The Olympus CH2 CHSP Petrographic Polarizing Microscope
Greg McHone, Grand Manan, Canada
As a geologist who likes to renovate microscopes, I have an interest in polarizing petrographic models that are specially equipped to study thin sections of rocks and other earth materials. As you may know they are also useful for other purposes such as forensic geology, organic crystallography, and materials science, as well any typical brightfield work. Other Micscape articles describe and illustrate some of them, which you can find via named links in this text or by searching the Micscape index.
In addition, I am particularly fond of microscopes made by Olympus after the 1970s, of which there are always several in my shop and lab including a big BX50-P made in the 1990s. It took me a long time to afford, acquire, renovate, and equip that one, which is the best microscope I have ever owned for sure. But most people do not need a major research machine like the BX50. A smaller, older, less complex, and much less expensive microscope will do just fine for most people for hobby and research investigations. This article is about one that came my way recently. As usual, it was a matter of making a good microscope using parts and pieces salvaged from several bad ones. I will describe steps to rebuild the instrument, and the parts that were needed. They are labeled in the first photo.
Photo right: Olympus CH2 CHSP after reconstruction, with special pol parts labeled.
Olympus Student Pol Scopes
Olympus produced a nice old-style student petrographic microscope from the 1960s called the POS, which I briefly described in this September 2008 article. It had a horse-shoe foot, mirror lighting, and single-objective clips with short-barrel objectives, all anachronistic today. But it was very well made and still listed in their 1996 catalog, along with the new BX-P (essentially my BX-50-P). A striking contrast!
However, the major modern-style Olympus "small microscope" was the CH model, which was developed in 1976 and continued after 1986 as the CH2. It resembled a smaller version of the bench-top BH/BH2 models including most of the same optics, and with a corresponding switch from a "warm gray" finish to "Olympus beige" around 1980. The CH/CH2 line was intended for use in clinical labs as well as student instruction, and for geology labs there was a CHA-P model to correspond with the BHA-P (the larger bench-top model for petrographic work). It must have come out after 1980 because I think it was always a beige color, but it used the BHA pol versions of condenser, stage, objectives, analyzer tube, and eyepieces as well as smaller 4x12 mm compensator plates. I have illustrated a CHA-P (with later long-barrel optics) on my Olympus microscopes website.
The CHA-P polarizing petrographic microscope is very rare; I have found only one and an instruction manual for it. I do not recall any of this model in the geology labs where I worked or visited from 1968 to 2008. Apparently they were not a stock item at dealerships but were a special order from the factory. The CH2 petrographic polarizing version, which came out after 1986, was based on the CHS version that used a 20 watt quartz halogen lamp rather than the 30 watt tungsten filament lamp of the CHT version. The 20 watt halogen lamp is directly beneath the field lens and provides a brighter light than the 10 watt halogen of the older CHA model, and more than the 30 watt tungsten lamp too. But it also puts out a lot of heat, which can be a problem for long periods of use. Olympus tacks on a P to the model name to designate the petrographic version. In the late 1990s Olympus replaced the CH2 line with new CX models that feature infinity optics and a fashionable new stand shape.
Parts and Pieces
An Olympus parts catalog with prices was issued in August 1992 for the BHSP, BHTP, and CHSP polarizing petrographic models. The CHSP system with 4x, 10x, and 40x objectives, binocular head, and modular petrographic polarizing parts (as opposed to a simple pol system with two polarizing filters added to a "normal" biological model) had a list price of $4,207 USD. Within that system, the intermediate tube with analyzer and Bertrand lens (part BH2-PA) cost $1,272. The same BH2-PA, WHK 10x eyepieces, and PO D Achromatic objectives were also standard on the BHTP model (system list price $5,467) and BHSP model (list price $9,897), but a BH2-CD condenser, BH2-SRG circular stage, BH-PRE centering nose turret, and CH-Bi45 binocular head were all modified at the factory for the CHSP model. These expensive parts and the trouble required to manufacture this microscope led to its relatively high sale price, which you may note was not all that much less than the large bench top BHTP model. Not to mention a long lead time to receive orders from the factory in Japan. No wonder if the CHAP and CHSP had low sales volumes. Yet, this is certainly a high quality microscope, very functional and desirable.
