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Carl Zeiss Jena 50/540 E ‘Optiksatz’ Lens Review

An experiment with a stopped down Takahashi FS-128 years ago proved that a 50mm refractor could be useful, if it was basically perfect. Since then, I’ve tried a few in the hope of finding a mini travel scope, but they’ve always been just a bit disappointing so far (no, even Borg’s 50FL wasn’t quite what I’d hoped).

Then recently I found a review suggesting that the original high-quality 50mm refractor (well refractor objective, anyway) – the Zeiss 50/540 - performs like a Takahashi FC-50 (a fluorite apochromat). Let’s find out if it does!

At A Glance

Telescope

Zeiss 50/540 ‘E’ lens (Borg OTA)

Aperture

50mm

Focal Length

540mm

Focal Ratio

F10.8

Length

47cm (OTA as configured)

Weight

~1 Kg (excl. rings)

 Data from Me/Zeiss.

What’s in the Box?

An original Optiksatz lens came in a little cardboard box with foam cutouts, bundled with eyepieces and focuser like this:

Design and Build

The 50/540 was manufactured over several decades, from the 1950s into the 1990s, as far as I can tell. It may have been incorporated into Zeiss’ own OTAs, but mostly it was sold as an ‘Optiksatz’ – an optics kit. These kits comprised the lens, a small helical focuser and two Zeiss eyepieces of Huygens type, an H-16 and H-25, all in a little box. Such complete sets do come up for sale, but I bought mine as just the lens cell.

The style of the cell is like a smaller version of the Telementor objective. Though the vast majority of Optiksatz lens cells are silver like this one (later examples are more polished and with a central band), a few of the last are believed to have been black.

To get a 50/540 built up into an OTA isn’t that easy, as I’ll explain. Here suffice to say I managed to create an OTA from Borg components.

Optics

This particular lens was a bit more expensive than the average, but I had to have it, what the heck. Only later did I find out why.

Earlier 50/540 lenses like mine with a serial number are so-called ‘E’ lenses. These earlier examples are air-spaced Fraunhofer achromats (with just-visible foil spacers) and may use slightly different glasses from later ones. As we will see, performance is astoundingly good, but I’m getting ahead of myself.

It turns out that later lenses have no serial number and are of a cheaper cemented doublet design, so-called ‘C’ lenses. These supposedly perform a little less well and are typically cheaper.

The plain but weighty aluminium lens cell is threaded unhelpfully only on the front (M52 for filters). The lens is held into the cell by a blackened lens ring. Some folks have undone this, removed the elements and turned them around to use the thread at the back for connection to an OTA. Personally, I would not do this without an optics bench – it would be all too easy to get things out of alignment or lose or damage a foil spacer.

Translation: “DIY telescope for hobby astronomy. Assembly for refractor 50/540.”

Tube

With no threads for attachment, exactly how Zeiss expected the ‘DIY’ astronomer to build the Optiksatz into an OTA is a puzzle. The original instructions show it snuggly fitted inside a tube, resting on a flange, but that would need very precise machining, or some lucky tubing finds.

I have only basic ATM skills and facilities, so I wasn’t going to make a Telementor-like tube from scratch, as some have done with great success. Others have incorporated the lens into a period tube, a cut-down reclaimed Unitron tube being one I’ve seen that is particularly effective.

Instead, I dug deep into a parts bin I know only too well – that from Oasis/Borg. The result isn’t period, but is quite in keeping with Zeiss, being an oversized tube with a helical focuser (The original Optiksatz kit focuser was a 0.965 helical).

The tube components I used were 8” and a 2” Borg 80mm tubes, snugly joined with their 77.6mm to 75mm connector. Another 2” tube section would have made the OTA too long, so I’ve resorted to their long 2” eyepiece holder (part 7509). This all comes to focus with a 1.25” mirror diagonal and eyepieces from 32mm to about 6mm. A 5mm eyepiece can be accommodated with a flat 1.25 adapter in the end of the ep holder.

To hold the lens cell, I used a combination of adapters on the end of an 80mm Borg tube – parts 7680 and 7459. These form a snug fit around the end of the lens cell and a lip for it to rest orthogonally on. Some careful shimming and a dab of glue make a solid connection that could easily be broken to remove the lens cell later.

I’m happy to say that collimation, my main concern, is spot-on.

The Zeiss Optiksatz lens mounted in a Borg tube assembly.

