American Optical Stereo Zoom; Accessory Thread

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As mentioned in a S.E.D. posting in the "Re: Digital Microscope purchase"  
thread on 23 June 2019, I recently purchased a well used Model 570 StereoStar  
Zoom. As it turned out, this unit had been dropped, cracking the housings  
where the erecting mirror boxes (one per eye) live. I purchased on eBay a  
for-parts-only model 570 body to get those housings, and was able to assemble  
a perfectly good model 570, plus a spare set of optical components. Missing  
are some knobs and screws.

By the way, one cleans the optics with 91% Isopropyl Alcohol from the  
drugstore, using cotton balls and/or Q-tips. Do not use any kind of paper, or  
anything sold for eyeglass lenses. Old 100% cotton teeshirts, washed and  
dried (use no antistatic stuff), are also used. And note that the mirrors are  
coated with pure aluminum on the front surface - very easily scratched.  
Remove fog, but particles here and there are harmless.

It is very useful to be able to attach things to the Accessory Port (the ~  
1-7/8" diameter threaded port in the nose of the body), but plain adapters  

thread, and with the right kind of lathe, one can make any thread, standard  
or not.

not talking, or still exist.

So, time to measure. Long story, but it turns out to be a mashup of  

posting is to publish the details of this thread, so others can also make or  
have made screw-in accessories.

The thread form is a standard American Unified 60-degree V shape. It is _not_  
55-degree Whitworth (widely used for microscope objectives.)

There are 36 threads per inch (tpi), regardless of diameter. It is _not_ the  
Metric 0.75mm pitch used for optical filters et al; this is equivalent to  
33.87 tpi threads, and so closely resembles 36 tpi threads, but they will not  
mate. Nor can one tell them apart by eyeball, unless one has a thread gage.

The nominal diameter is 1.875" (47.625 mm), not 48 millimeters.

The above emphasis on the non-metric thread details is because the modern  
stereo microscope accessory thread is M48x0.75. This is often wrongly called  
two inch, which is close (to this and a few other kinds), but a M48x0.75  
external thread will not screw into an AO Stereo Microscope accessory port.

The thread length is 0.125" and there must be a shoulder (where the  
diameter increases abruptly from 1.875 to 2" in diameter) against which the  
adapter will bottom, to prevent screwing the adapter in too far and damaging  
the optics within.

I have machined such an adapter, and the threads mate perfectly. In the  
newer, cleaner body, the adapter turns freely but is a snug fit. On the older  
body, the fit is slightly loose, but is still secure.

For the record, I measured the Pitch Diameter (PD) of the newly machined  
adapter thread using the three-wire method and micrometers. The measured PD  
is 1.874". This will be useful to anyone machining the thread, to tell when  

This adapter is now used to mount a 56-LED ring light, which works better  
than the original oblique illuminator. AmScope WR63HWR on Amazon, $26 or so.  


Joe Gwinn

Re: American Optical Stereo Zoom; Accessory Thread

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Ouch. The Pitch Diameter is *not* 1.874", it's 1.845".

Joe Gwinn

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