Edmund Optics article on Reflective Objectives

[Macro_Cosmos]

Interesting article, saw it on Photonics Online.
https://www.edmundoptics.com/knowledge- ... bjectives/


- Reflective objectives offer several benefits over refractive objectives.
- Offers chromatic correction from deep UV to deep IR.
- Refractive objectives only are usually corrected over a narrow and specific range, imaging applications outside of this range could suffer from compromising chromatic aberration.
- Reflective objectives are usually cheaper.
- High quality mirrors minimise transmitted wavefront error.

- Infinity conjugate reflective objectives are perfect for collimating light beams with a large range. (i.e. multiple LASERs)
- Finite conjugate reflective objectives are great for imaging.
- Infinity conjugate reflective objectives can be used for imaging with the aid of a tube lens.

Many IR imaging microscopes use a reflective condenser (such as spectra-tech).

[Scarodactyl]

- Finite conjugate reflective objectives are great for imaging.

These objectives are always tempting, but has anyone actually found this to be the case? I've seen plenty of posts from disappointed buyers but none from ecstatic ones. But I Want To Believe.

[rjlittlefield]

A related question...

I expect there are integrated systems that come with these objectives already installed. What are some examples of those?

--Rik

[Scarodactyl]

I got an FTIR microscope (nonfunctional alas) which uses one. I should probably try that objective on my normal stand.

[ray_parkhurst]

Looks like I will need to get out my Beck reflective objectives, 36x and 15x I believe, and check them out again. As I remember they were interesting, and had promise, but I'm not using them, so there must be a reason I don't remember...

[Chris S.]

I expect there are integrated systems that come with these objectives already installed. What are some examples of those?

I think I have such an objective. If so, it came on a derelict Biorad UMA 300 FTIR microscope that I bought for its opto-mechanical components.

It's doubtful I'll ever experiment with this reflective objective. If anyone would like to do so--especially if you can post your findings here--it's yours for the cost of shipping. (I'm in Ohio, USA, ZIP code 44632. The optic weighs 2.2 lbs, or 1 kg.)

Edit to add:

Here is a rather poor picture from the Internet of an instrument similar to the (incomplete) one I purchased. I've added a red bracket to show the vertical dimension of what I take to be the reflective objective.

Also notable in the image are a pair of Olympus BHMJ focus blocks, which attracted me to the item I bought. There is also an RMS turret, and a nice bunch of other bits of interest to optics tinkerers.

--Chris S.

[jmc]

I wrote an article on reflecting objectives for the Quekett Journal a few months back. Having tried a few different ones out, my conclusions where that they do not offer the same high image quality as a well corrected refractive objective, but they do offer some benefits vs refractive objectives such as mentioned above - long working distances, correction from UV through to IR, etc - which means that sometimes they are the better option. Also keep in mind depending on the design you can get some star shaped diffraction patterns from the spider holding the front mirror.

Alignment, and how easy or difficult it is to change that is another area where they are very different to refracting objectives. Some manufacturers make it easier to realign them if needed, others do not, and some require realigning very frequently.

Interesting to note that while Zeiss have made reflecting objectives, in the Zeiss optical systems brochure from 1971, in the section on Ultrafluars they said "We do not manufacture any monochromats or even reflecting objectives, because these offer no advantages over Ultrafluars, but only have disadvantages". I presume here they were mainly referring to their use over a wide spectral range rather than the working distance, as large working distance is certainly something that reflecting objectives do offer, even at high magnification.

[Lou Jost]

Interesting to note that while Zeiss have made reflecting objectives, in the Zeiss optical systems brochure from 1971, in the section on Ultrafluars they said "We do not manufacture any monochromats or even reflecting objectives, because these offer no advantages over Ultrafluars, but only have disadvantages".

I have a gigantic fancy silver one made by Zeiss, with an iris (unusual for reflectors) and with another ring whose function I cannot determine (the ring says "Nullpunkt", what's a nullpoint in this context?).

[jmc]

Interesting to note that while Zeiss have made reflecting objectives, in the Zeiss optical systems brochure from 1971, in the section on Ultrafluars they said "We do not manufacture any monochromats or even reflecting objectives, because these offer no advantages over Ultrafluars, but only have disadvantages".

I have a gigantic fancy silver one made by Zeiss, with an iris (unusual for reflectors) and with another ring whose function I cannot determine (the ring says "Nullpunkt", what's a nullpoint in this context?).
_1210417..jpg

Ooooh lovely. Do you know when that was made? Zeiss did make reflecting objectives earlier in the century as I have a couple, but I have not seen that one before.

Have tried contacting the Zeiss archive to see if they have anything on that one?

[Pau]

Interesting to note that while Zeiss have made reflecting objectives, in the Zeiss optical systems brochure from 1971, in the section on Ultrafluars they said "We do not manufacture any monochromats or even reflecting objectives, because these offer no advantages over Ultrafluars, but only have disadvantages".

I have a gigantic fancy silver one made by Zeiss, with an iris (unusual for reflectors) and with another ring whose function I cannot determine (the ring says "Nullpunkt", what's a nullpoint in this context?).
_1210417..jpg

Lou's objective seems to be Zeiss Jena (post war DDR made), while Jonathan's brochure seems to be of Zeiss West Germany, this would solve the apparent contradiction.

[rjlittlefield]

they do offer some benefits vs refractive objectives such as mentioned above - long working distances ...

Back in 2007 I bought an old 15X Beck-Ealing specifically to get long working distance. Unfortunately that objective was useless for my application due to stray light problems, so I ended up returning it.

I do not recall the specifications of the old Beck-Ealing, but today I see that Edmund Optic's High Performance ReflX objectives are 15X NA 0.28, with 23.75 mm working distance.

I'm sure a spec like that would have impressed me in 2007, but today I also note that Mitutoyo 20X NA 0.28 are WD 30.5 mm, and 20X NA 0.42 are WD 20.0 mm.

Comparing against the Mitutoyo's, the reflectives don't seem to have an advantage in WD.

Have I overlooked some better performers?

--Rik

[ray_parkhurst]

From what I've seen on several Beck objectives I've owned, the stray light issue relates to the VERY touchy nosepiece, and the specific spacing and diameter of the entrance hole. I had one that was missing the nosepiece, and it was impossible to get it to work properly. With the nosepiece it is much better as I remember.

[Lou Jost]

"Do you know when that was made? Zeiss did make reflecting objectives earlier in the century as I have a couple, but I have not seen that one before."

No, I don't know much about it, though I did once find an old Zeiss brochure online that seemed to be describing it. I have not been able to find it again....

[rjlittlefield]

...specific spacing and diameter of the entrance hole... With the nosepiece it is much better as I remember.

Mine did not have a nosepiece, but I experimented with adding a conical hood. That worked sort of OK in terms of controlling the stray light, but only when the hood extended so far from the objective that it eliminated the nominal advantage in working distance.

I notice that the design posted above shows an internal tube labeled "Stray Light Baffle". I did not recall that my old Beck-Ealing had such a tube, but I just now located the documentation that I had at the time, and apparently it did contain one. Still, even according to the diagram there was ample opportunity for light from far outside the field to go directly into the tube, and that seems to be true for the new design also.

Documentation for old objective:

Diagram shown above, annotated:

--Rik

[chris_ma]

(the ring says "Nullpunkt", what's a nullpoint in this context?).

Nullpunkt in german is usually a point where you start at.
No idea what it means in this context, I'd guess something like the default position, the calibrated position, the position from which adjustments are made from etc.