Repair of an old 1.4 N.A. oil immersion objective; with objective view, now.

[apochronaut]

One of the forum members, Jim Wheeler, known here as apatientspider asked me if I could attempt a repair of a fairly old and unfortunately non-functional Spencer 90X 1.4 N.A. apochromat. The front lens, otherwise undamaged was floating free, having departed it's housing. I had done this a few times already, so up it came from Texas. Jim suggested and belatedly I realized that the process might be of interest to some other forum members, so I documented the process with some lousy pictures from the point of having secured the front lens in place, on. I didn't get a picture of it's original condition but it is easy to imagine; there was a hole in the front lens housing where the font lens should have been.
Spencer jumped around a little on how they secured the front lens but I can say that there are quite a few objectives out there with no front lens, so either their estimation of the level of attachment necessary or that of just what these lenses would be used for was a bit off. You see them on ebay all the time. There is an otherwise very nice and shiny 4mm for sale right now, without a front lens.

Jim tells me that they used a sealant around the front lens for the oil immersion versions at one time but at a certain point in time they ceased that practice and then later, you can again find a sealant around the lens. It is likely that their method of fitting the lens into the front housing proved poor and they went back to sealing them in.

The lens for this objective appears to have been pressure fit into a small bore that goes part way through the metal of the front housing. There is a lip at the bottom, which stops the lens from going all the way through. When I first encountered the objective, there was a considerable amount of residue inside the housing, I initially presumed to be either sealant or dried oil, or a combination of both. Subsequently, I have decide it was dried oil only, so this lens most likely came from the factory with a pressed in front lens.

The front housing and all the other lens cells( 4 more) are fitted internally into the objective shroud. Spencer ubiquitously used a threaded in cap for the front lens of it's achromats up until the 1930's but the apochromats from about the first W.W. on were designed with all lens cells fitted internally. This required that the objective be split in order to get the front lens cell out. The lens section is in 2 parts on the 90X and the threads are often stubborn on those. This one was seized, so required very delicate treatment with a tiny amount of penetrant to loosen up the threads. It took quite a few weeks but eventually it came a part. The lens pack then pushes out the back, like a piston in a cylinder. The tiny little 1.4 N.A. lens just fell out, upon complete disassembly.

The pictures were taken rather haphazardly and afocally through whatever eyepiece I could put my hands on that would give me enough field to cover in some cases a fairly large object, such as an entire objective casing imaged through a stereo microscope. Most were done through a Wild 6Xk H or a Kyowa 2.5X photo eyepiece. Neither are well corrected for a Cycloptic, so there is lots of ca and dust on the photo lens. I was more interested in fixing the objective than making perfect pictures.

Picture one and two show the objective barrel. Early Spencer apochromats were lacquered brass and then for about 4 decades they were gold plated. For a very small period between those two incarnations, they did that pictured; a brass upper section and a platinum plated lower section.
Picture three shows the other possibility when too much pressure is applied to a front lens. That's the front lens from one of the gold plated 90X 1.3 N.A. 0bjectives, that had a startling end to it's life.
Picture four shows the 1.4 N.A. front plano/convex lens fitted into the bore. The brown colouring is the wood beneath the glass stageplate of the stereo microscope.
Picture five shows the front of the housing after the lens has been refitted and this time cemented. The lens had become too loose in it's bore and also the lip at the front surface was too degraded to adequately stop the lens from pushing right through. The thread sized needle laying over the lens is the smallest needle I have here, out of about 200. It turned out to be an indispensable tool for both manipulating the lens into place and transferring cement to the seam between the lens and it's housing.

[apochronaut]

A few more pictures.

Picture 6 is the inside of the front lens cell with the lens heavily cemented to secure it and keep it aligned.

The initial analysis of the objective indicated that it's sole problem was the detached front element, however upon separating the other 4 lens cells , it was immediately apparent that fourth cell was badly de-laminated. Since I had not the opportunity to test the objective previously, this came as a disappointment. It seemed that the objective was euchred. Why had I not noticed this when I looled down through the lens stack when still assembled? Re-assembling the stack, I proceeded to peer down through it in increments through the stereo. No de-lamination. It seems that the concentric form of the de-lamination in that particular cell, just brushes the extreme edge of the image circle arriving from the previous lens. The apochromats apparently use very large lenses, with relatively less in the center being utilized to pass the image circle. Just one of the techniques employed to limit off axis distortion. It seem the objective is o.k., even with 70% of the fourth element showing de-lamination.
Picture 7 shows all five lens cells. The de-laminated one is readily apparent in the upper right.

