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40mm Rodenstock Questions

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#1 SomebodyElseEntirely

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Posted 21 February 2019 - 10:12 PM

Call me crazy, but I’m seriously thinking of getting myself a 40mm Rodenstock eyepiece. Before I commit, I’m hoping that folks who have (or have had) this eyepiece can answer a couple of questions for me:

 

1) Does this eyepiece actually have a field stop and, if so, is it sharp?

The cutaway diagram of the eyepiece on the BW-Optik website does not show a field stop, whereas Chris Lord, in his “Unusual Eyepieces” document states that the eyepiece has a sharp field stop. (I have found no other references to the field stop in my searches.) Is anyone able to help with resolving this contradiction? Chris, if you’re reading this, I would love to have your input.

 

2) What kind of coatings does this eyepiece have?

BW-Optik describes the coatings as multilayer. Everyone else (including Chris Lord) says that they are single layer. And apparently the eye lens is uncoated on its outer surface.

 

Clearly, I am hoping that, with regard to Question 1, BW-Optik is wrong and that, with regard to Question 2, BW-Optik is correct.

The field stop issue is a potential deal-breaker. The coating issue is, in comparison, quite minor — I find that, at large exit pupils, differences in transmission are not as noticeable as they are at small exit pupils.



#2 BillB9430

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Posted 22 February 2019 - 10:21 AM

The 40 mm Rodenstock seems to be a military eyepiece remounted in a nicely anodized large barrel housing, adapted to 2" focusers at the bottom. Removing two small Allen set screws (1.5mm hex) allows the inner barrel with all lenses to be extracted. It has seven engraved, white paint filled lines and a + and - along the upper outside of the inner barrel, as would be used on a diopter scale, but the numbering is missing. That numbered scale was likely on the removed military focusing housing. A 30 mm wide band of multiple start threading from the original military usage appears below the diopter lines but is not engaged in the Rodenstock adapter at all. The clear aperture of concave surface bottom of the field lens is 52 mm. The focal plane of the eyepiece is about 2cm below the bottom of the military housing, as would be logical for a military eyepiece with a reticle in that position. The 2 inch OD lower barrel of the adapter is threaded inside at the bottom for filters and threaded along its inside length to decrease reflection. The very top inside of that barrel has a slight ring constriction that may serve as a field stop when the eyepiece is in that housing. I cannot get my caliper jaws in that far to measure its diameter, but it is just slightly less than the 46.8 mm ID of the threading in the barrel. This field stop does not appear perfectly sharp to my eye, but becomes so if the inner eyepiece is lifted a little (about 7 mm for my eyes) in its housing before tightening the two set screws holding the anodized adapter in place. For me, it is hard to get close enough to see the field stop with glasses on, but easy without them. For comparison, the Rodenstock has a sharper edge-of-field image than with a 2" adapted Kaspereit 38mm eyepiece from the M71 WWII Sherman tank sighting telescope, but the M71 eyepiece has sufficient eye relief for use while wearing glasses while the Rodenstock does not (for me, anyway). 

 

I have not disassembled the lens stack, but looking at reflections of a ceiling light within the eyepiece shows some blue, some deeper purple (multicoated?) and some clear white reflections, so the coatings may be mixed on the various lenses on my sample. The outer surface of the eye lens is indeed uncoated, as you said. Hope this helps a bit.  I bought this eyepiece used several years ago, so new ones could be different. - Bill


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#3 SomebodyElseEntirely

SomebodyElseEntirely

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Posted 22 February 2019 - 09:26 PM

Thank you very much indeed, good sir, for such a detailed response. You have answered my questions, and more!

 

Great to hear from someone who is clearly very interested in out of the ordinary eyepieces.

 

Looks like I’ll be spending some money. smile.gif




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