Jury-rig a Betrand Lens?

[linuxusr]

Never having used a Betrand lens, I want to experience it. I know I can look down the ocular, observe the intermediate image plane, and adjust my condenser diaphragm aperture but I want to see if I can achieve better Koehler illumination with a Bertrand lens.

Unfortunately, I just learned that my Zeiss Primostar 3 is not designed for a phase telescope, for example, a slider, and that I would need to buy the next model up which I nave no intention of doing. It always seems, the more you learn, that there's a "better" microscope around the corner.

So, do I have any option to employ a Betrand lens or must I forget about this?

[dtsh]

I have both a bertrand lens and a phase telescope. I would think adapting a bertrand lens not initially designed for the scope would take quite a bit effort and perhaps a bit of luck, but that's speculation. I don't know why a typical phase telescope wouldn't work, it merely replaces an eyepiece and unless the tube were obstructed or surprisingly short, I can't see why there would be any issue. The phase telescopes I've seen are longer than the usual eyepiece, so perhaps that's the issue as I am not familiar with your microscope; otherwise the only snag I can imagine is getting one with the correct diameter.

Lastly, there's a method that I am aware of but haven't used myself involving stacking one eyepiece on top of the other as a make-shift phase telescope.

[linuxusr]

I have both a bertrand lens and a phase telescope. I would think adapting a bertrand lens not initially designed for the scope would take quite a bit effort and perhaps a bit of luck, but that's speculation. I don't know why a typical phase telescope wouldn't work, it merely replaces an eyepiece and unless the tube were obstructed or surprisingly short, I can't see why there would be any issue. The phase telescopes I've seen are longer than the usual eyepiece, so perhaps that's the issue as I am not familiar with your microscope; otherwise the only snag I can imagine is getting one with the correct diameter.

Lastly, there's a method that I am aware of but haven't used myself involving stacking one eyepiece on top of the other as a make-shift phase telescope.

Thanks for this, dtsh. You have given me a lead. I did not know that a phase telescope and a Betrand lens were different. I thought that the terms were synonymous. Now I think I should check for the diameter of my eyetube and then start looking at phase telescopes that might fit. I am blind in one eye and maybe the only silver lining in that cloud is that I keep one ocular removed and capped off, so it's easy for me to fiddle with the eyetube or look down it as I need to do anyway.

I am assuming that the Bertrand lens is designed for the microscope (I could be wrong) but that the phase telescope has a different purpose that will also suit microscopically. Can you clarify?

[linuxusr]

(1) I just went to Google Images and searched "phase telescope" and I see that for the same, sometimes "phase contrast" and "phase centering" are used. Are we talking about the same device except that sometimes the adjectives "contrast" or "centering" are added?

(2) I see that this device is also used for PC and not just Koehler illumination. BTW, we are familiar with BF, DF, PC but is there a short form for Koehler illumination such as KI? It's a big chunk to write out. And how about polarization?

[Dubious]

Lots of descriptions do use the terms "Bertrand lens" and "phase telescope" interchangeably when talking about a phase telescope. My understanding is that if you consider them as separate things, the Bertrand lens works together with the eyepiece, while the phase telescope works in place of the eyepiece--but to accomplish the same purposes. I just googled the issue and here's an explanation that more or less agrees with mine:
https://www.microscope-antiques.com/pteles.html

I agree with Dtsh. I don't see how the Zeiss Primostar 3 could not work with a phase telescope--but I've never used the Primostar 3 so maybe there is something about it I don't understand. If it has removable eyepieces, you should be able to use just about any cheap phase telescope that fits where the eyepiece goes--you take out the eyepiece and put in the phase telescope and adjust it to see the back focal plane of the objective--doesn't need to be from the same manufacturer as the microscope.

[linuxusr]

Lots of descriptions do use the terms "Bertrand lens" and "phase telescope" interchangeably when talking about a phase telescope. My understanding is that if you consider them as separate things, the Bertrand lens works together with the eyepiece, while the phase telescope works in place of the eyepiece--but to accomplish the same purposes. I just googled the issue and here's an explanation that more or less agrees with mine:
https://www.microscope-antiques.com/pteles.html

I agree with Dtsh. I don't see how the Zeiss Primostar 3 could not work with a phase telescope--but I've never used the Primostar 3 so maybe there is something about it I don't understand. If it has removable eyepieces, you should be able to use just about any cheap phase telescope that fits where the eyepiece goes--you take out the eyepiece and put in the phase telescope and adjust it to see the back focal plane of the objective--doesn't need to be from the same manufacturer as the microscope.

Now we can agree that "phase telescope" and " Bertrand lens" are used interchangeably because they accomplish the same purpose but do so my different means where the latter fits, perhaps via a slider, within the ocular, whereas the former swaps out the ocular and replaces it with the phase telescope.

I suspect that the Betrand lens is more convenient and perhaps sophisticated as well for a microscope set up for PC illumination.

