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Why there is a tube lens in my finite microscope?

 
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kit1980



Joined: 18 Aug 2016
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Location: Bellevue, WA, USA

PostPosted: Fri Feb 23, 2018 8:38 pm    Post subject: Why there is a tube lens in my finite microscope? Reply with quote

I use Nikon CF 160mm finite objectives on a finite 160mm Omax microscope.

I'd like to eliminate all Omax optics from the photo image path. Now I do a "direct projection" on a micro four thirds camera, but there is still some kind of tube lens inside the microscope in the head (and also some thick glass in the nosepiece, bit it appears to be plain glass, not a lens). I suspect this lens adds various aberrations.

I roughly measured distance from objective rear end to the camera sensor (which is parfocal with the objectives), and it seems to be ~200mm instead of 150mm. So it looks like the lens makes the tube optically shorter than it actually is. When I removed the lens, I could get image in focus only for lower-power objectives, and the working distance was greatly reduced.

Why the lens is incorporated in the design? Maybe the body was designed first for 210mm metallurgical objectives and supporting 160mm was afterthought?
Can such a lens be a significant source of aberrations? Any ideas how to test/improve?
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Alan Wood



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PostPosted: Sat Feb 24, 2018 12:59 am    Post subject: Reply with quote

Please post a photograph of your microscope.

If it has a binocular or trinocular head, there will be prisms in the light path. The refractive index of glass is higher than air, and lenses are needed to achieve an apparent tube length of 160 mm.

Olympus designed their BHT microscope with a non-reversed nosepiece. When they introduced the BHTU with a reversed nosepiece, they had to add a glass element to compensate for the increased tube length. With a proper design, this has no effect on image quality.

Alan Wood
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kit1980



Joined: 18 Aug 2016
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Location: Bellevue, WA, USA

PostPosted: Sat Feb 24, 2018 1:08 am    Post subject: Reply with quote

Alan Wood wrote:
Please post a photograph of your microscope.

If it has a binocular or trinocular head, there will be prisms in the light path. The refractive index of glass is higher than air, and lenses are needed to achieve an apparent tube length of 160 mm.

Olympus designed their BHT microscope with a non-reversed nosepiece. When they introduced the BHTU with a reversed nosepiece, they had to add a glass element to compensate for the increased tube length. With a proper design, this has no effect on image quality.

Alan Wood


Thanks!
Yes, it's trinocular: https://www.amazon.com/gp/product/B00UCEPH70
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ChrisR
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PostPosted: Sat Feb 24, 2018 1:52 am    Post subject: Reply with quote

I have noticed what appears to be parallel-ended block of plain glass in a couple of "finite" microscope models: the Olympus CH3 and the Nikon Diaphot inverted. The Nikon's is integral with the turret. I posted a picture of the Oly's which is or the order of half an inch/13mm thick, some time ago.
I suspect it's a tube-length correction but I don't know exactly what it does. Surely it would have an effect on all the relevant aberrations.
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Pau
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PostPosted: Sat Feb 24, 2018 4:49 am    Post subject: Reply with quote

As Alan says this must be to match the mechanical tube length with the optical tube length (no needed in infinite corrected microscopes)
To do so it must be a very weak divergent lens system. At first look it could seem just flat glass but this has no sense. If of good optical quality it will do nothing about aberrations at least within the 18mm image circle typical of this kind of microscopes, IMO.
Very likely the microscope will use the same binocular heads than non inverted turret models like in the Olympus BH series case, as anyway inside the optical path of the binocular head there is in many cases this kind of correction.

Because of the presence of this optics you can't accurately measure the mechanical length, for direct projection use you can find the right position as the one where different magnification objectives of the same series are parfocal.
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zzffnn



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PostPosted: Sat Feb 24, 2018 6:36 am    Post subject: Reply with quote

Yes, prism or weak lens to correct tube length.

Per Curtis Sleve (who deals with Nikon scopes a lot), the lens in Diaphot inverted scope's turret is for correcting tube length. I have seen such lens in an Optiphot turret that leans scope arm (instead of the other way; said "the other way" provides a shorter tube length).
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Choronzon



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PostPosted: Sat Feb 24, 2018 11:59 am    Post subject: Reply with quote

n phase corrector on those cylindrical glass tubes in heads and prism turrets. If they were not there, with Pol and DIC, the light speed would be different from the path with a prism.
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Pau
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PostPosted: Sat Feb 24, 2018 12:49 pm    Post subject: Reply with quote

I seriously doubt that a high school grade microscope like the Omax would have such plate, it isn't designed for quantitative pol nor DIC
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Choronzon



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PostPosted: Sat Feb 24, 2018 1:38 pm    Post subject: Reply with quote

Thats because its a lens, not a phase corrector
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Choronzon



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PostPosted: Sat Feb 24, 2018 4:48 pm    Post subject: Reply with quote




This is the exploded view of the inner optical prisms of an early 50's Zeiss Jena Seidentoph binocular head. The glass cylinder is the phase corrector element. It has no diopter power, just a refractive index matching block to fill the air space on the side with less glass. Its purpose is to equalize the wave front speed.
Inverted microscopes with light path selector clusters with prisms, need the same glass cylinder for the same reason when the light path is going straight through to the bottom, or up to the viewing head.
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ChrisR
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PostPosted: Sun Feb 25, 2018 3:17 am    Post subject: Reply with quote

Would the "phase correction alone" explanation fit for a pretty basic Olympus CH30 too? The glass part looked similar to that one.
I have a couple which came with video cameras above, and just s 2x objective fitted.
When I examined the glass part at the time I found it, I convinced myself it had no diopter
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enricosavazzi



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PostPosted: Sun Feb 25, 2018 3:46 am    Post subject: Reply with quote

ChrisR wrote:
I have noticed what appears to be parallel-ended block of plain glass in a couple of "finite" microscope models: the Olympus CH3 and the Nikon Diaphot inverted. The Nikon's is integral with the turret. I posted a picture of the Oly's which is or the order of half an inch/13mm thick, some time ago.
I suspect it's a tube-length correction but I don't know exactly what it does. Surely it would have an effect on all the relevant aberrations.

Zeiss did the same in the Photomicroscope models. The slider on the head contains a glass block with parallel faces in the position that sends light to the external camera mounted on top of the head.

I believe the microscope optics are corrected to eliminate aberrations along the paths to the eyepieces and internal camera (both of which contain prisms), and the glass block simulates the optical effect of the prisms in the path to the external camera.
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