Thus my interest at seeing a CH2 on the popular internet sales site that sported a circular stage, centering nosepiece, and crosshair eyepiece, all hallmarks of a petrographic microscope. It had the trinocular version of the CH2 head, but no intermediate analyzer tube or lower polarizer. The buy-it-now price of $299.90 USD was cheap for a petrographic microscope, but this was in such rough shape that the seller advertised it "for parts only". It certainly looked like a basket case in the sale photos. The two DPlan objectives (4x and 10x) with it were also rough and not the pol versions, although they cleaned up well. However, standard DPlan and A achromatic objectives are usually about strain free and work well with polarizing applications. I no longer worry about the "PO" designation for using them.
It had the critical circular stage that I knew to be durable, and in my spare parts bins I had an early-style Olympus intermediate analyzer tube, additional objectives, WHK eyepieces, and compensator wave plates. I also owned two CH2 stands with various problems but still good for parts. With a decent potential for renovation or reconstruction, I bought it.
The microscope arrived packed pretty well with bubble wrap, but I could see why it was listed with a warning of "parts only" condition. The finish on the stand and every part on it had been badly mauled by acid or some other destructive gas or liquid, with bubbled paint, corroded plating, and rusted metal including all screw heads. I cannot imagine how anyone could use it in such a caustic environment, or why an expensive instrument had been abused this way. However, the stage rotated smoothly as did the nose turret, and the optical surfaces were not bad. The lighting system worked except that the dial to adjust intensity did not turn. I always try to restore projects as near to "like new" as possible, but there was no way to get this one very close to that. But it could still be decent and functional. I took the microscope apart for cleaning and fixing, starting with the electrical base.
Photo left: Corroded centering nose turret.
Photo right: CHS lighting base interior.
At first I thought the light intensity dial would not turn because the rheostat was stuck, but I found that the problem was actually in the axis where the dial is fastened to the base. I took the dial off and removed corrosion with steel wool, then added a bit of grease, and then it worked fine and turned smoothly. If only all fixes were so easy! The plastic pulley band is the most fragile part here, but this one was pretty good for its age. The interior escaped most of whatever corroding fumes damaged the outer surfaces.
The stand would require a complete paint job to look decent, but with my tools and ability it would be far from a real factory finish and probably not a perfect color match either. Another serious problem with it was the focusing mechanism, which no longer functioned as smoothly as it should. It was an obvious decision to substitute one of my spare parts CH2 CHT stands, which appear to be identical except for the lighting base and plate on the back. The parts came off the damaged stand and went onto the better stand. Simply by installing the 20-watt halogen base and swapping the name plates, my CHT parts stand became a CHS pol stand.
The next step was to remove and repaint the BH2-SRG circular stage. The stage is made in upper and lower sections that easily detach, and this is a simple design that uses a greased surface connection for rotation in the upper section. The modification made by the factory to put a BH2 circular stage onto a CH2 stand was immediately apparent, as you can see below. On the left is a lower stage section made to clip the SRG circular stage onto a standard BH2 stage holder (this one is missing one of the stage centering bolts). On the right is the modified base section from the CHSP stand. The standard ring-shape of a BH2 stage holder allows even the rectangular biological stage to be rotated a fair amount by loosening a thumbscrew. But the CH2 stage holder is completely different as it uses a 4-screw attachment for its common biological stage. Also, the base section of an unmodified circular stage would not sit low enough on the CH2 stage holder. The drastic factory modification was to cut away a section of the circular stage base at a width to slip snugly over the CH2 stage holder, and drilled for the stage screws. However only two of the normal four screws are used‚ a compromise, but still secure enough.
And so far, so good.
The stage surface was in poor condition. I removed the top section and the centering bolts, cleaned it, and smoothed the surface and bolts as much as possible with fine steel wool. Then the outer rim section with its graduated degree marks was masked off with plastic tape, which can be stretched around curves. I used good-quality flat black spray paint on this surface, and after drying the tape was removed and the piece baked in a toaster oven for a half hour at 225 °F. It made a smooth durable surface on which thin sections will slide easily. I used a semi-gloss black enamel paint to touch up the scars and scrapes around the graduated edge and the round surface below it. The photo below shows the stage before and after repainting.
