20 replies to this topic

[Masvingo]

What is the lens arrangement in the ubiquitous 0.965" SR4mm eyepiece that came with the Towa? I've been unable to find a diagram on line, but a cautious peek into the eyepiece suggests that both the field and eye lens are symmetrical, but given the small size of the lens I can't really tell if the curvatures on either side of the lenses are the same. Before I fully dismantle and clean the lenses I want to check whether there is a 'front' and 'back' or whether it doesn't matter which way round each lens goes.

[wfj]

SR = Symmetrical Ramsden

Two plano convex lenses. Same focal length and diameter, thus symmetrical

Eye and field side flat. In between curved facing each other.

[bremms]

SR = Symmetrical Ramsden. FWIW. Your mileage may vary. I accept no responsibility for damages incurred as a result of dismantling...... Yada Yada.

[Masvingo]

That's what is confusing me, as far as I can tell, each lens appears to have two convex sides, they don't seem to have a plane side, and the field lens has a large diameter than the eye lens

Maybe it's time to get the magnifying glass out!

[wfj]

I often find mis-assembled eyepieces in used scopes.

If there's an meniscus lens, it's probably part of a HM eyepiece.

If its convex on both sides, it may not be even from an eyepiece - possibly a microscope objective.

Check the focal lengths for a hint.

[Masvingo]

Thanks William & Marc

I'll have another look tomorrow, it's difficult with such small lenses!

[wfj]

By the way, context is important here. If there are other eyepieces/optics/gadgets surrounding, they may also be parts of a much larger puzzle.

Or, if it's a "standalone", the source/sink of the other parts may be inaccessible.

Where this happens, is someone takes apart all of the lenses and either leaves them in a pile or randomly reassembles them. Seen both.

[Masvingo]

Well, the SR4 is part of a set of 5 that came with a second hand Topic 80mm (aka Towa 339). As purchased the eyepieces were very dusty especially the 22 mm which was missing its case. I started cleaning with the longer focal length eyepieces, the Kellner 22 mm and three Huygenian Mittenzwey ones, 12.5mm, 9mm and 6mm and all went fine, with the lens shape and layout being as expected.

The SR 4 shows no evidence of being tampered with as the paint seals were intact. Having a quick look now (before dashing off to work) at the assembled eyepiece the eye lens certainly looks as if it has a convex outer surface (although I might have reversed the lens - despite trying not to - when I dismantled the eyepiece a couple of days ago). The field lens looks similar although it is harder to tell as it is recessed in the barrel and I didn't have time to unscrew anything.

However, the SR 5 mm from by SNS badged Eikow looks like it has plane surfaces on the outer side of both the eye and field lens, as expected from the Ramsden diagram in your post above.

If I get time tonight I plan to have another closer look at the SR4,

[Terra Nova]

I have found that well made SR 5mm and HM 6mm eyepieces are not bad when used within the 60X per inch limit. I have a couple of Royal Astro and APLs that are really quite good.

[SteveNH]

The SR 4mm that came with my Jason 313 was frustratingly dirty, enough for me to brave the task of taking it apart and cleaning it. Judging by the cleaness of the interior surfaces, I don't think it was ever taken apart after manufacture.

I don't know why they called this a symmetric design, but it surprised me that both the eye lens and field lens were double convex, albeit of different diameters. The eye lens is symmetrical, and field lens appears to be more convex toward the field stop; but since I have no tools to measure this on such a tiny lens, I don't know for sure.

I made a small paint brush out of a tiny piece of rolled newspaper and used it to blacken the interior of the field stop barrel. After cleaning the lenses by soaking in alcohol, I reassembled everything and found a marked improvement in flare control. As Terra notes, these can give very decent images, aside from the almost non-existent eye relief. It became one of my favorite high power "experimental" eyepieces (until I sold it with the 313). A Unitron Or. 4mm has now taken its place.

Here are some photos: (the ruler does not indicate the proper spacing between lenses)

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[bremms]

Funny, not really a ramsden and not really symmetrical. Maybe they mean... Ramsdenesque and symmetricalish

[Masvingo]

Thanks Steve, that looks very similar to my layout from memory, epecially the double convex eye lens.

Marc, well, the eye lens is symmetrical! Maybe it's a Surprising Ramsden!

[Masvingo]

I have found that well made SR 5mm and HM 6mm eyepieces are not bad when used within the 60X per inch limit. I have a couple of Royal Astro and APLs that are really quite good.

Hi Terra, that's encouraging. As acquired both the Topic/Towa and SNS/Eikow lenses were quite dusty so the views weren't that great.

I've also just bought a hybrid diagonal so I can try my other eyepieces and am also thinking of getting the Vixen 36.4 mm to 1.25" adapter.

[bremms]

Had a celestron 0.965 Ortho 9mm in need of a good cleaning.
Took it apart and it was a Plossl or achromatic symmetrical. not an Ortho. Felt a liitle cheated..But it still gave a good image. The 10mm Celestron Halloween 0.965 (still have)plossl was clearly better.

[SteveNH]

You're welcome, James, glad to be of help!

[wfj]

Steve, James,

I think what you've both got is a longer focal length eyepiece fixture that someone used up the inventory for.

So what they did was order up lenses to fit "good enough" - they made a "modified" Huygens out of.

If you test it, I suspect you'll find a) it's longer than 4mm EFL, b) its got the AFOV of a Huygens, and c) its got significant field curvature.

A case of "I'm short parts and need to make shipment".

[SteveNH]

You're probably right - it struck me as a cheap substitute for custom made optics when I first saw the use of double convex lenses - just like the surplus ones in those old Edmund experimental lens kits! I can't test for focal length anymore because I no longer have it, but don't recall too much noticeable field curvature as Jupiter or Saturn drifted from center to edge, not really having to refocus. (My eyes might have compensated, but I doubt it - my eyeballs have hardened to the point that I have a very shallow focal range.)

I've known the symmetric design to have two identical simple lenses, as if it were a non-achromatic Plossl. The SR lenses were nowhere near that design, although one known design for a simple magnifier does employ two double-convex lenses spaced inside their focal lengths from each other, and the arrangement doesn't appear to exhibit terrible field curvature.

[Masvingo]

Interesting, a MH (Modified Huygenian) instead of an HM (Huygenian Mittenzwey)!

Here's mine disassembled, looks pretty much the same as Steve's.

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[Masvingo]

And the lenses, field to the left. Whichever way round I had the field lens, in photos the right side always seemed to be more convex!

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[SteveNH]

James, I'm glad you posted photos of your SR4mm - interesting to see that it's pretty much identical to the one I had. At the time, this must have been the least expensive way to design and build a 4mm eyepiece with adequate performance. Blackening the interior of the field stop bushing really made a difference in flare control on mine, eliminating almost all of it.

[Masvingo]

Yes, very similar. I'm glad to see mine isn't the only unorthodox Symmetrical Ramsden! Thanks for the tip about blackening.

We're now in London visiting our son where my Topic scope lives, but, unfortunately, it's overcast tonight so no chance to see how the cleaned lenses perform. Probably just as well after a 12hour trip with delays due to Bank Holiday traffic/accidents and frequent stops for the dog to stretch her legs!