing than what I have. Not that the ones I have are bad by any means but I have been looking at different ones and was wondering if there were any real advantages with a 2" verses the 1.25" eyepieces. I have not had the opportunity to try any other than the 1.25" I already have. Could someone explain the differences to me? Thanks! smile
Posted 09/24/2011 09:23PM | Edited 09/24/2011 10:00PM #1
A 2" barrel allows a wider potential field of view with eyepiece focal lengths longer than about 30 or 32mmm; that is their only *advantage* vs 1.25" barrels. 32mm & shorter focal lengths have similar fields of view regardless of barrel diameter, because the useful fields of view for those focal lengths are fully accomodated by smaller barrels (so a larger barrel size in this case merely fits larger focussers without using an adaptor). Your scope must also be able to deliver a wider cone of light for such longer focal lengths with wider AFOV (which is why actual trial is preferable). In the shorter focal lengths, 1.25" eyepieces are usually less cost vs comparable 2" barrel sizes & there is greater variety of types, focal lengths, & brands to select from. Determine the magnification of any eyepiece/scope combo by dividing eyepiece focal length into scope focal length. Divide the resultant magnification into the scope's primary diameter to get the diameter of the exit pupil of light; more than 5 to 7mm exit pupil is wasted low power as your eyes' night-adapted iris cannot enlarge enough to see it all (this equates to using a smaller telescope as some of the primary aperture is wasted).
Posted 09/27/2011 02:14PM #2
Hi Jack,

I also have an 11” Celestron (Nexstar 11 GPS) and agree with all the posts recommending a low power eyepiece since you will likely use this more than any other eyepiece.

One way to do this is to purchase a long focal length 2” eyepiece like my 30mm Widescan III that gives about 93X with a 0.9 degree field of view in my NS11.

Another option is to purchase an F6.3 reducer/corrector and use a longer focal length 1-1/4” eyepiece like my 20mm Widescan III that gives about 88X with a 0.95 degree field of view with the R/C in place. The R/C will also improve the off-axis image by reducing the field curvature that is inherent in older SCT designs – stars at the edge will appear sharper.

Gook luck, Tom