Reflective objectives - reflachromat

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JohnyM
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Joined: Tue Dec 24, 2013 7:02 am

Reflective objectives - reflachromat

Post by JohnyM »

Some time ago i've bought cheaply an Reflachromat 15x/0.58 which was forgotten and only after recent discussion on this forum i've dusted it out.
On paper it's quite impressive:
-15mm working distance.
-Cover glass compensation from 0mm to 3mm within scale and much greater behind it.
-15x magnification with 0,58 NA providing ~F13.
-All mirror design.
-Finite 143mm tube lenght (infinite versions available).
-Huge image circle. I've aquired image with uniform sharpness* from center to corner with 12x on Sony A7RII full frame sensor.

Thing is - objective is very broad, and any external illumination is very difficult. It would rather work with trans or epi ttl illumination. It's much easier to illuminate for BE Plan 10x with just 6,5mm WD.
It's designed for 142mm tube, yet designed magnification is achived with much shorter tube, tho correction collar is NOT accurate in this instance. It seems to be accurate when mounted on 160mm tube microscope, but provided magnification greater than 20x.
*Contrast is VERY low, extremely low. Image is hazy and blurred. I've tried to run some test stacks, but there is nothing to show. Upscaled Mp-E65 beats it in every aspect. Unusable. I've tried trans, external and ttl epi illumination, fluorescence - nothing is usable.

I wonder, does anyone have experience with similar objectives? Or maybe whole IR microscope? My understanding of reflective optics is limited, but on the scheme below, do you think that this curved mirror might be some kind of compensator?
Image

PS: Funny aspect of this objective - when mounted on DIA microscope, you can get any kind of oblique illumination just by closing down or decentering the condenser aperture. If stopped down enough, it will provide darkfield. Also it provides perfect conoscopic cross in polarized light and works with my version of Nikon DeSenarmont DIC.

Lou Jost
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Post by Lou Jost »

Sorry to hear this lens was so bad. Did you try making extreme lens hoods? Look at the enormous folding lens hood built into this reflective objective:
https://www.ebay.com/itm/HV-Spectra-tec ... 0005.m1851
and in this broken one:
https://www.ebay.com/itm/SPECTRA-TECH-R ... 2974711536

enricosavazzi
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Post by enricosavazzi »

I have had the same experience with a Reflachromat 15x, and also thought that adding baffles (both around the front opening of the barrel and around the hole in the primary mirror) would be the only solution worth attempting, but did not try because of lack of time and other priorities. Just by peering through the front and rear openings of the objective I can see plenty of optical paths that allow stray light to directly enter the microscope tube or hit the primary mirror, so operation without baffles is out of the question.

The concave mirror near the sensor in your scheme functions in the same way as an eyepiece (or probably it is more appropriate to call it a collector or concentrator) to focus IR onto a one-pixel IR sensor. Probably a mirror is used instead of a lens because few materials are available to build refractive lenses that work in a broad wavelength range of IR without introducing a substantial axial chromatic aberration. There is apparently no bandpass filter or diffraction grating in the scheme, which would be necessary to isolate a narrow range of wavelengths as necessary for IR reflectance/transmittance analysis (unless the concave "mirror" is actually a concave diffraction grating). It is also possible that no filter or grating is used in the microscope because the illumination source (not shown in the diagram) contains a variable monochromator to illuminate the subject with a narrow band of wavelengths.

I too thought that the mismatch between the magnification and tube length engraved on the barrel is puzzling. Your specimen of Reflachromat (and mine too) are finite versions, so no tube lens should be necessary.

My specimen has a compensating ring for cover glass thickness with a very large range (from 0 to over 10 mm). The objective gives a somewhat corrected (but washed-out) image at 15x (with tube length around 109 mm) if I set a 2mm cover glass thickness on the compensating ring (and use it without cover glass). This suggests that the objective is being used outside its optimal magnification range at 15x. My best guess is that one should stick to a tube length of 160 mm, regardless of the specifications engraved on the objective.

PS - The Reflachromats at the eBay links in Lou's post are probably condensers for transmitted illumination, rather than objectives, but likely use just the same optics as the objectives of same magnification. These condensers do have an internal baffle (either around the secondary mirror or around the hole in the primary mirror), which might eliminate the need for corresponding baffles in the objective. I have also seen pictures of Reflachromat objectives with an external conical baffle at the front of the objective, which reduces the already short working distance but might improve contrast sufficiently to allow imaging with some form of peripheral reflected illumination.

PS - An article published in the 1950s ( http://adsbit.harvard.edu/cgi-bin/nph-i ... etype=.pdf ) describes the design of reflecting objectives and ways to correct their low contrast. One of them is the use of a three-mirror system. This opens up the possibility that the oblique concave mirror in the diagram by the OP is an indispensable part of the optical design, and might explain the low contrast of the Reflachromat objectives when used without this third element, as well as the mismatch between tube length and magnification.
--ES

JohnyM
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Joined: Tue Dec 24, 2013 7:02 am

Post by JohnyM »

@Lou Jost
Those 10x are even more impressive (on paper) than 15x. That's what makes me sceptical about it. It seems that those 10x objectives were not used for imaging, but frequently appeared as condensers.
Here is one mounted in condenser carrier:
https://www.ebay.co.uk/itm/NICOLET-MICR ... Swkp1az-fI
That baffle seems to be an adjutable aperture stop.

I didnt tried any baffles other than standard one. I've gave up on this objective when it provided equally bad images in epi and epi-fluorescent microscope. I'll try to tinker with it again.

@enricosavazzi
Interesting paper, thanks for the link.
Overall i've found very little information about those objectives or microscopes. There must be some kind of trick to it. I cant imagine any researcher making any use of this otherwize.

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