Eyepiece behaviour in two different scopes.

[mdstuart]

I have a 90mm refractor now which has a 800mm focal length.

I am using a 25mm BST 66 deg FOV eyepiece.

Views are great in both the refractor and my 10 inch 1200mm focal length dob..

BUT

When using the refractor you need to get your eye just right over the eyepiece to see the FOV. If you move your eye to close you lose some of the FOV. In the dob its much more forgiving and you can see the whole FOV even moving your eye off target slightly.

Is this to do with the light cone??? or some other effect.

Does the eye relief change in the two scopes due to the nature of the focal length and different size lens/mirror?

Any thoughts welcomed.

Mark

[Moonshane]

hi Mark

It's to do with exit pupil. in the frac it's about half that of the newt so 'more difficult' to find.

[estwing]

i knew newts were better

[mdstuart]

OK so should I get a different eyepiece that will deliver a larger exit pupil?

Maybe a 32 or 40mm plossl?

Mark

[Moonshane]

you could do (a 32mm would be better as it shows the same field at higher magnification) although 2.7mm is pretty fine for an exit pupil I think. a 32mm would produce quite a large exit pupil with the newt (6.8mm) but still usable from darker skies (which I think you have?).