My microscope, Zeiss Primostar 3, is a teaching microscope rather than a research lab microscope per se, although I have upgraded it to full Koehler illumination. Therefore, it lacks oculars suitable for a Betrand lens whereas the next microscope a step up in the Zeiss line, the Axio (not the full name), does have this capability. Nevertheless, my microscope will take a PC turrett, has PC objectives available, etc.

I think I'm good to go for a phase telescope. The diameter of my eyetube is 30 mm. I don't know if this is inside or outside diameter but I can measure.

Any leads on where I should buy?

[dtsh]

I don't know I am using the terms correctly, but as I understand it when the lens is mounted below the microscope head, it's a bertrand lens and when used in place of an eyepiece it would be a phase telescope/centering telescope/etc.
The bretrand lenses I've seen are mounted on an arm to allow them to swing in/out of the optical path, which saves some time and hassle, but only works in that microscope.

I bought my phase telescope through eBay, I kept watching for a month or so and eventually one came up at a price I was willing to pay; mine is a LOMO for 23.2mm eyepieces.

[Dubious]

Here's a 30mm one on Ebay for $80.

https://www.ebay.com/itm/292913460623?h ... Swld9dHe7D

[patta]

Some DIY phase telescopes

If you happen to have a camera with a macro lens, you can mount it over the phototube (no eyepiece or other lenses)
And focus it close, on the objective aperture plane.
it works as phase telescope; you either look in the camera screen/viewfinder, of hook it up to the computer.

Also a phone camera, one of the new expensive types with telephoto, stitched over the photo port in the same way, should work ok.

Two eyepiece method: take out one of the eyepieces; twist it back; hold it in front of the other eyepiece, and look in it. Adjust the distance until the phase ring is in focus. Very unstable, but works

Lens method: take a 10x loupe, look with it inside the eyepiece. At some distance, the phase ring is in focus; it is the same as the "two eyepieces"

Telescope/monocular method: if you happen to have a binocular, monocular or telescope that can focus very close, like 150mm. Stick it over the photo port, focus close, and look in it; it is a phase telescope.
If you have around a low-power lens (like, eyeglasses 5+, or a magnify glass), can be glued in front of a normal binocular, so it focuses close enough to be used as phase telescope.

All the previous tested working by yours dearly.
However a true centering eyepiece / phase telescope, as previous posts, is much more practical than those DIY makeshift.

[linuxusr]

I just realized I'm probably mistaken in using the diameter of my eyetube to determine phase telescope size. My ocular is already fitted into the eyetube properly, and a phase telescope will be swapped in for the ocular, so this tells me that I need the diameter of the insertion end of my ocular.

[linuxusr]

@patta

Thanks for the DIY suggestions. Why phototube and not eyetube? My camera is mounted. It would be a problem removing the camera everytime I needed to check.

[patta]

Why phototube, well should be the same.
With the first setup, will be a mess to mount a camera with macro lens over the eye tube, just that. The "double eyepiece" is on the eyetube; crude, but works.

If your eyepiece fitting diameter is 23mm, you can find what you need with maybe some less money than the 30mm.
Keywords: "centering telescope", "Phase telescope", "centering eyepiece"

[linuxusr]

I just realized I'm probably mistaken in using the diameter of my eyetube to determine phase telescope size. My ocular is already fitted into the eyetube properly, and a phase telescope will be swapped in for the ocular, so this tells me that I need the diameter of the insertion end of my ocular.

OP update: I'm probably wrong about this. I just started looking for phase centering telescopes. At the website accu-scope.com it notes that

Inserts into eyetubes with 30.1 mm I.D.

So, yes, I need my eyetube diameter. Also I learn that "I.D." = Interior Diameter

[patta]

Take out the eyepiece and measure its barrel diameter from the bottom with a ruler

[Dubious]

It appears Zeiss sells a 30mm phase telescope for the Primostar 3, called for some reason an "auxiliary microscope":

https://www.micro-shop.zeiss.com/en/ar/ ... 1900400000

I suspect the Zeiss price is way too high; so if it were me, I would just grab a cheap 30mm telescope from Ebay or similar. You could also try one of the ad hoc solutions suggested by Patta--you definitely already have two eyepieces for the back-to-back option.

BTW, I remembered my IMT-2 actually has a selectable built-in Bertrand lens. I did try it a few times after getting the microscope but then went back to using the phase telescope, which I prefer.

[linuxusr]

It appears Zeiss sells a 30mm phase telescope for the Primostar 3, called for some reason an "auxiliary microscope":

https://www.micro-shop.zeiss.com/en/ar/ ... 1900400000

I suspect the Zeiss price is way to high; so if it were me, I would just grab a cheap 30mm telescope from Ebay or similar. You could also try one of the ad hoc solutions suggested by Patta--you definitely already have two eyepieces for the back-to-back option.

BTW, I remembered my IMT-2 actually has a selectable built-in Bertrand lens. I did try it a few times after getting the microscope but then went back to using the phase telescope, which I prefer.

