Question about filter slots on Olympus BH2-RLA vertical illuminator

[microcosmos]

In another thread, @mrsonchus posted this image of an Olympus BH2-RLA vertical illuminator that shows three filter slots. One is about 6/7 mm thick and has a BD filter inserted, while the other empty slots are narrower, about 4 mm thick:

Does anyone know what filter mount fits into those two narrow slots? Has anyone ever seen one? I have an RLA and am trying to make use of them. I couldn't find any information anywhere in the Olympus documentation or elsewhere online. Olympus' own brochure just shows the empty slots, saying they are filter slots, and doesn't list any part that fits in them.

I know these slot shapes are found on some other microscopes for you to simply drop in a generic round filter that then rattles around, but did Olympus make a specific mount for these slots?

[PeteM]

Your choice - a polarizer, color filter, etc.

I've seen mounts similar to the BD filter shown, with a squared top to ease putting them in and out. Other makers use a little post threaded into the peripheral ring. Anything that fits and has a way to fish it back out should work.

[microcosmos]

Thanks for your comments. The ring with the threaded post works, but it drives me crazy from a mechanical engineering and design point of view. There is enough space inside the tube for the filter to accidentally roll, pitch or yaw out of perfect alignment, even to the point that it becomes parallel to the illumination axis.

The best design I have seen for this kind of slot is something like what they use for the drop-in filters on supertelephoto camera lenses:

Screenshot 2021-08-21 at 09.33.55.png (211 KiB) Viewed 6007 times

The tab completely stops it from moving in any of the three dimensions, a very elegant engineering solution. It looks nicer than the rectangular tab of the Olympus' BD filter, as it maintains the tubular look of the whole setup. It doesn't have to have the locking mechanism shown in the image, since the microscope is stationary and does not get manoeuvred like a camera lens.

I will try to design a 3D-printed version using CAD software and update again when I get to it.

[PeteM]

I believe (would have to double check) the BH2-RLA a groove in the filter slot position. So if you make up a circular mount for a filter that's the right thickness and diameter, it should stay in place.

The plus of a ring with the threaded post design is that it allows rotation of a polarizer/analyzer or a wave plate.

[microcosmos]

I went to take another look at my RLA. There is no groove! They could have designed it with a groove, which would have been a simple fix that allows the mount to be of slightly larger diameter than the bore of the tube.

I will have to think about how to incorporate a rotatable polarizer, which I actually need.

[microcosmos]

I am thinking of this elegant design for the polarizer:

https://www.ebay.com/itm/324223325680

It immobilizes the whole filter mount so it doesn't move in directions you don't want it to, but you can still rotate the polarizer, and there's a degree scale.

This particular one is too big for the RLA and the tab doesn't widen out low enough.

Olympus made a BH2-MPO polarizer that supposedly came "standard" with the RLA, but there are no pictures of it anywhere, not even in the Olympus brochure for their metallurgical microscopes that shows an RLA without any filters.

Does anyone have this BH2-MPO or know what it looks like?

[microcosmos]

As I have been unable to find the original filter holders, I decided to draw them in CAD software with a slightly nicer design (smaller rectangular tab whose long edge is flush with the outer circumference of the mount instead of slightly raised) and get them fabricated. Here're the blueprints so far:

Screenshot 2021-09-28 at 21.22.33.jpg (45.89 KiB) Viewed 3239 times

There's one for the thinner slots and one for the thicker slots.

I was planning to simply fix the polarizer in place in the E-W position, as my analyzer is rotatable 180 degrees so I will still be able to control the degree of extinction. I have seen a photo of the original Olympus polarizer and it is fixed as well.

Instead of sticking it down with E6000, I might try small bits of Nano Magic Tape which is extremely sticky but does not leave any residue after a long time and can be picked off with a toothpick without using solvents.

I am also toying with the idea of incorporating a snake-eye screw ring that screws into the mount to hold the filter in place so I don't have to glue it down. The filter I am getting is 2mm thick while the available depth in the thick mount that goes into the slot meant for the pol filter (I have switched from a BH2-RLA to a BH2-MA so the number and position of slots are different) is 6mm so there's 4mm of allowance for the screw ring. But this is probably going to be expensive and not worth it besides giving me some training in drawing screw threads.

[microcosmos]

I've sent the designs to be 3D-printed using multi-jet fusion of nylon PA12 dyed black, which gives a matt black finish to minimize reflections and does not have visible 3D-printing lines. Unlike the modified filter holder and modified dovetail mount, I didn't go for CNC this time as the components are non-load-bearing and/or do not require high precision - Olympus' own holders fit quite loosely.