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A Very “Old School” Eyepiece
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First off, I'll admit to being a lover of old optics. Vintage scopes and accessories hold a special appeal to me. So, when I was clearing out some old space at work the other day and found something that looked tarnished and brassy, it caught my eye. It seemed like a short piece of tubing, sitting abandoned on a window ledge. I picked it up, thinking it might be useful as an adapter tube or something similar. That's when I noticed the small piece of brass tubing had lenses inside. What's this?? Turning it over, it became apparent that the chunk of brass was, in fact, an old eyepiece. A pocket ruler confirmed the barrel was 1.25" diameter.
There were no inscriptions or designations on the barrel. The top of the ocular's eyecap was anodized black. Everything was coated with a thick layer of dust.
This mysterious old eyepiece must have come from a telescope, but that was a long time ago. Based on it's antique appearance, it seemed to be at least 80, maybe 100 years old.
I decided to take it home and clean it up. Not only was the exterior coated with dust, but the interior of each lens had a scummy deposit. Fortunately, the old eyepiece was very easy to disassemble. Some Windex and lint-free tissue removed all the grunge.
The next task was to determine it's design and focal length. Upon disassembly, it was obviously a Ramsden design. This consists of two plano-convex lenses, mounted with their convex sides facing each other. The field lens measured 25mm across and the eyelens came in at 10mm diameter. Compared to other, low power eyepieces I own, the focal length appeared to be 42mm, give or take a bit.
Apparent field of view is estimated at a modest 40 degrees.
The lenses are uncoated and water-white in color. They look extremely well polished. No scratches or scuffs visible.
So, the old ocular was undamaged and has antique charm, but how does it perform?
Since I knew it was a Ramsden, I wasn't expecting much. My earlier experience with cheaply made samples had led me to believe the design was synonymous with poor quality.
Happily, the ancient brass eyepiece had a surprise in store...
In my 100mm F/9 refractor, the image was sharp to the edge, used with or without a TeleVue 2.5x Powermate. No spurious color visible and no ghost reflections. I had a look at Vega and then Arcturus. No matter where I put those bright stars in the field of view, there were no extraneous reflections. Amazing for an uncoated eyepiece!
Using a Goodwin Barlow at 3x with the ol' Ramsden was less satisfying. The apparent field of view narrowed to only 25 degrees. This eyepiece didn't get along with the Barlow! However, no such vignetting of the field stop was seen using the 2.5x Powermate.
I looked at a few deepsky objects with the Ramsden and Powermate combo. M13 was lovely. Many individual stars could be seen glimmering across the cluster. I wondered what kind of light thoughput the uncoated lenses were delivering. A quick calculation shows the transmission is approx. 85%. Modern, multi-coated oculars will do better... typically 93 to 96%.
OK, the ancient ocular isn't miraculous, but it’s still very useful. Such old relics are often written off as junk by today’s observers. You might see them at a swap table for a few bucks each, or they might end up in the trash when a club or organization upgrades their equipment. But, if this Ramsden is any indication, some elderly "brassies" can still deliver the goods. There's also the added bonus of enjoying a genuine artifact of eyepiece history.
In this case, the history is still a mystery. According to some resources, the late 19th/early 20th century U.S. telescope maker, John Brashear, was the first to impliment the 1.25" format as a standard offering with his scopes. Perhaps the 42mm Ramsden is a Brashear ocular. Whatever it is, it's a fun antique I'll continue to use and enjoy.
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