Awhile back we discussed UO orthos, baffling, and field stops. I was somewhat handicapped at the time since I was going on memory, but I recently got an 18mm UO Abbe ortho for a finderscope project I'm working on, and I have some comments about it that relate to our past discussion and a new observation.
First of all, the 18mm UO ortho has a fine field stop, as good as any you'll find in any eyepiece. It has a diameter of 13mm and resides about a cm below the top of the chromed barrel. This is not to say that the eyepiece is well baffled, however, since the field element retaining ring and the spacer ring between the eye lens and field triplet have shiny surfaces despite being black colored. Unfortunately, I don't see an easy user fix. Painting them would give only marginal benefit since even flat black paints are fairly reflective at low angles. It would be better if these rings had a grooved, threaded, or at least textured flat black finish, and the retaining ring doesn't need to be as wide as it is.
I measured the apparent field and came up with 46.8 degrees, which is close enough to regard the quoted 46 degrees on the Kokusai Kohki web site as correct. Measuring the exit pupil also produced results consistent with an 18mm focal length, and if anything it might be a tad longer. I was surprised, however, that the field stop is only 13mm in diameter. To be "orthoscopic" means (among other things) to have low angular magnification distortion, but in this case the angular magnification distortion is positive and significant. The apparent field is about 11% wider than it should be for an 18mm eyepiece with a 13mm field stop and no angular magnification distortion. To get a feel for how far off this is, the 20mm Tele Vue Plossl with 50 degree afov and a field stop of 17.1mm has an apparent field only 2.1% wider than it should have to achieve zero angular magnification distortion.
In other words the 20mm Tele Vue Plossl is a lot closer to being orthoscopic than the 18mm UO Abbe "ortho". Go figure. I would be careful, however, about extrapolating about shorter focal lengths based on the 18mm.
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Mike
First of all, the 18mm UO ortho has a fine field stop, as good as any you'll find in any eyepiece. It has a diameter of 13mm and resides about a cm below the top of the chromed barrel. This is not to say that the eyepiece is well baffled, however, since the field element retaining ring and the spacer ring between the eye lens and field triplet have shiny surfaces despite being black colored. Unfortunately, I don't see an easy user fix. Painting them would give only marginal benefit since even flat black paints are fairly reflective at low angles. It would be better if these rings had a grooved, threaded, or at least textured flat black finish, and the retaining ring doesn't need to be as wide as it is.
I measured the apparent field and came up with 46.8 degrees, which is close enough to regard the quoted 46 degrees on the Kokusai Kohki web site as correct. Measuring the exit pupil also produced results consistent with an 18mm focal length, and if anything it might be a tad longer. I was surprised, however, that the field stop is only 13mm in diameter. To be "orthoscopic" means (among other things) to have low angular magnification distortion, but in this case the angular magnification distortion is positive and significant. The apparent field is about 11% wider than it should be for an 18mm eyepiece with a 13mm field stop and no angular magnification distortion. To get a feel for how far off this is, the 20mm Tele Vue Plossl with 50 degree afov and a field stop of 17.1mm has an apparent field only 2.1% wider than it should have to achieve zero angular magnification distortion.
In other words the 20mm Tele Vue Plossl is a lot closer to being orthoscopic than the 18mm UO Abbe "ortho". Go figure. I would be careful, however, about extrapolating about shorter focal lengths based on the 18mm.
--
Mike