By chance I had a prized pol-type mechanical stage, Olympus part C-FMP made to fit this BH2-SRG circular stage, which is smaller than the ball-bearing BH2-SRP circular stage that was an option for the BH2 models. It works well except in certain positions it can interfere with (bump) the tips of longer objectives‚ users need to take care about that. It appears in the photos of the finished project at the top and bottom of this essay. I like mechanical stages but many petrographers are just as happy to move the slide around by hand, secured when needed by stage clips.
The centering nose turret is attached by two screws to the stand beneath the head. It should be four screws but two are spaced too far apart on the stand to fit the turret. This is a standard BH-PRE turret with 3 of 4 objective holes centered via small spring-loaded side screws (see the photo higher up), with the dovetail connector removed. Olympus provides small thumb-bolt wrenches for the centering screws, but since those tools are easily lost, two small screwdrivers will work well enough. Once centered they stay pretty well. I do have a spare centering nose turret which looks very pretty, but one of the centering holes has a bad spring. The corroded turret works OK so I decided to use it after cleaning and removing as much rust as possible, then painting the curving outer face a semi-gloss black and baking it like the stage. It is not as nice as an original brushed-chrome finish (the part is actually made of plated brass) but as I say, it won't be possible to return this microscope to "like new."
Left photo: Three Olympus pol scope condensers. The left one is the CHSP condenser modified from a BH2-CD by the factory; center front is a 1970s era BH-POC pol condenser with a flip-up n.a. 0.9 lens and rotating polarizer; right rear is a U-POC pol condenser for my BX-50P, similar to the BH2-POC.
The CHSP came with a BH2-CD achromatic condenser (n.a. 1.25) that was modified to fit a ring holder with a larger 38.8 mm ring rather than the old CH standard 36.8 mm ring. It has a metal tube neck with a groove for the set screw of the condenser holder. This one was in decent optical condition although the upper plastic surfaces were pretty rough. But it has no flip-up top lens that is very helpful to concentrate light for a good conoscopic image. And no attached polarizing filter; instead the CHSP used a simple pol filter placed over the field lens (and the filter was not part of the purchase). In my opinion this was a big step down from the nice BH-POC pol condenser that was used in the 1970s to early 80s for both the BHAP and CHAP microscopes.
But by chance that older type of condenser came up on good old eBay. Although rather expensive I did a "buy it now." It arrived in very good condition except for a small mark or two on the pol filter, which is nothing of consequence. It fits the 36.8 mm ring of the condenser holder of my "new" CH2 CHSP stand so I did not have to swap that part of the holder nor renovate the old holder. I painted a red dot on the condenser holder to show the proper location above a red dot on the condenser, so that its pol filter is aligned properly at the zero graduation.
Right photo: an older BH-PA intermediate analyzer tube (left) and larger BH2-PA tube (right), both with dial-in/out Bertrand lenses, sliding analyzer filters, and graduated rotating polarizing filters. Respective wave compensator plates are in front of each tube.
Another part that did not come with my CHSP purchase was its extremely fine (and expensive) BH2-PA intermediate tube, which has the slide-in analyzer (upper polarizer) and also a focusing Bertrand lens. But I already had one, as well as an older and slightly smaller BH-PA version from previous model years. That older one has a slot to use smaller 4x12 mm wave compensator plates, and I happened to have some of those too. I decided to save my BH2 version for a future project and use the older version here, even though it was not original equipment for the CHSP. It works well and I think it is still appropriate for the CH2 line.
The trinocular CH2 head that came with my CHSP was not listed as an option in 1992, but it worked well despite a lot of cosmetic damage. I spent extra time cleaning and smoothing the finish. I doubt this is the original head because both eye tubes had diopter focusing rings, and no slot for the pin that holds the crosshair objective in line with the microscope axis. You do not need two diopter tubes when using the WHK crosshair reticle eyepiece, and it has one built into the eyepiece for focusing on the crosshair. Once that is sharp, you can focus on the sample view for that eye by the microscope knobs, and then use the diopter in the other eyepiece tube to focus the view for its standard WHK 10X.
