American Optical Series 20 illumination

Everything relating to microscopy hardware: Objectives, eyepieces, lamps and more.
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PeteM
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American Optical Series 20 illumination

#1 Post by PeteM » Thu Dec 26, 2019 10:51 pm

I recently started to fix up an old A.O. 20 ("H20") microscope, with a bit of help from my friends as they say, and learned a bit in the process.

The A.O. Series 20 is similar to the Series 10 in age and style. What it adds is a bright 100 watt halogen lamp -- something that normally puts one into Olympus BHS, Nikon Optiphot, or Reichert Diastar territory. So, it's a good candidate for methods like darkfield, phase contrast, and even DIC in the case of the A.O. H21.

The Series 20 also adds two convenient filter carousels. This is of less importance in the digital camera age, but perhaps someone has found a good use for them?

The H20 is mostly rugged and reliable design. The adjustable stage lets it accommodate anything from short barrel to DIN standard objectives. Either a Series 10/20 or (if you can find one) the slightly larger field series 110/120 trinocular head can be fitted to it. One could even configure it as a Reichert Diastar with a change of objectives and head. It has A.O.s unique lens protection system. So, it's a pretty useful (that 100 watts) and attractive (to my eye) base for a variety of optics.

The surprise for me is that my H20 had a cracked collector lens. It put a dark line through the field, especially annoying at 4x, and 10x magnifications. Turns out this must have been a common problem, as learned in receiving another H20 carcass with a different collector lens (thanks, Kurt!).

The older and revised versions are shown below. The newer one has four standoffs, a plastic backing piece to take up the extended space, a redesigned collector lens, the addition of a heat absorbing filter, different and larger heat baffles, etc. All designed to keep the collector lens cooler. It looks, for the most part, that the newer type can be retrofitted to the old, once a hole is drilled and tapped to hold the plastic backing piece. In my case, I'll also need to patch some of the plastic parts with epoxy & stainless mesh.

There were also two versions of the AO 2050 lamp holder. One, likely older, illustrated in one manual with a tiny bit of frosted glass secured between the lamp and collector lens. The other, bought separately on Ebay, without out the glass but adding a better spring-loaded contact mechanism.

The bottom line is that if you happen across an H20, be sure to carefully check the collector lens. If it's an older style and happens to be good, I'd either add a heat absorbing filter, convert it to LED or pipe in light from a fiber optic illuminator. If it's the newer style, you might be good to go. Since the usual 100 watts scopes tend to be $500 up, one of the later version H20's still seems a viable option. I plan to use mine with one of the old A.O. darkfield condensers in our "Micronaut" program. A.O.'s darkfield condensers are also generally a bargain ($100-200 less) compared to their Nikon, Olympus, etc. counterparts.

In the pictures below, left side is the old design. Right side is the newer design.
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KurtM
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Re: American Optical Series 20 illumination

#2 Post by KurtM » Fri Dec 27, 2019 1:19 am

Did the one I sent you also have a cracked collector lens? If it did, it sure escaped my inspection...
Cheers,
Kurt Maurer
League City, Texas
email: ngc704(at)aol(dot)com
http://sawdustfactory.nfshost.com/microscopes/
https://www.flickr.com/photos/67904872@ ... 912223623/

PeteM
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Re: American Optical Series 20 illumination

#3 Post by PeteM » Fri Dec 27, 2019 1:31 am

Hi Kurt, Yours is the good one on the right. Thanks!

What the newer design (yours) showed is the A.O. had to reengineer the lamp housing. Every change seen in your example (the standoffs, added heat filter, new cooling fins) seemed aimed at keeping the revised collector lens cooler.

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rnabholz
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Re: American Optical Series 20 illumination

#4 Post by rnabholz » Fri Dec 27, 2019 1:49 am

Hey Pete

I had the exact same issue when putting together my 20. Discovered the two versions with and without the stand offs.

I wrote about it starting at entry 105 of this thread: viewtopic.php?f=24&t=776&start=90

Worth the bother though, great stand.

Good luck with yours.

Rod

apochronaut
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Re: American Optical Series 20 illumination

#5 Post by apochronaut » Fri Dec 27, 2019 1:51 am

I guess you now see what I meant by the dark grey collar over the illuminator/body seam separating one version from another, now, Pete?

As much as I like the 20, and I have had 5 or 6 go through my hands, it isn't as versatile an instrument as the 120. They can use the same optics but if one wants to fit a 20 out with 45mm parfocal objectives, there is very little room left under the stage. They also have the curious quality of only being fitted with an abbe condenser. There are workarounds for that , using a 1240 condenser body with either a .90 achromat or a 1.30 achromat. The 20 is Koehler, the 10 is a modified Koehler.

The 20 was envisioned when transmitted fluorescence was still very much used(1964 or 5). The filter carousels are available, primarily for glass fluorescence filters, although a series of N.D. filters in there is helpfull when one wants to reduce the illumination . You can't back off 100 watt halogen much below 90% without ruining the bulb. They also provided an incident fluorescent unit with a 100 watt lamp, which used a similar but different socket.

PeteM
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Re: American Optical Series 20 illumination

#6 Post by PeteM » Fri Dec 27, 2019 2:46 am

Thanks, Rod. Looks like you encountered (and solved) the same cracked collector lens problem.

Phil, good info as always.

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