The Novitiate’s Odyssey

Episode Six: How not to see double. Collimating the older Spencer Model 20 Series stereomicroscopes.

by G. Joseph Wilhelm, Florida Keys, USA

( Editor's note: Previous episodes - part 1, part 2, part 3, part 4, part 5. )

By the time you read this, a new year will be upon us and as happens every year in late December after the 25th the local Florida Keys community suffers from holiday season postpartum depression. They discard the comforting values of Christmas and revert back to the general consensus that life is way too important to be taken seriously and beer is a food group. This usually happens just in time for New Year’s Eve. It was the interaction, on January 1^st, with several individuals who were obviously over-served the previous night and could not focus or see straight that led to my convoluted inspiration for this commentary.

In previous articles I have touted the virtues of the venerable Spencer Model 20 Series Stereo-Scope. The availability, price, simplicity, versatility and quality of these instruments make them ideal subjects for the home enthusiast to tinker with. The disassembly and cleaning of the upper end optics (eyepieces and prism housing assemblies) was shown in “Episode Two” and a breakdown of the variant stands and available objectives/eyepieces was given in “Episode Five Pt 1”.

As near as my Hercule Poirotesque investigative abilities have been able to deduce, this model series had a production run of approximately twenty years from the late 1930s to the late 1950s. While the stand had improvements and minor developments during this time, all of the objectives, eyepieces and major components appear interchangeable. This article is specific to the Spencer Greenough design and unlike most of the latter 20^th century stereomicroscopes; the objectives are not an integral part of the overall stand.

(For a full explanation of the Greenough style and its comparison to the more modern Common Main Objective, or CMO, design, please see Mr. David and Ian Walker’s excellent article and the cited MicroscopyU article.

While the Spencer paired objectives appear quite robust, they are easily removed or replaced from the rotating nosepiece (or fixed single mount). This ultimately results in them being knocked about, dropped or otherwise neglected with a loss of collimation as the usual end consequence. It is to this particular state of malfunction that I offer some redress.

If I may quote Spencer Care publication of 1938 which states:

“If the stereoscopic microscope fails to give a fused image or causes eye strain the objectives may have become de-centered and this may be tested readily by focusing on a ruler or some other object with a straight line. Move the line to the extreme left side of the right eye’s field. Then notice whether the line is in the corresponding position in the field for the left eye. Test the lens also by moving the ruler to the top or bottom of the field. If the line is not in corresponding positions in the two fields the objective should be sent to the factory for proper adjustment. The lenses should be adjusted only by a person who has had factory training.”

Now since the above qualified personnel are probably resting in peace beneath the sod and if not, the cost of the service should exceed the value of the optic we have the perfect rationale for a DIY solution.

Now as you can see in (Fig.1) below there are few differences between the early (black) and latter (gray) objectives. The independently focusable lens element assemblies are pared in a cast metal housing, which has three centering setscrews for collimation (Fig.2). These setscrews are either .050 Allen head or slotted head. In either case, before attempting to loosen these for adjustment I highly recommend placing just enough PB Blaster to wet the screw head and threads projecting from inside the housing. This is way less than a drop and can be accomplished with a small syringe fitted with as fine a needle as you can obtain. Then wait at least 24 hours. Remember, these screws have probably not been moved for 50-60 years and may require a repeat of this application before these recalcitrant fellows acquiesce to external coaxing. The slotted head screws were in the earlier manufactured objectives and seemed to be seized tighter than the Allen head screws. The slot is less of a mechanical advantage, requires a good quality jeweler’s screwdriver and in extreme cases will require drilling out and re-tapping, usually to the next size up.

Figure 1

Figure 2

Without benefit of the standards for alignment used by the Spencer factory it is entirely up to the user as to which lens should be adjusted to the other. At such low magnifications having the paired objectives par center with other paired objectives is not critical. The deciding factor for me is which set of the three adjusting screws I get loose first. Collimation is then accomplished by having a stable reference point secured to the stage just inside the extreme left and top field of view in the stationary lens. Loosen and adjust the other as you would for centering a stage/condenser until the images are identical. As can be seen in (Figs. 3&4) the side adjusting screws are visible and the front and back screws are cleverly hidden behind the name and magnification plates. These plates are either screwed in place or held on with contact cement.

Figure 3

Figure 4

Of the ten paired objectives I have, only two required collimation and none required any focus adjustment. All of them seem happy on any of the three 20 series stands, which also speak well of the ruggedness and consistent alignment of the upper end optics.

And so ends this simple but uncomplicated procedure.

Regards,

Joseph Wilhelm

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Published in the January 2011 edition of Micscape Magazine.

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