I have found four or five 30 mm telescopes available for < $100.00. Yes, the price of the Zeiss is high. The only question is optics quality as the ones I have found are un-named brands. Still, might be a good idea to try one of the cheaper ones. Since I am blind in one eye, I keep one ocular removed and the tube capped off. I could, therefore, keep one ocular and one phase telescope in place permanently and, thus, very easy to check when fine-tuning Koehler.

I spent a pretty penny for a Zeiss C-mount as I was concerned about optics quality. I'm not sure in this case how optical quality or the lack thereof plays out for this function.

BTW, do you use your telescope for Koehler or PC or both?

[linuxusr]

Take out the eyepiece and measure its barrel diameter from the bottom with a ruler

I will check that. However, the interior diameter of my eyetube is 30 mm and as I look at telescopes, that appears to be the standard, really, the most available choice. I suspect that the diameter of the ocular base will be slightly less, 29.75, or something like that. I'll check anyway.

[Dubious]

Yes, the two standards are 23mm and 30mm, and yours is 30mm. I would just grab one of those <$100 options and give it a try. Mine is a no-name simple slider-type telescope and it works just fine.

I don't think optical quality is really a consideration. The phase telescope is a very basic device doing a very simple job. Even if there is a significant difference in the optical quality of the lenses of one versus another, I doubt you could tell it in normal use.

I use the phase telescope mostly to adjust the phase rings, much less to aid in adjusting Kohler illumination. I once used it to focus on the internal pieces of glass of an objective from Ebay that looked fine from the outside but seemed to lack contrast, and found cracks in the glass--the objective had been dropped once too many times, I guess. It's a useful tool to aid your eye in doing a few things, but that's about it; in fact, you may find using it a bit anti-climactic. As noted by Patta, you could use handheld back-to-back eyepieces; it would just be harder to keep things in focus.

[linuxusr]

Yes, the two standards are 23mm and 30mm, and yours is 30mm. I would just grab one of those <$100 options and give it a try. Mine is a no-name simple slider-type telescope and it works just fine.

I don't think optical quality is really a consideration. The phase telescope is a very basic device doing a very simple job. Even if there is a significant difference in the optical quality of the lenses of one versus another, I doubt you could tell it in normal use.

I use the phase telescope mostly to adjust the phase rings, much less to aid in adjusting Kohler illumination. I once used it to focus on the internal pieces of glass of an objective from Ebay that looked fine from the outside but seemed to lack contrast, and found cracks in the glass--the objective had been dropped once too many times, I guess. It's a useful tool to aid your eye in doing a few things, but that's about it; in fact, you may find using it a bit anti-climactic. As noted by Patta, you could use handheld back-to-back eyepieces; it would just be harder to keep things in focus.

Ha! "Anti-climactic" is probably the word! However, there's just no substitute for experiencing something as opposed to thinking about it. Yes, given its simple function I think you're probably right that optical quality is not of great import. I see that some phase telescopes have a gnurled knob and others do not. Is that for focusing? I appreciate Patta's many DIY ideas. For me, since I am blind in one eye and only use one ocular anyway, it makes more sense to keep a telescope in my "unused" eyetube permanently--very easy to check condenser aperture when I need to. Finally, yes, the CT phase telescope is used principally for PC illumination and not Koehler although for the latter it's a way to get that contrast/resolution ratio as precise as possible. I have also found a reference to using it in DF to center the stop.

[crb5]

patta » Wed Aug 18, 2021 10:53 am

Some DIY phase telescopes

Thanks patta about the tip of using two eye-pieces to make a phase telescope. I made the set-up stable using a 3 inch tube cut from an old vacuum cleaner pipe which had just the right diameter to fit snuggly around the eyepiece barrels. I found a 5x ocular in the microscope and an inverted 10x ocular in the tube gave a good magnification, and by sliding the 10x ocular in the tube I could focus all the way from the light source, through the back focal plane of the objective and into the prism surfaces of the trinocular head. Excellent for checking which surfaces are in need of cleaning.

[linuxusr]

@Dubious
@et al

Dubious, I laughed when you said that when I finally purchased my phase telescope that I might find its use anti-climactic. I finally bought a CT from Fein Optic through microscopeworld.com for about $100.00. At first I thought I wasted my money because I didn't understand what I was doing. Now that I understand it, I'm very happy with this addition. Since I am blind in my left eye, I keep the CT permanently in my left eye tube.

When I look through my eyetube (ocular lens) to achieve one step in Koehler illumination, I am looking at my Abbe condenser diaphragm closed down partway. I then move my condenser lens in the Z direction until the perimeter (lines) of the polygon I am looking at, actually a duodecagon, are razor sharp. I managed with my CT to achieve the same in my left eyetube, then to turn the setscrew to tighten, then to use my diaphragm lever to open the diaphragm to about 75% of FOV diameter, the two parts being light through the vertical axis and the diaphragm's perimeter.

Now I can achieve one more increment in better resolution. All the tutorials on Koehler illumination never say to adjust the condenser level according to that scale but to always look down the eyetube. With my poor vision I couldn't do that before; now I can very easily. Most recommend opening the Abbe condenser diaphragm between 75 and 95%. I like 75%. The latter worked particularly well for histological specimen slices (on microtome).