Focuser

A big advantage of the large Borg helical focuser is no risk of vignetting. The main problem with this focuser is its short travel, but by swapping 1.25” adapters I can accommodate most eyepieces. For short focal lengths down to about 5mm, a low-rise 1.25” adapter works, but for the longest FLs (i.e. a 32mm Plössl), a high-hat 1.25” adapter is required.

Mounting

I obtained a pair of Borg’s lightweight 80mm tube rings that slide into place then lock (the rings don’t open). These are nice light rings, but slightly dangerous – their mount holes are drilled through, so it would be all too easy to mar the tube.

A Takahashi 80mm clamshell is perhaps a better, though more expensive and heavier, option.

The Optiksatz OTA as configured is very light, so will go on any mount. I needed to take the little Zeiss up a local hill to get onto a few low planets, so I mounted the rings on an old Stellarvue plate with ¼-20 mounting threads so it would go on a fluid photo head.

The Zeiss/Borg mounted on a Takahashi PM-SP.

Accessories

The Optiksatz came with two Zeiss eyepieces, an H-16 and an H-25 and that little helical focuser. A likely period upgrade would have been a Zeiss Jena orthoscopic for higher powers. So for this review I obtained both an H-25 and an O-16.

In Use – Daytime

The daytime view is excellent – crisp and sharp and contrasty, with low levels of false colour in visual use. My usual test of viewing branches in silhouette against a bright cloudy sky at ~100x yields a completely sharp image with a mild wash of purple in focus and some muted green/purple fringing either side. This is firmly in semi-apo’ territory.

Field flatness is also excellent, with a flat-field eyepiece (Ethos) giving a sharp view off axis right up to the field stop. Surprisingly, brightness is good too. Powers of 100x or more are perfectly usable and the little Zeiss beats, for example, a Tele Vue Ranger in this respect. The Zeiss opticians did their job well.

However, the news isn’t quite as perfect using the 50/540 as a telephoto lens, where very high contrast areas get washed with false colour, especially parts of the scene that are slightly defocussed. Still, daytime telephoto shots of most things come our very well indeed – super-sharp and with great coverage on APS-C. As usual the image below is straight from the camera.

You can’t expect a fifty year old design to cater for the extended violet sensitivity of a modern camera sensor, so no fault here. And for general photography it’s really good.

So, no, for terrestrial use the 50/540 is not quite the same as an APO like the 50FL or FC-50, but the difference is very subtle, visually at least.

With an APS-C chip, the 50/540 makes a very sharp telephoto lens.

In Use – Astrophotography

A 50mm lens made in the Fifties or Sixties for imaging? Please!

But, in fact, the 50mm Optiksatz lens, in the Borg tube with a 2” VB, covers an APS-C chip very well and is pretty flat too. Compared with larger aperture achromats, violet bloat is very well controlled.

It is interesting how much better the 50/540 performs than its bigger sibling the Telementor, for false colour in imaging, the same as for visual use. Just 10mm of aperture make a big difference here. The 50/540 really is almost like a semi-apo, whilst the Telementor is in more familiar achromat territory – a good lens, but limited for imaging or high-powers by significant chromatic aberration.

Zeiss 50/540 takes a surprisingly good image of the Moon (cropped and contrast pushed slightly).

M45 through the Zeiss 50/540 – Fuji XM-1 APS-C, 110s at ISO 3200. Violet bloat, but good coverage.

In Use – The Night Sky

General Observing Notes

The Borg tube lacks focus travel, but is otherwise a very refined piece of engineering. The focuser doesn’t like big cameras or heavy eyepiece setups, but for the type of eyepieces the 50/540 was designed for it’s smooth and accurate.

The lens is very close to the end of the cell and so subject to veiling flare (and dew). A lens hood would be a good addition.

The original H-25 eyepiece is a bit dimmer, narrower than a modern 25mm Takahashi MC Ortho, with a more yellowish cast due to its older glasses, but it’s every bit as sharp on-axis. The same is true of a Carl Zeiss Jena O-16, which would have been a common upgrade.

Cool Down

With such a tiny mass of glass in a thin aluminium tube, cool-down is virtually immediate.

Star Test

At 108x the 50/540 has one of the most perfect star tests you’ll ever see – identical either side of focus with very clearly defined Fresnel rings and a perfect Airy disk with diffraction rings in focus. Oddly, considering the way I mounted the lens, collimation is spot-on too.

