Do Reichert objectives need corrective optics?

[Lou Jost]

Does anyone know if objectives designed for the Reichert ME F2 "Universal Camera Microscope" (250mm tube length) need corrective optics between them and the camera? If so, do you think the correction is just to flatten the field (hence not needed for stacking) or do they correct also for chromatic aberrations?


I've just reviewed all mentions of Reichert objectives on the forum, and find some people (eg Morfa) have used them successfully without corrective projection lenses, but others say that correction is definitely needed, at least for higher-m lenses. Some suggest that uncorrected CA can be removed in Photoshop. Anyone have any experience especially with the 250mm tube length Reichert objectives?

Thanks very much in advance,
Lou

[Lou Jost]

I asked about this because I just bought this weird Reichert objective (40x NA=.52 long-working-distance)
https://i.ebayimg.com/images/g/wb8AAOSw ... -l1600.jpg
for a reasomnable price. Supposedly it is designed to image hot metal through a 1.5mm thick quartz window, and it uses internal mirrors to increase working distance so as to avoid heat damage.

Here is a link to some details of this lens, from an auction in 2014:
http://www.ebay.com/itm/Reichert-40-0-5 ... 1419883708

[enricosavazzi]

I think the main idea with these objectives is that reflective elements are immune from chromatic aberrations and usable from NIR to UV. This benefit is somewhat negated when reflecting elements are combined with refractive elements (which need correction for chromatic and other aberrations), but still better than refractive-only optics.

The main application should be multispectral (mainly VIS and UV) and UV-only imaging. Resistance to heat is probably not one of the strong points of these optics.

[Lou Jost]

Enrico, yes, lower chromatic aberration is one advantage of these lenses, but in this particular lens, the purpose of the mirror design is not particularly to avoid chromatic aberrations but rather to provide very high working distance with high NA, because the lens has to image hot metal in an oven/vacuum chamber through a thick quartz window. Short working distance is not an option. Mirror lenses are especially good for this purpose.

See for example :
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5134545/

Tropel, which also makes catadioptric objectives, lists the following advantages:"The µCAT design incorporates reflective and refractive elements, resulting in a micro-objective with high numerical aperture, long working distance and low obscuration."

Long working distance is also one of the advantages of pure reflecting objectives; GMA (n aobjective manufacturer) lists these advantages:

"No chromatic aberration, due to all mirror design.
Longer working distances relative to the magnification.
Large numerical apertures for improved light gathering power.
High throughput from UV to far infrared due to all mirror construction.
Coatings are available to enhance performance at specific wavelength regions."

[enricosavazzi]

There are similar, more modern objectives made by SpectraTech (you can google for "Reflachromat"). They are mainly designed for incident or transmitted IR microscopy of subjects at ambient temperature with simultaneous VIS observation of the subject. The IR after passing through the objective is diverted to a spectrophotometer for analysis, while the VIS goes to an ordinary binocular or trinocular head. The idea is that VIS observation is used to make sure that the objective is pointed to the desired subject area, so that spot analysis can be performed, e.g. in forensics.

There are two series of Reflachromat objectives, an older finite one and a more recent infinity corrected one. The latter of course needs a tube lens (apparently the tube lens in Olympus UIS bi/trinocular heads is adequate for VIS, but I have no idea of what beam splitter and tube lens are used for the IR part). Two or three magnifications were made in each series, with 15x the most common now on eBay. There is/was also a matching condenser built in much the same way (possibly just using the same optics as the objective, but mounted in a different barrel), for transmitted IR and VIS illumination.

The Reflachromat are purely reflective optics and use aspheric mirrors, so there is no need for correction in the eyepieces, but the finite series does require the nominal tube length (which I have not been able to find out, unless it is 142 mm as one of the markings suggests). Some have a correction ring to allow for different thickness of the cover glass.

I have found no information about the Reichert objectives, but if they are purely reflective (except for the - probably removable - cover glass at the front), then they intrinsically have no chromatic aberration, which partly answers your question. They may still need correction for spherical aberration (I don't know whether they use spheric or aspheric mirrors) and/or field flatness, though. My best bet is that they require whatever eyepiece correction was used in conventional Reichert microscope objectives of that time.

Catadioptric objectives mix reflective and refractive optics (usually by employing back-surface mirrors), and all bets about correction are off.

I have seen other legacy objectives of similar designs (possibly Zeiss, Leitz and/or Ealing), but did not keep notes. Opinions about these objectives were generally not high. Edmund and Thorlabs are marketing current models of the same types, usually optimized for UV.

[Lou Jost]

Thanks for that information, Enrico. I had seen those Spectra-Tech lenses, in two tube lengths (142mm and one other length) and infinity-corrected (much rarer)...and similar designs from other companies. I liked the Reichert lens since it seems to be designed for visible light and long working distance. I've seen people apparently getting good photos with some Reichert objectives without using dedicated correction lenses, but others say correction is needed.

I've read the bad reviews...but some of the complaints seem resolvable if there were a good enough reason to make the effort. In my case, the extra working distance makes it worth the effort. The quartz window this Reichert lens requires can also work in my favor, as perhaps I can substitute an optically-equivalent amount of liquid (my work mostly involves specimens in liquid).

[JohnyM]

This is not much, but in Pluta "Advanced light microscopy" this objective is mentioned. It's designed by Jerzy Nomarski (known from DIC) and it's made from 2 mirrors AND one lens.

[Lou Jost]

Thanks JohnyM. Do you mean there is only one single glass element or that there is a multi-element glass lens behind the mirrors? Was there any other information about the lens' strengths and weaknesses?

Edit: It is quite amazing that you found or knew that! I am impressed.

[JohnyM]

I've read the whole book, it's filled with knowledge about everything considering microscopy, that was known by the release date. Lots of forgotten knowledge that's resurfacing nowdays (Oblique light variations, most interesting is Zeiss own "Plas DIC" technique described, holographic microscopy etc.)
There is achromatic doublet behind mirrors. Upper scheme is Reichert, lower is Carl Zeiss Jena:

Text rougly translates to (simplified):
"Earlier mentioned optical faults of mirror objectives (cental vignetting, coma and astigmatism) can be significantly reduced even to 10% by catadioptric designs but added lenses need to be achromatic or apochromatic. They need to be made from quartz and fluorite.
Important advantage of those objectives is their long working disance. Typical refractive objectives working distance usually cannot exceed it's focal lenght. But mirror objectives can have up to 3x focal lenght working distance...
..."C.Reichert Wien is producing objective designed by Jerzy Nomarski, as high temperature chambers microscope accesory, 40x/0,52 objective with 8mm working distance...
...Shown on lower scheme is Carl Zeiss Jena Planapo 40x/0,5 objective with 18,8mm working distance which is included in microspecrto laser analyzers and some metallographic microscopes."

I dont know if it's a typo lacking "1", or working distance is only 8mm? If not, seems that this Zeiss is much more impressive with 18,8mm and just slightly lower NA.

Even so, nowdays mitutoyo have such impressive working distances.

Edit: There is quoted patent to this lens, interestingly google search found it:
https://patentimages.storage.googleapis ... 1570-2.png

[Lou Jost]

I'm very impressed with your knowledge. Thank you very much for all this information!