13mm Nagler Type 6 F.O.V. ?

I have used the new 13mm Nagler for about 2 months in my 13.1" F/4.5 dob. I
find that with my eye centered and almost pressed against the eyelens I
cannot see the edge of the field. To see the edge in any direction I must
move my eye position slightly off center to the opposite side. The usuable
field appears about like a Radian and not as sharp on axis. Has anyone else
noticed this? Don

In article
,
"Don Fritz" wrote:

I have used the new 13mm Nagler for about 2 months in my 13.1" F/4.5 dob. I
find that with my eye centered and almost pressed against the eyelens I
cannot see the edge of the field. To see the edge in any direction I must
move my eye position slightly off center to the opposite side. The usuable
field appears about like a Radian and not as sharp on axis. Has anyone else
noticed this? Don

I have the same experience with Naglers, at least the ones I've used. I
am unable see the full AFOV with my eye in the optimum position (iris
coincident with the exit pupil). I own one Nagler - a 7mm Type 1 which
happens to be my favorite eyepiece for DSOs through my 8" f/6. Still, I
suspect that a 7mm Pentax XL with a 65 deg AFOV would show essentially
the same view and be just as satisfactory. If there was a 7mm Radian,
it might do the job, too. As for sharpness on axis, I really don't have
a firm preference between the quality eyepieces. For me, the telescope
objective (a Zambuto mirror in my case) is the main determinant of image
quality.

The extreme AFOV of Naglers (and Meade UWAs) obviously satisfies a lot
of amateurs so you can't argue with success. However, when you
calculate the true field from the field stop of these eyepieces and
relate it to the focal length and magnification I think you find that
the 82 degree apparent field is somewhat of an illusion and is enhanced
by the pincushion distortion inherent in the design. If there were no
pincushion distortion, I believe the AFOVs would be in the 70s.

John

I agree... but then Televue is not marketing the "usable" portion of the
field, they are marketing the "spacewalk" effect... which is fairly unique
to Naglers.

"John Bevan" wrote in message
...
In article
,
"Don Fritz" wrote:

I have used the new 13mm Nagler for about 2 months in my 13.1" F/4.5
dob. I
find that with my eye centered and almost pressed against the eyelens I
cannot see the edge of the field. To see the edge in any direction I
must
move my eye position slightly off center to the opposite side. The
usuable
field appears about like a Radian and not as sharp on axis. Has anyone
else
noticed this? Don

I have the same experience with Naglers, at least the ones I've used. I
am unable see the full AFOV with my eye in the optimum position (iris
coincident with the exit pupil). I own one Nagler - a 7mm Type 1 which
happens to be my favorite eyepiece for DSOs through my 8" f/6. Still, I
suspect that a 7mm Pentax XL with a 65 deg AFOV would show essentially
the same view and be just as satisfactory. If there was a 7mm Radian,
it might do the job, too. As for sharpness on axis, I really don't have
a firm preference between the quality eyepieces. For me, the telescope
objective (a Zambuto mirror in my case) is the main determinant of image
quality.

The extreme AFOV of Naglers (and Meade UWAs) obviously satisfies a lot
of amateurs so you can't argue with success. However, when you
calculate the true field from the field stop of these eyepieces and
relate it to the focal length and magnification I think you find that
the 82 degree apparent field is somewhat of an illusion and is enhanced
by the pincushion distortion inherent in the design. If there were no
pincushion distortion, I believe the AFOVs would be in the 70s.

John