Seven Tenths Where One Focal Length Should Be

The rattling started maybe fifteen eyepieces ago. One of the six Plössls at the bottom of the $15 estate sale bin had something loose inside it. Didn’t notice until I got home and the barrel made a soft clicking noise when I shook it.

Figured the retaining ring had backed off a turn. Happens with old optics. Unscrewed the eye lens end — smooth thread, barely any friction — and lifted it off.

The entire optical train came out in my hand. Field lens group. Brass spacer. Field stop. Eye lens group. All modular, press-fitted, threaded together like a parts diagram I’d never bothered to imagine actually existed.

It’s 2:47 AM. I’ve disassembled five of them now and I can’t stop.

What cleaning revealed

The loose one was a 25mm marked “JAPAN” in faded silver ink on the barrel. Late ’70s, maybe early ’80s, based on the coating style. Someone had loved this thing — the barrel threads were clean, no cross-threading — but they’d stored it somewhere humid. Every internal air-glass surface was clouded with fungal growth, a pale spiderweb pattern that you couldn’t see unless you held it up to a lamp and looked through at an angle.

I know from cleaning microscope optics that you can’t use solvents on coated glass without consequences. Modern anti-reflection coatings are 1–2 wavelengths thick — maybe 500 to 1000 nanometres total — and they evaporate under acetone or isopropyl faster than the oil you’re trying to remove. You get one cleaning method: distilled water, lens tissue, patience.

Took ninety minutes to clean all four elements. The fungus came off in grey streaks, but underneath, the coatings were intact. Ruby-purple on the convex surfaces, pale amber on the concave ones. Multi-coating, which wasn’t cheap in 1978.

Reassembled it and looked through at the kitchen light fixture. Clear. Sharp. No ghosting.

The transformation is absurdly satisfying considering I didn’t make anything. Just removed what shouldn’t have been there.

Spacing that looks wrong but isn’t

Three of the eyepieces are marked “Ramsden” in script on the barrel. Two plano-convex lenses, flat sides facing out. Jesse Ramsden, 1782. I read about these once in an optics textbook but never held one.

Here’s what I didn’t expect: the lens separation isn’t one focal length. It’s noticeably less — maybe seven-tenths, if I’m judging the curvatures correctly. That seems wrong. Ramsden’s design is supposed to correct chromatic aberration by spacing the lenses at exactly one focal length.

Turns out: exactly one focal length puts dust on the field lens into perfect focus. Disturbingly, edge-to-edge sharp. So the manufacturing standard became 7/10 to 7/8 of one focal length — close enough to correct the colour fringing but far enough that dust stays soft. Intentional imprecision. The design looks broken until you understand what problem it’s solving.

Checked all three. Same story. Dust particles on every field lens, blurred just enough to ignore.

The field stop nobody looks at

Inside each Plössl there’s a brass ring pressed between the two doublet groups. Looks like a washer, black anodized, maybe 12–15mm inner diameter. That’s the field stop. The narrowest aperture light has to pass through. It defines how much of the sky you see.

One of the cheap Plössls had a field stop that someone had drilled out — maybe half a millimetre wider than the others — and filed smooth. Amateur work, but careful. Probably trying to increase the apparent field of view without buying a new eyepiece.

I tested it. Set up my refractor (80mm aperture, 600mm focal length, lives in the closet because light pollution) and compared the modified eyepiece against the stock one. The drilled-out version shows maybe 5° more sky, but the edges are soft. Coma, I think. The lens design can’t cover the wider field cleanly.

The modification works if you’re okay with compromised image quality at the periphery. Some people are. I put it back in the bin. But I understand why someone tried it.

What I haven’t fixed yet

The 10mm Plössl has 7mm eye relief. I measured by holding a ruler next to my face while looking through it. Uncomfortable doesn’t quite describe it — you’re mashing your eye socket against the lens barrel, and any head movement loses the image.

Eye relief scales with focal length in a Plössl design. About 70–80%. A 10mm eyepiece gives you 7–8mm of working distance, and anything under 10mm is miserable. The orthoscopic designs (Ernst Abbe, 1880) have longer eye relief for the same focal length, but they’re four-element systems. You can’t just swap in an ortho eye lens group and call it fixed — you’re redesigning the entire optical path.

I could sell the 10mm and buy a longer focal length. Or wear it as penance for buying unseen eyepieces at estate sales.

Both options remain on the table.

It’s 3:24 AM and I’ve lined up all six on the bench in front of me. Five work. One is uncomfortable but functional. None of them are optimized for my telescope’s focal ratio (f/7.5), and one has a modification I didn’t make that failed in exactly the way I would’ve expected if I’d tried it myself.

But they’re clean. And I know how they’re built now. Same discipline as grinding telescope mirrors: precision optics you can service with your hands if you’re careful and patient and willing to work at scales that don’t forgive mistakes.

The clouds clear tomorrow night. Maybe.