Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

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hans
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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#121 Post by hans » Sat Aug 29, 2020 3:00 am

Zuul wrote:
Sat Aug 29, 2020 12:52 am
The light exiting the objective being focused to infinity has nothing to do with the point/plane of focus in front of the objective.
Sorry, I don't follow, and I am not sure what parts of the MicroscopyU page you are considering relevant. When a point at the normal working distance is imaged by an infinity-corrected objective rays from the point are parallel between the objective and tube lens. To image a point further from the objective rays must converge past the objective and to image a closer point rays must diverge, just like if the objective were a simple, thin lens? I don't think "infinity-corrected" changes anything fundamentally, it just indicates that the complete optical system (including the rest of the stuff after the objective) is designed such that aberrations are minimized (corrections most effective) when rays from the imaged point are parallel leaving the objective. If you still disagree do you mind starting a new topic with some further explanation and maybe a ray diagram of what you are thinking happens when an infinity-corrected objective images points not at the normal working distance?

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#122 Post by Zuul » Sat Aug 29, 2020 4:31 am

hans wrote:
Sat Aug 29, 2020 3:00 am
I don't think "infinity-corrected" changes anything fundamentally, it just indicates that the complete optical system (including the rest of the stuff after the objective) is designed such that aberrations are minimized (corrections most effective) when rays from the imaged point are parallel leaving the objective.
The ray diagram is in the article I linked where they discuss the difference between infinity and finite tube systems. “Infinity corrected” in no way indicates that aberrations are minimized. There is no optical superiority inherent to infinity corrected systems. The only advantage is that the distance between the objective and tube lens doesn’t matter. This allows accessories to be inserted into the optical path without disturbing the optics. If you don’t believe me, please ask one of the members you trust.

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#123 Post by hans » Sat Aug 29, 2020 6:08 am

Zuul wrote:
Sat Aug 29, 2020 4:31 am
The ray diagram is in the article I linked where they discuss the difference between infinity and finite tube systems.
"Figure 2 - Finite and Infinity Optical Systems" shows the objectives imaging points at their intended working distances. If "Object" is moved relative to "Objective" why should the rays in the region labelled "Parallel Optical Path" remain parallel?
Zuul wrote:
Sat Aug 29, 2020 4:31 am
“Infinity corrected” in no way indicates that aberrations are minimized. There is no optical superiority inherent to infinity corrected systems.
I am talking about varying the positions of components within one particular system, not making a comparison between systems. Infinity-corrected system are designed to minimize aberrations when rays from the imaged points leave the objective parallel. Finite tube length system are designed to minimize aberrations when rays from the imaged points focus at the intended intermediate image plane corresponding to the tube length.
Zuul wrote:
Sat Aug 29, 2020 4:31 am
The only advantage is that the distance between the objective and tube lens doesn’t matter. This allows accessories to be inserted into the optical path without disturbing the optics.
Yes, these are some practical advantages.
Zuul wrote:
Sat Aug 29, 2020 4:31 am
If you don’t believe me...
I believe many of the things you are saying I just don't see the relevance to this question:

If we shift around components in an infinity-corrected microscope in order to image points not at the normal working distance of the objective, are rays from the imaged points still parallel between the objective and tube lens? I think the answer is no.
Zuul wrote:
Sat Aug 29, 2020 4:31 am
...please ask one of the members you trust.
I already suggested starting a separate topic, which you may do if you like.

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#124 Post by Scarodactyl » Sat Aug 29, 2020 8:16 pm

You could change working distance by changing the distance of the tube lens to the sensor. In that case the tube lens would no longer be focused at infinity so yeah, things will be screwy.

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#125 Post by hans » Sat Aug 29, 2020 9:23 pm

Scarodactyl wrote:
Sat Aug 29, 2020 8:16 pm
You could change working distance by changing the distance of the tube lens to the sensor. In that case the tube lens would no longer be focused at infinity so yeah, things will be screwy.
And more generally, it seems pretty fundamentally impossible, in terms of basic geometric optics at least, to image points at different working distances without altering convergence/divergence on the other side of the objective, regardless of what exactly is happening in the following optics?