I had a binocular CH2 head for parts, and I took out its straight right eyepiece tube along with the attached interior prism, and swapped it for the right side diopter tube and its prism in the trinocular head. Then I carefully filed a notch into the bottom edge of the new eyepiece tube to fit the eyepiece peg (photo to the left). I worried that I might have wrecked the collimation or view alignment in this head by swapping major part assemblies, but it looks and works fine. Olympus doesn't need special strain free heads for its pol scopes as required in many other brands, because there is a quartz filter built into the BH/BH2-PA intermediate tubes that removes extraneous polarization. The filter only allows through the light that is polarized in the proper vibration directions, which are not affected by the prisms as oriented in the head. Very nice.
Large patches of black finish were etched from the metal eyepieces. I swapped the standard WHK 10x left eyepiece with a better one from my collection, but I wanted to re-use the crosshair eyepiece so I had to touch it up to improve how it looks. It is not perfect but not bad either, and the optics are fine.
A Camera System
The great value for a trinocular head is of course for photography. I have been experimenting with small and relatively inexpensive consumer cameras for my Olympus microscopes, as described in a previous Micscape article on camera gear. One that works well on this microscope is the Olympus C-5050 Camedia (5.0 MP). I like its swivel view screen, infrared remote control, and high quality. Olympus made microscope adapters for this camera, starting with a C3040-ADL screwed into the camera filter thread, then a U-PMTVC c-mount adapter connected to a flange adapter that fits over the trinocular top tube (see the photo at left). For a projection lens an older Olympus WF10x eyepiece is currently inserted into this tube adapter, but in fact a BH2 type Olympus eyepiece might work a bit better as it will make a final correction for color aberrations.
The photomicrograph images (photo to right) are of a basalt thin section using this camera system, in plain (single filter) polarized light on the left, and crossed polarized light (both filters) on the right. Most of the minerals are strongly birefringent pyroxene (augite) grains, while the pale gray low-relief mineral at the top center is plagioclase feldspar. The image is satisfactory for the most important purposes of mineral identification, also for textures and structures.
Pricing it Out
Perhaps you can now appreciate why petrographic polarizing microscopes are so much more expensive than standard biological versions of the same models. It is because of all those special parts! List prices in 1992 for the BH2-POC condenser ($785 but not what I am using), intermediate analyzer tube BH2-PA ($1,272), whole-wave compensator plate AH-TP530 ($231), circular rotating stage BH2-SRG ($465), mechanical stage C-FMP ($511), and quadruple centering nosepiece BH-PRE ($459) total an impressive $4,182 in US dollars, which is the equivalent of $7,104 in 2014. And that does not include the (strain-free) objectives, eyepieces including a crosshair reticle type, trinocular head, and stand with an extra-bright lighting system. I added two Olympus objectives from my parts box, an A 20x and a DPlan 40x, which are a good match for this system and exhibit no problems such as strain effects.
I could never afford all this new, and maybe not even used except that I maintain a "hobby fund" by selling projects and surplus parts. My cost for the rough microscope and the replacement pol condenser came to $525 USD, plus shipping and taxes. I do not know what the camera system cost when new, but the pieces now would total about $400. More to the point, it would not have been practical to reconstruct this CH2 CHSP as it came, except that I have been collecting parts for quite a few years and happened to have the ones needed to make this into a good petrographic system. And those parts are valuable. This microscope is not for sale, but if it was it might fetch $1,600 to $1,900 USD (less than in "like new" condition), or more with the camera system, depending on interest in what I think is a waning market. Or if I sold the parts and pieces separately, several hundred dollars additional, but I would rather build them up than tear them down. It is not a business for me now, just a great hobby!
Feel free to contact me at greg "at" earth2geologists "dot" net with questions or comments.
J. Gregory McHone, PhD, CPG, Grand Manan, NB Canada
The finished CH2 CHSP project microscope.
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Published in the September 2014 edition of Micscape.
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