Russell Optics 65 & 85 mm EP

Has anyone used these EP's on a Big Dob? I'm looking for a wide field, low power EP to take in the Veil, Pleadies, North American, etc. I'm still a newbie so any help is appreciated

I suggest viewing the North American nebula by using a good pair of binoculars with a generous objective end, say 12x60 or so, under dark skies. For me, they seem to work better than wide angle EPs on a scope. With practice, using the binocs, you can later see it with the naked eye if your skies are good. You're right on with the Veil or Pleadies, however.

Most Big Dobs have focal ratios around f5, even less. With an f5 scope, the 65mm eyepiece will give a 13mm exit pupil, and the 85mm eyepiece will givea 17mm exit pupil. If you're young and healthy, your pupil might dilate to as much as 7mm. You're wasting a LOT of light. Wasting a little light may not be so bad to trade off for a wide field, lots of people "cheat" with 7 or 8mm exit pupils even if their pupils aren't that big, but 13 - 17mm? That's getting a little extreme. And, while I still don't understand all that determines this effect, with really large exit pupils you might start seeing the secondary mirror as a shadow in your view.

You need the right tool for the right job, and big Dobs don't really do the Pleiades - not easily, if at all. Buy a little rich field scope for that, IMO.

Others have pointed out the problem with the size of the exit pupil as well as problems with the diagonal showing up as a dark hole.

But there is another factor to consider, the field of view of a 2 inch eyepiece is limited by the barrel diameter. This means that going to a long focal length eyepiece like a 65 mm or 85 mm won't buy you more field of view than an optimized 41 mm widefield design. The barrel limits the field stop diameter so you are pretty much stuck with a maximum field stop of 47 mm or so.

Consider 3 eyepieces, the Russell 65mm, the 41mm Panoptic and the 31 mm Nagler all being used in an 10 inch F5 scope with a 25% central obstruction (diagonal)and an observer with a maximum exit pupil of 7mm.

Russell 65mm:

Magnification: 19.5X
Maximum possible AFOV: (47/65)*57.3= 41 deg.
Maximum Possible TFOV: (41/19.5)= 2.1 deg
Exit pupil: (65/5) = 13 mm
Hole in exit pupil 13mm*.25= 3.25 mm
Effective aperture: 10*(7/13)= 5.4 inches

41mm Panoptic:

Magnification: 31x
AFOV: 68 degrees
TFOV: 2.1 deg
Exit pupil: 8.2mm
Exit pupil hole: 2.1 mm
Effective Aperture: 8.5 inches

31mm Nagler:

Magnification: 41X
AFOV: 82 deg
TFOV: 2.0 degrees
Exit pupil: 6.2 degrees
Exit pupil hole: 1.5 mm
Effective Aperture: 10 inches.

So all these eyepieces have essentially the same True Field of View but the shorter focal length designs make better use of the aperture and do not suffer from the hole in the exit pupil problem except during the day.

For a final comparison:

Same observer but using a 5 inch F5 scope with the Nagler 31mm:

Magnification: 20.5X
AFOV: 82 deg
TFOV: 4.0 degrees
Exit pupil: 6.2 degrees
Exit pupil hole: 1.5 mm
Effective Aperture: 5 inches.

Comparing this scope with the Russell 65 mm in the 10 incher, one sees the following:

The smaller scope has essentially twice the field of view at essentially the same effective aperture.

It is pretty easy to see why owners of fast scopes like widefield eyepieces in the 30-40mm range, they provide maximum True Field of View while making maximum use of the aperture.

Slower scopes, F10 -F15 are really what eyepieces like the 65mm and 85 mm's are designed for. In an F10 scope, a 65mm eyepiece will provide about the brightest possible image, in an F15 scope, the 85 mm will provide about the brightest possible image.

Jon