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#126 Post by MichaelG. » Sat Aug 29, 2020 9:24 pm

I’m not sure how much this helps, but I have just been playing with the simple lens formula
[ignoring the modern convention where u is negative] ...

1/u + 1/v = 1/f can be rearranged as u = (fv)/(v-f)
... which is easy to pop into a spreadsheet or calculator, for some spot checks.

Taking a 5mm lens : reducing the primary image distance to 16,700mm would only increase the object distance from 5.000 to 5.001mm.
[obviously I could not perform the calculation for v=infinity, but the answer for that is ‘defined’]

I doubt if we need worry about a one micron shift.

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#127 Post by Scarodactyl » Sat Aug 29, 2020 9:35 pm

If small shifts mattered a lot you couldn't adjust eyepiece focus. Infinity space is handy but of course in reality there is some loss of image quality with extended distance and eventually vignetting.

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#128 Post by hans » Sat Aug 29, 2020 9:55 pm

I have not tried to come to any conclusion about whether the effect would be significant. The discussion regarding infinity space was just explaining why I asked apochronaut about accessories, since we were talking about intentionally shifting things to get some corrective effect. I have no idea if this contributes to the corrective effect with the 145 eyepiece, but:
apochronaut wrote:
Wed Aug 26, 2020 4:44 pm
The focus needs to be raised about 200 microns.
I think this is not a huge distance, but also not obviously negligible.

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#129 Post by hans » Sat Aug 29, 2020 10:14 pm

Scarodactyl wrote:
Sat Aug 29, 2020 9:35 pm
Infinity space is handy but of course in reality there is some loss of image quality with extended distance and eventually vignetting.
In addition to the effect of distance alone, if rays are converged/diverged in what used to be infinity space, there will be some spherical aberration and other effects from even flat glass elements. Again, no idea if significant, that's just why I asked about accessories.

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#130 Post by MichaelG. » Sat Aug 29, 2020 10:37 pm

hans wrote:
Sat Aug 29, 2020 9:55 pm
.
[…] but:
apochronaut wrote:
Wed Aug 26, 2020 4:44 pm
The focus needs to be raised about 200 microns.
I think this is not a huge distance, but also not obviously negligible.
Taking my 5mm lens, and the simple lens formula ... that would mean that the primary image lies at v=130mm instead of infinity. ... Presumably I have misinterpreted the quoted statement.

MichaelG.
.

P.S. Happy to use a different focal length if you prefer.

_______________

!! CORRECTION !!
Edit: or just an alternative interpretation : see subsequent discussion

Raising the focus 200 microns would mean u is reduced to 4.800
... not increased to 5.200

therefore apochronaut’s primary image is beyond infinity

[ the Buzz Lightyear setting ? ]
.
A7E1AEAC-AFC7-4BBA-BD74-207AD4BA0F20.jpeg
A7E1AEAC-AFC7-4BBA-BD74-207AD4BA0F20.jpeg (66.38 KiB) Viewed 3438 times
Edit: :oops: Just realised that I’m displaying the new cells [rows 8 and 9] in the wrong colums ^^^
... it was past bedtime ... I will tidy-up the presentation
Last edited by MichaelG. on Sun Aug 30, 2020 6:38 am, edited 2 times in total.
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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#131 Post by hans » Sun Aug 30, 2020 1:22 am

MichaelG. wrote:
Sat Aug 29, 2020 10:37 pm
Presumably I have misinterpreted the quoted statement.
No, you're right, 200 um is potentially much larger than I was thinking if that is objective working distance, but it depends on magnification. Obviously the objective is far from "thin" but in a rough sense your calculations look reasonable to me. With 200 mm reference length 5 mm would be 40X magnification, is that why you used 5 mm?

However I just realized, since objective magnification was not specified, perhaps 200 um is the distance the eyepiece is raised from being parfocal with the binocular in order to focus on the sensor, in which case it is quite small.