The Moon

Just after full, with Mare Criseum near the terminator, the Moon through the little Zeiss was a wonderful view. A cold hard ball, full of detail at low power, I was surprised to find the view still absolutely crisp at 108x with 5mm Nagler, with none of the grainy fade-out I’d seen with the Borg 45ED. False colour was very well controlled too, with just a faint trace of purple off the limb, a similar level to many fast apochromats.

The views through a fluorite apochromat nearby – Takahashi’s excellent FC-60 – were generally comparable. The TeleVue Ranger was out too and showed a bit more resolved detail, but quite a lot more false colour.

The embayments around Mare Criseum, Proclus with its bright walls and strange rays, the central peak in Hahn and rough floor of Gauss right on the limb, all looked like they do in larger scopes. Here was that stopped-down big scope view I had had with the FS-128 all those years ago. The little Zeiss is definitely the best 50mm I have used for the Moon.

I got up early on a frosty November morning to find excellent (8-9) seeing. A last quarter Moon hung high in the sky just off the meridian, with Albategnius on the terminator. After testing a pair of 60mm Takahashi fluorite apochromats, I got out the little Zeiss to see how it would compare and found that now I could really push the magnification.

A 3.5mm Nagler giving 154x showed extraordinary detail for such a tiny aperture. I could make out the craterlets along Rima Hyginus, normally the preserve of larger scopes.

Running south down the terminator, I viewed Mons Piton shining alone in the mare, the sharply defined peaks of the Apennines, Rima Ariadaeus full of shadow and running off into the lunar night, Alphonsus with its offset central peak and hummocky ridge and the slumped walls of Tycho in extraordinary detail before ending with Clavius and its arc of craters.

Rilles and craterlets with a 50mm?! I was astonished that a 50mm achromat could deliver so much, only a little less than the Takahashi 60s nearby and with just a minor lilac false colour washing the bright lunar highlands at the highest powers (far less than a Tele Vue Ranger alongside).

Venus

At an altitude of 18° and a bright magnitude -4, Venus displayed a crisp gibbous disk 14 arcsecs across at 108x. In a bright dusk sky, there was some purple and green either side of focus and muted violet stray light around the planet in focus.

Compared directly with two other scopes generally regarded as ‘semi-apochromats’, the 50/540 showed much less false colour on Venus than the TV Ranger, but more than a Takahashi FS-60C (a 60mm F5.9 fluorite doublet). However, I thought the 50/540 gave a crisper and more contrasty view of the actual planet, whereas the FS-60C seemed a bit soft.

Saturn

Though small and low in the dusk sky, Saturn still displayed its rings nicely.

Deep Sky

Really perfect optics means star colours are intense, star fields beautiful. At 77x, Albireo was a gorgeous firey orange and gas-flame blue. Star fields are flat to the field stop and wonderfully pinpoint, if not as bright as a larger aperture allows.

I tried to split Epsilon Lyrae, expecting to fail. At 108x, the components were certainly dumbbells.

DSOs were less successful. This is a single-coated 50mm after all. I struggled to find M57 (the Ring Nebula) and M15 (a bright globular cluster near star Enif) resolutely stayed in dim blob mode at 60x. On extended objects at low power the Zeiss fared better. M31 was much the same as through any small refractor at 17x with a 32mm TV Plössl.

Summary

The Zeiss 50/540 surprised me. It was, after all, sold as an inexpensive DIY kit fifty years ago. Of course, it is only 50mm. But the Zeiss Optiksatz E lens shows what I discovered all those years ago with a stopped-down Takahashi FS-128: that a 50mm aperture can do surprising things, if it’s near perfect.

Indeed, with near perfect optics and excellent false colour correction it is the best 50mm refractor I have tested so far (I have yet to try an FC-50, but plan to). It takes powers of 100x and more with aplomb, night or day. Every view is crisp and sharp and vivid. It often behaves like a larger semi-apochromat and puts most premium birding scopes to shame at powers above 70x. It works especially well for the Moon and planets.

I bang on about false colour and apochromats, but in truth when an ‘achromat’ is this good, an apochromat really isn’t required for most things. Only Venus revealed significant false colour that a full-on apochromat would not have.

But should you buy Zeiss 50/540 lens? The trouble is mounting it. The tube I put together from Borg components works very well indeed, but it doesn’t really suite the vintage character of the Zeiss lens. Then again, a vintage focuser like the original, perhaps 0.965” only, would not allow the little Zeiss lens to perform to its full potential.

The Zeiss 50/540 E is a fabulous tiny refractor lens which performs extraordinarily well for such a small aperture. But ideally, it needs a talented ATM to create a period OTA for it.

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