Picture 8 shows a closeup of the de-laminated lens

Picture 9 shows a closeup of a stack of the 4 secondary lenses magnifying a sheet of paper towel upon which they rest. No de-lamination visible.

I have since reassembled the lens briefly to test it and it seems to be fine. However, to be sure that the cement used has gassed off sufficiently, I have disassembled it and will wait another week before the final assembly.
I will post some pictures through the actual objective in a week or so.

[MicroBob]

Hi Phil,
thank you for describing the repair! This is really a beautiful old objective with the nice stained paint on the brass and the convex knurl. It's a pity that these convex knurls are not used any more - beautiful and nice to touch.
I have a first series Leitz Planapo 100 objective with the same problem and one with a delaminated lens group (It is said to be the Leitz objective most prone to delaminations). So I will have a go at a repair when time allows.

Bob

[MichaelG.]

Very interesting ... Thanks

MichaelG.

[wporter]

Nice salvage job, Phil. Well worth it, for this old objective, looks like a gem.

I wonder if you will be able to see the delaminated parts of the lens using a Bertrand lens, once you get it reassembled. Also be interesting if an image taken through the objective shows loss of edge of field contrast or other effect. Like you say, it may be outside the fov. But people didn't worry much about the edge-of-field in those old apos, anyway, right?

[apochronaut]

I didn't check it with a bertrand lens but I will once I put it back together again. The last picture shows the lens stack focused but as I backed off the focus, the de-lamination wasn't apparent, which surprised me. It seems, that at the point the cone of light is passing through that smear of wonky cement in that triplet( I think it is the first joint), that the image circle is just a fraction inside the de-lamination zone. Clever of them to make a lens that de-laminated from the outside in concentrically wasn' it! I hope it stays put.

[PeteM]

impressive repair, Phil. Not sure my fumble fingers could even pick up that front lens element, much less carefully re-cement it back in place.

[apochronaut]

Well. I have the shakes, Pete. In my case it seems like it has been coming on as part of the Borreliosis circus. That tiny needle in a pair of small vicegrips ended up being the primary tool for moving the lens here and there until I was ready to install it. I used the needle with a tiny hardened epoxy pad on it's point to roll the lens onto a tissue, where it was cleaned and left under a small flap, until it was dumped unceremoniously into it's mount. The needle again was used to rotate it, flip it and move it into it's bore once in the housing. I pressed it in with the shaft of a Q-tip, cut to a precise angle and hollowed out some.
I didn't pick it up once and I'm guessing , were I to have, the objective would still have a hole in it's end.

[PeteM]

Even more impressive, Phil.

[apatientspider]

....It seems, that at the point the cone of light is passing through that smear of wonky cement in that triplet( I think it is the first joint), that the image circle is just a fraction inside the de-lamination zone. Clever of them to make a lens that de-laminated from the outside in concentrically wasn' it! I hope it stays put.

I can sense a twinkle in your eye as you type that, Phil, but your quip may not be far from the truth. People who have done laminating work in various materials know that it is often possible to create the joints so as to behave in certain ways should they de-laminate for some reason. As a woodworker by trade, who often had to build up wide boards by edge gluing small ones together, I found that if the wood continued to shrink because it wasn't quite dry enough to begin with, it would do so at the ends first - particularly in thicker pieces. So I began using sprung joints. That is, I would make the joint edges very slightly concave along their length instead of a perfect fit. If the ends shrank a bit the wood's natural springiness would hold them together instead of breaking the joint at that point. (Wood usually dries and shrinks much more quickly through its ends than along its length.)

I'm sure the glass will stay put for the relatively short length of time I'm likely to have it. Still it might behoove me to keep the lens either on the nosepiece or in something other than a brass can like it originally came in - just to keep it aired out so to speak. I have one of those plastic boxes for it.

Jim

[Scarodactyl]

This was a great read! Thanks for sharing.

[apochronaut]

So, I cured the cement and took some pictures through the repaired objective and a few others for comparison. I didn't take the time to select an ideal photo eyepiece . I had a Wild 6XK H handy, so I used it....not quite perfect for the apochromats but o.k., nonetheless. The condenser was an unoiled D.I.Y. condenser, I profiled in this thread viewtopic.php?f=12&t=1173&hilit=condenser+quest. Not exactly sure but it probably works at about .95 N.A. Colour correction seems excellent and it was adjusted for Koehler. The higher N.A. objectives were at a slight disadvantage in this demonstration due to the unnoiled condenser but I followed up by testing several oiled condensers of better performance with the one 1.4 N.A. repaired objective, in this subsequent thread, viewtopic.php?f=5&t=7213

I took 5 pictures of a selected diatom and a tag along, through 5 different objectives. The diatom is about 100 microns. Of more telling importance are the sub micron details along the rounded edge in the other partially out of focus diatom.