To put some numbers on what Scarodactyl was saying about eyepiece focus I just messed with my 410 a bit. The total range of the adjustable eye tube is about 8 mm. Empirically, in terms of working distance, that is roughly 460, 70, 4 um with the 4, 10, 40x objectives, respectively. (Going by the scale on the fine focus dial which was within 10% against a dial indicator when I checked a while ago.) Those numbers agree reasonably well with calculations like you were doing, except here I am considering the objective and tube lens together, with focal length calculated to match the magnification, using baseline 200 mm intermediate image distance, and finding the change in intermediate image distance:
  • 4X: 200 - 1/(1/200 + 1/50 - 1/50.460) = 7.0 mm
  • 10X: 200 - 1/(1/200 + 1/20 - 1/20.070) = 6.7 mm
  • 40X: 200 - 1/(1/200 + 1/5 - 1/5.004) = 6.2 mm

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#132 Post by hans » Sun Aug 30, 2020 3:47 am

MichaelG. wrote:
Sat Aug 29, 2020 10:37 pm
Raising the focus 200 microns would mean u is reduced to 4.800
... not increased to 5.200
All these AO/Reichert infinity bodies actually do focus by moving the objective, not the stage, so taking "focus needs to be raised" literally your original interpretation would be correct.

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#133 Post by MichaelG. » Sun Aug 30, 2020 6:16 am

hans wrote:
Sun Aug 30, 2020 3:47 am

All these AO/Reichert infinity bodies actually do focus by moving the objective, not the stage, so taking "focus needs to be raised" literally your original interpretation would be correct.
.
Thanks for confirming that, Hans
Hopfully apochronaut can tell us what objective he uses

Yes, my 5mm was just a guess ... a stake in the ground

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#134 Post by MichaelG. » Sun Aug 30, 2020 11:24 am

.

Here’s a tidier version of my little spreadsheet
.
Revised layout
Revised layout
68E02CEC-E7CC-441C-96B3-85CC40B6977C.jpeg (41.1 KiB) Viewed 3387 times
.

As presented :

D7 = D3

C5 = (D3 x E4) / (E4 - D3)

E9 = (D7 x C8) / (C8 - D7)

and it’s showing 1:1 magnification
... which is a convenient check on the calculation formula

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#135 Post by hans » Mon Aug 31, 2020 12:02 am

The color correcting doublet is separated. When I went to open the glass tube it was in the plastic cap was securely solvent-welded in place (obviously, I'm not a chemist) so I had to carefully hacksaw and split away the cap. With it out of the solvent I could see a bit of delamination creeping in from the edges (could actually see interference fringes) but they were still firmly attached. I tried shearing force with some pliers and could see the interference fringed moving around but not much progress and I was worried I would shatter one of the elements. I then tried "thermoshock" as described here but heating more aggressively with a heat gun. After about 10 increasing hot cycles there was a surprisingly loud pop while heating, then the elements fell apart when I dropped them in the solvent.

The chlorinated carburetor cleaner appears to have very effectively removed the anti-reflective coatings, which surprised me a bit, but otherwise the elements are still in reasonably good condition. There is residual adhesive on the plano-concave element which I have not tried to remove yet.

I measured 52 mm for the focal length of the plano-convex element. That agrees with the index and radius given in the patent for color correcting doublet VI:
29.95 mm / (1.589 - 1) = 50.8 mm
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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#136 Post by MichaelG. » Mon Aug 31, 2020 10:12 am

Your dedication is rewarded, Hans

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#137 Post by hans » Sat Sep 05, 2020 5:58 pm

