• BirdForum is the net's largest birding community dedicated to wild birds and birding, and is absolutely FREE!

    Register for an account to take part in lively discussions in the forum, post your pictures in the gallery and more.

1.25inch fixed mag Eyepiece Recommendations?? (1 Viewer)

mayoayo

Well-known member
I Understand that you cant see the whole field of view?.. Try to find the eye relief sweet spot.. Too close you start to get blackouts and too far you loose FOV.. If You wear glasses, that might be a problem,depending of the fit of the frame or glass thickness. Some eyepieces have very wide eyecups and the Flat Surface is not very ergonòmic, but the BST conical eyecups are quite narrow and offer good eye placement. The eyecup is quite thick though and takes away some usable eye relief. The rubber eyecup is lightly glued to the screw-up hood.. Try to remove the rubber and play a bit with eye position then. Be careful if you use glasses,so not to scratch them.
 

giosblue

Well-known member
BST
Starguider 15mm, not the widest fov, but good contrast and tack sharp on my Pentax PF65 and PF 80.
Highly recommended.
 

Ridge Walker

Well-known member
I Understand that you cant see the whole field of view?.. Try to find the eye relief sweet spot.. Too close you start to get blackouts and too far you loose FOV.. If You wear glasses, that might be a problem,depending of the fit of the frame or glass thickness. Some eyepieces have very wide eyecups and the Flat Surface is not very ergonòmic, but the BST conical eyecups are quite narrow and offer good eye placement. The eyecup is quite thick though and takes away some usable eye relief. The rubber eyecup is lightly glued to the screw-up hood.. Try to remove the rubber and play a bit with eye position then. Be careful if you use glasses,so not to scratch them.


Hi thanks for your advice which helped. I don't wear glasses, and yes, you understanding on the FOV is spot on. I removed the rubber eyecup as you advised, which BTW wasn't glued. FOV is much better. I can now nearly get the Full FOV. Just a Q! Would I get the full FOV with a higher mag, or lower mag. Focal length of my scope is 480. 18mm gives me around x28. I will have to play around with it. Can you get thinner rubber eyecups for this eyepiece ?
Thanks again :t:
 

mayoayo

Well-known member
Glad it helped.. However, not using eyeglasses the eyepiece should have enough eye relief with or without the rubber eyecup, but facial features and structure are going to make some difference too. I use eyeglasses and sometimes i have to reverse the rubber eyecups on binoculars to gain even 1 or 2 mm. 1 mm can make the difference between a crisp full field stop or not.. And 2 mm too short will already eat away some of the view..
 

jring

Well-known member
Hi John,

yes, for the larger scope, the Petzval radius will be larger and thus we have less curvature.

The curvature is counteracted by the defocus sensitivity of the scope which is proportional to the focal ratio squared (and thus identical for both of our f6 scopes). See the text around equation 26 on the following page for an explanation and example (for f10).

https://www.telescope-optics.net/defocus1.htm

You could then calculate the Petzval radius, the sagitta of a circular segment with the Petzval radius and a chord length equal to the field stop diameter of your eyepiece and the allowed defocus for your instrument and 0.8 Strehl and compare the two. If the sagitta is smaller than the allowed defocus, the edges of the field will be "sharp enough" for 0.8 Strehl.

Joachim
 

Arek

Well-known member
I have good experience with Vixen LVW eyepieces. I use them succesfully with my Pentax PF80 scope.
 

Users who are viewing this thread

Top