1) 97X 1.25 coated AO/Spencer achromat from about 1955. This was close to the last AO 160mm 1.8mm oil immersion objective made.
2) 1.8mm 1.30 N.A. Spencer objective from about 1910. No marked magnification but probably around 95X. Probably an achromat.
3) 1.8mm 95X 1.25 N.A. Spencer Fluorite. From around 1925.
4) 2mm 90X 1.30 Spencer Apochromat . This one was remade from 2 different objectives, selecting the best lens cells from each.
5) 2mm 90X 1.40 Spencer Apochromat as profiled in this thread.

[Hobbyst46]

Impressive and successful repair job!
Although I do not quite understand how you overcame the worn out metal front tip of the barrel. Neither as how you aligned the cemented parts along the optical axis.

There are several locations in image no 5, which seem to be all in focus, indicating planarity of FOV vs the images from the other objectives.
The apparent CA in the major diatom may also be in favor of the repaired (vs the other) objective, since diatoms refract light.

[apochronaut]

Thanks for the reply and comments. The front lens is being held in place simply by adhesion. The fit in the bore once reset all the way to the front surface was reasonably firm but not enough to accept any pressure. The lens is quite tall due to it's 1.4 spec., so I was able to get quite a bit of cement around the side surface where there would be no optical interference. Seems quite firmly entrenched and secured right now.

Most objectives have been scribed for optical alignment when assembled. As long as the same shimming is used, where it exists, and the alignment marks are adhered to , there should be no mis-alignment upon reassembly. However, where one might be in doubt as to whether the objective is marked or not; it is always wise to mark them upon dis-assembly. As far as the alignment of the repaired front lens goes; it is a plano convex lens with the back surface in absolute symmetry. As long as the front surface of the housing is undamaged, it can be relied on as a mechanical reference point with which to align the lens to be reset to the rest of the lens stack, given that the front housing has been included in the original alignment scheme. There really wasn't much of a necessity to use a beam in order to align the parts. They were pretty much self aligning.

The ca being noted at the tips of that 100 micron elipsoidal diatom is due to the Wild eyepiece lacking a degree of correction relative to the requirements of the Spencer apochromats. In actual use through the eyepieces, the ca captured by the sensor isn't anywhere as nearly objectionable .

The subsequent pictures taken through a group of oil immersed condensers gives a more complete idea of how the objective performs.

[apatientspider]

Hi, Phil:

Very nice work! Though I was eagerly awaiting these photos, I've held off commenting on them until I could study them carefully and train my eyes a bit by considering things pointed out by others. I don't always know exactly what to look for in image quality as you more experienced guys do, but I do know when one image appears better to me than another - just can't always put my finger on why.

Unless I missed it, you didn't mention whether or not the objectives were oiled to the slide - or does that matter here? The condenser used is not oiled, so the best n.a. it delivers is only 1 or slightly less. In which case, doesn't that mean that regardless of n.a. of the individual objectives, the best any can deliver is n.a. 1.0 or slightly less - whether oiled or not?

To my eye the images produced by the two achromats don't appear to be any different from each other - regardless of their difference in age. The fluorite is a different matter. I think it produces a sharper, slightly more detailed, and more pleasing image. I may be prejudiced here, as I own a fluorite I believe to be the same make, model and vintage; it's a great objective - even when used dry.

The last two photos - those taken through the two apochromats - reveal the superiority of the apos over the others in general. Especially revealing are "... the sub micron details along the rounded edge in the other partially out of focus diatom..." The other three objectives don't seem to even resolve these. Or perhaps that is just due to the apos having a bit higher n.a. than the other three.

Jim

[apochronaut]

I think you are spot on , Jim. Those little micron or sub micron details really show up the difference. ... might not seem important but being able to resolve them does make a huge difference to the observation of very small organisms and communities of organisms.

To some degree it is a case of N.A. but the second achromat of the two is a 1.30, albeit a very old one. Oddly, through the eyepieces it appears to pass a better image than the newer 1.25 achromat; the ca doesn't seem so objectionable but in the pictures it falls behind a bit and in the case of it's N.A. twin the apochromat, quite a bit.