I thought it would be interesting to try an approximate calculation of "percent lateral color" due to the telelens and color correcting doublet based on the specs in the patent, mainly as a way to check understanding. Another reason is to get an idea, for the design given in the patent, how much of the 0.6% lateral color coming from the objective is corrected by the telelens vs. the doublet. To summarize some context, the patent (US 4,715,697 - Microscope body system) says:
By use of the present invention, a microscope body having typically a lateral color correction of 0.6 percent can be coupled with a microscope objective having a deliberately designed-in lateral color aberration of 0.6 percent such that the resultant combination of the microscope body with the microscope objective can substantially cancel out a major portion of this deliberately-introduced lateral color in the microscope objective. In addition, the present invention does not introduce an appreciable amount of axial color aberration. With the invention, as disclosed, a minimal number of optical elements can be used at a lowering in overall cost of the microscope system such that a high-performance microscope body is available at a substantially lower cost than the prior art.
By the terminology used in the patent, the "microscope body" (more commonly referred to as the head) contains optics functioning as a "telescope lens system" -- "telescope objective lens" (usually tube/telan lens, but Wayne Butter's shorthand "telelens" seems logical in this context), "color correcting doublets" (which are the windows just below each eyepiece), and various flat glass pieces (prism, splitter, glass plug). The patent does not state explicitly how 0.6% lateral color correction is distributed across the components but the name "color correcting doublet" seems like a pretty good hint that they would be responsible for a majority of the correction. It seems to be widely agreed that the 181 eyepieces themselves are not strongly compensating in terms of lateral color and that is consistent with what I have seen. In #84 I crudely estimated lateral color after the telelens by comparing the red and blue channels in one of the direct projection test photos and got a result close to 0.6%. So far everything seems consistent with the correction being done mainly in the doublet.

The patent gives refractive indices at the Fraunhofer D line and specifies dispersion using Abbe numbers. That is only enough information to determine the difference (n_F - n_C) and not the absolute refractive indices at the F and C lines. (I believe the same Fraunhofer lines F (blue) and C (red) used in the Abbe numbers are also used to define lateral color in the patent.) There may be some common approximation or rule of thumb I am not familiar with to estimate n_F and n_C from n_D and V_D but with approximations the calculation ended up depending only on the difference anyways. The elements are labeled in figure 1 of the patent and the specifications are given in table I, both attached below for convenience. The (n_F - n_C) differences for the elements of interest in the order they appear along the optical path are:
  • 0.0061, element I of telelens, biconvex, ((1.49694 - 1) / 81.60)
  • 0.0162, element II of telelens, negative meniscus, ((1.63964 - 1) / 39.59)
  • 0.0096, element VI_1 of color correcting doublet, plano-concave, ((1.58904 - 1) / 61.23)
  • 0.0144, element VI_2 of color correcting doublet, plano-convex, ((1.58909 - 1) / 40.91)
Numbers I am using for the geometry:
  • 100 mm from aperture stop in objective to telelens (measured approximately with no accessories installed)
  • 183 mm reference focal length (Wayne Butter)
  • internal interface of color correcting doublet positioned 35 mm before the intermediate image (not specified in patent, measured)
  • 20 mm diameter field stop located in the eyepiece at the intermediate image plane (marketing literature, also measured)
As a sanity check I calculated thin-lens zero-spacing focal length for the telelens doublet from the numbers in the patent and got a result close to 183 mm:
1 / ((1.49694 - 1) * (1 / 195.41 mm + 1 / 69.54 mm) + (1.63964 - 1) * (-1 / 65.297 mm + 1 / 119.25 mm)) = 190.2 mm

First I estimated where the principal ray from the center of the aperture stop in the objective rear focal plane to the edge of the field stop in the intermediate image plane intersects the telelens and doublet while ignoring the optical effect of the doublet. (At the Fraunhofer D line the refractive indices of the elements of the doublet are closely matched so it should behave like a flat plate.) A ray through the center of the telelens to the edge of the field stop is angled 55 mrad (10 mm / 183 mm, small angle) relative to the optical axis. The principal ray should be at the same angle in the infinity space and therefore hit the telelens 6 mm (55 mrad * 100 mm, small angle) from the optical axis. The principal ray should then cross the doublet 9 mm (6 mm + (10 mm - 6 mm) * (1 - 35 mm / 183 mm)) from the optical axis.

