Jump to content

2" lenses vs 1.25" lenses


Recommended Posts

Hi ...

I have a 80ED refractor which takes the usual 2" accessories ... although comes supplied with 1.25" lenses.

What are the benefits of 2" lenses and barlow lenses etc over the small and often cheaper 1.25" accessories ...

Thanks :)

Link to comment
Share on other sites

Wider true field of view. The true field of view is equal to eyepiece field stop diameter ÷ telescope focal length x 57.3

So 2" eyepieces have a bigger field stop which means a greater true field.

Link to comment
Share on other sites

What umadog said. Also being more solid I've heard is useful for "photovisual" eyepieces when you'll be hanging a camera off the end of them. There's less need for the 2 inch format at short focal lengths.

(As a side note, thanks for that formula umadog! My small scope has a .965 inch focusser and I've long wondered what the lowest useful magnification would be, if I got a hybrid diagonal and some new EPs say. Now I've worked it out - for a Plossl something like a 25 mm would give 28x magnification and a 2 degree true field.)

Link to comment
Share on other sites

A 2" eyepiece has no advantage optically at focal lengths of less than about 17mm as the maximum field stop needed is less than 1.25". It's only when you get to the longer focal lengths that you can take advantage of a larger field stop. In fact at less than about 17mm a 2" eyepiece is a bit of a disadvantage as you are paying for larger glass lens elements whereas with a 1.25" eyepiece you can get higher quality glass for the same money.

John

Link to comment
Share on other sites

A 2" eyepiece has no advantage optically at focal lengths of less than about 17mm as the maximum field stop needed is less than 1.25". It's only when you get to the longer focal lengths that you can take advantage of a larger field stop. In fact at less than about 17mm a 2" eyepiece is a bit of a disadvantage as you are paying for larger glass lens elements whereas with a 1.25" eyepiece you can get higher quality glass for the same money.

John

Well I did NOT know that!

Thanks John - good skills!

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.