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instruments. But the rich powdering of stars in Sagittarius was very subdued.

  Curiously, I could see none of the usual magenta color flare at night, and bright white stars stayed bright and white...not yellowish, like viewing earth objects during the day.

  Steve loaned me his precision photometer, which he set up to have a 2-degree restricted field of view, with about a 4 mm aperture. Naturally, the objective of this photometer must be placed in the middle of the exit pupil of the binocular, else vignetting would occur and low transmission calculations would result. But since the field is small, it doesn't actually need to be placed at the location of the Ramsden Disk, so for stability I pushed the photometer head right onto the eyelens, to square it up and center it by eye.

  The measurements correlate with the impression of low brightness.

  The 'white light' transmission of the 30x180 comes out at only 32%! This, mind you, with those 'golden coatings' removed. Presumably, with those coatings, the brightness would be cut in half again. Indeed, viewing Sirius with the original coatings gave proof of that.

  For comparison, I measured the transmission of my Nikon 10x70  (6.5 deg FOV) binoculars, to which I have glued spectacle lenses to the eyepieces,  to be 76%. Since these are only MgF coated, this is a reasonable value.

  A thought: Fresnel losses alone give an pessimistic estimate of transmission for binoculars, since some of that loss is reflected back into the eye rather than being reflected out the front of the binocular. This may account for measurements on uncoated binoculars being higher than simple Fresnel losses would suggest. Mind you, this additional brightness is not a good thing! It is stray light!

  So there we have it: the 30x180 has a brightness only 42% as great as ordinary MgF coated binoculars. The cause of this is principally the yellow glass used in its elaborate but otherwise excellent eyepieces. Russian binoculars are infamous for their frequent use of yellow glass, which while it may have virtue for daylight use, is a decided handicap for stargazing.

  It is worth noting that a low-transmission, large exit pupil binocular is superior to a small aperture binocular with high transmission. The contrast of an image, provided the optical are otherwise excellent,  is proportional to the size of the exit pupil, even though binoculars don't approach 'diffraction limit'. The 30x180 shows star clusters with a clarity not obtained in similar power, but small exit pupil systems. The importance of diffraction on contrast is also evident with roof prism binoculars, being noticeably poorer if the roof has not been silvered or given a phase-correcting dielectric coating. So, there is some value to these big binoculars, even though it's a shame transmission is so low.

  Regards, Dick Buchroeder.


I continue to find some good posts on the DO web site 'bulletin board':


Posted by Deutsche Optik on 08/12/03

>>How do I open up a 10x80 “Flakfernrohr

Without such proprietary tools as grip wrenches, spanner wrenches, and a healthy dose of experience, we would not advise opening a flak glass on your own. Among other things, the gearing system that times the placement of the different filter assemblies is exceedingly difficult to reassemble, and you'll often find haze and spots on the prism elements that you cannot get to without removing the entire prism clusters. Thus, we advise considerable caution in taking on such a project.



Binocular List #272: 18 October 2003


From: Peter Abrahams

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