Next I estimated the angular deviation of an F (blue) ray relative to a C (red) ray after passing through each element using prism deviation angle with small-angle approximations, (n - 1) * a, where "a" is the apex angle in radians. Apex angle is approximated from the radii (sign convention in the patent is the same as the linked Wikipedia page) as a = (1/R1 - 1/R2) * r where "r" is the radial distance from the optical axis where the ray intersects the element. The F-C deviation is then (n_F - n_C) * (1/R1 - 1/R2) * r and with these sign conventions apex angle and F-C deviation are positive for positive (converging) elements with positive (n_F - n_C) difference, which deviate blue towards the optical axis relative to red. The F-C deviations for the four elements listed above are:
  • 0.71 mrad, element I, (0.0061 * (1 / 195.41 mm + 1 / 69.54 mm) * 6 mm)
  • -0.67 mrad, element II, (0.0162 * (-1 / 65.297 mm + 1 / 119.25 mm) * 6 mm)
  • -2.88 mrad, element VI_1, (0.0096 * (-1 / 29.95 mm) * 9 mm)
  • 4.33 mrad, element VI_2, (0.0144 * (1 / 29.95 mm) * 9 mm)
Finally I estimated lateral color by projecting the angular deviations into the intermediate image plane and normalizing by the field radius. The lateral deviation is 7 um ((0.71 mrad - 0.67 mrad) * 183 mm) due to the telelens and 51 um ((4.33 mrad - 2.88 mrad) * 35 mm) due to the color correcting doublet. As a percentage of field radius the deviation is 0.58% ((7 um + 51 um) / 10,000 um), pretty close to 0.6% specified in the patent. But is it agreement, or just lucky cancellation among a series of grave conceptual errors...? Comments or questions regarding the validity/accuracy of the approximations are welcome.

If this understanding of the function of the doublet is correct I think the main conclusion of practical interest is that the corrective effect of the doublet scales with distance from the intermediate image, as I had speculated earlier, and so its position relative to the intermediate image probably cannot be modified by more than a few mm (eye tube diopter adjustment is +/- 4 mm, for example) without disturbing the cancellation noticeably.
Attachments
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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#138 Post by hans » Sat Sep 05, 2020 8:23 pm

Just noticed a dimension in the patent I had overlooked before:
The lens I such as a simple line, is spaced a distance S1, typically 86.15 mm, from objective shoulder 14...
Referring to the distance from the objective shoulder to the surface of the telelens. I think that is roughly consistent with the 100 mm aperture stop to telelens I estimated for use in the lateral color calculation, considering the thickness of the telelens doublet (about 15 mm) and that as I understand these longer, more highly-corrected objectives like the 45 mm plan achros have the rear focal plane somewhere inside the objective, below the shoulder. And obviously that distance changes when accessories are inserted into the infinity space.

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#139 Post by hans » Wed Sep 09, 2020 4:58 am

Finally, some less theoretical progress:
Attachments
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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#140 Post by PeteM » Wed Sep 09, 2020 5:41 am

Be very interested to see how this works out, Hans.

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#141 Post by MichaelG. » Wed Sep 09, 2020 7:43 am

hans wrote:
Wed Sep 09, 2020 4:58 am
Finally, some less theoretical progress:
Very devious, Hans ... I like it !

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#142 Post by apochronaut » Wed Sep 09, 2020 1:09 pm

Very nice consruction. Expecting an earthquake?

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#143 Post by hans » Wed Sep 09, 2020 9:29 pm

Thanks, everyone. The design is a bit odd, but I was trying to make sure alignment didn't rely too much on precise fabrication (using hacksaw, files, hand-held drill) with enough adjustment points to make up inaccuracies. The ends of the square tubes that interface with the big plate and eyepiece were filed then sanded on a flat plate to be accurately square after hacksawing, everything else is looser tolerance. The three little bits of 0.001" Kapton tape visible in the first photo on the end of the larger square tube set the contact points to avoid rocking. The earthquake-proof thickness of everything is mainly for the convenience of being able to drill and tap holes wherever necessary.

