1.25 diagonal - Vignetting ?

[andrew63]

I haven't  got/used one for a while, but remember reading about them vignetting with wide field of view eyepiece. How does the view look with a 32mm plossl and 24mm 68 deg and  do some 1.25" diagonals offer a wider field ?  

Thanks, andrew

[John]

Some 1.25" dielectric diagonals, eg: William Optics and clones, have a rather thick ring that joins the eyepiece holder onto the body of the diagonal. I found that this did cause some loss of the full field of view when I used eyepieces with a field stop larger than this ring. You can see the ring if you look down the eye piece barrel of the diagonal. I found it an issue with 1.25" eyepieces with the largest field stop size eg: 32mm plossl, 24mm super wides, 16mm ultra wides. It appears as a small reduction in the field of view and a fuzzy edge to the field rather than the sharp one that you should normally see when the eyepiece field stop defines the field edge rather than the ring in the diagonal.

I found diagonals that don't have this feature include the newer Revelation 1.25" dielectrics and the Tele Vue Everbright 1.25" diagonal though the last one is a very expensive option of course.

[andrew63]

Thanks John - i don't like fuzzy edges !

andrew

[John]

Here is a photo of a WO 1.25" diagonal showing the ring I was talking about. Pity they had to make it so thick:

[cs1cjc]

At some expense the Baader T2 type diagonals have minimal vignetting.   However decent 2" diagonals, like the GSO or WO, are quite cheap and are generally to be preferred to a 1.25" unless weight or light path length are issues.