At the height shown assembled the eyepiece on top is parfocal with the binocular, for afocal coupling. Loosening the two thumb screws and sliding the inner square tube gives a range relative to parfocal of about 10 mm down (mainly for curiousity/experimentation) and 30 mm up to experiment with direct projection.

Using the old camcorder tripod shown in the first post of the thread is a huge pain. It has to be tilted by making one leg longer in order to get the camera in the right place, but then using the height adjustment crank shifts the camera horizontally at the same time. I am not too eager to take more comparison photos until I figure out a better way to mount the camera, with easier vertical adjustment. Would also be nice if it could also move out of the way to mess with the eyepiece, then be put back precisely into the same position without a lot of messing around.

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#144 Post by MichaelG. » Wed Sep 09, 2020 9:59 pm

hans wrote:
Wed Sep 09, 2020 9:29 pm
Would also be nice if it could also move out of the way to mess with the eyepiece, then be put back precisely into the same position without a lot of messing around.
If you’re up for a bit more construction work, Hans
That can be achieved with a simple Kinematic mounting arrangement known as Cone/Groove/Plane

I will see if I can find a decent explanation on-line, to save myself some effort.

MichaelG.
.


Edit: This should help: https://wp.optics.arizona.edu/optomech/ ... orial1.pdf
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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#145 Post by hans » Wed Sep 09, 2020 11:19 pm

Thanks for the link, Michael. I was actually planning something like figure 2.6(b) with five additional little pieces of Kapton tape (omitting the sixth in the XY plane) between the inner and outer square tubes on the eye tube mount but it turned out to not be necessary. After removing high spots with some gently lapping of the outer surface of the smaller one and careful filing on the inside of the larger one the contact was much better than I was expecting for raw extrusions. (I bought the two square tubes from McMaster-Carr, not the cheapest option, but you get what you pay for I guess.) I like the idea of something like the three-V-groove variant shown on the right in figure 3 for camera, easy to fabricate.

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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#146 Post by BrianBurnes » Wed Oct 21, 2020 5:06 pm

To breathe some life back into this thread: I spent quite some time reconstructing the Microstar IV microscope body and objectives from Reichert/Cambridge patents in order to perform a full spectral optical simulation of the whole system. What follows is a fully virtual reconstruction, based on real lens and glass specs.

I'm showing a zoomed out image of the entire simulation below (click for full size):

Image

As a basis, I used the "Microscope body system" (US 4715697A) patent. For the objective, I used the five-component objective from US Patent 4417787A, which is claimed to be a 50x 0.85NA semi-apochromatic objective.
I determined the focal length of the objective by passing collimated light through it from the back and noting the focal point. Then, I determined the focal length of the tube lens by aiming light from a pinhole emitter through the objective and the tube lens and noting where the light was imaged the sharpest.

Somewhat interestingly, the focal point of both the objective and the tube lens are different than claimed in their respective patents. For the objective, the focal point was 0.0046mm lower than claimed (this seems small, but at 0.85NA it is quite significant). Similarly, the focal point of the tube lens seemed to be a 172mm from the top of the tube lens vs the 193.725 claimed in the patent.

A bigger challenge was the illumination system: Lacking the description of the exact illumination system and condenser used in the Microstar IV, I created a DIY epi-illuminor. It uses an EdmundOptics aspheric condenser lens as the collimator and a Thorlabs AC254-100-A Achromatic Doublet as the field lens. Light from a virtual filament is projected at infinity by the condenser, and then refocused at the back focal plane of the objective before passing through a beamsplitter. The use of epi-illumination allows use of the objective itself as the condenser, working around the missing Reichert condenser description. I'm also using two iris diaphragms to control field and aperture of the system, placed at the correct points in the illumination system.

The total system uses a smattering of 12 (!) different types of glass. I spent quite a bit of time trying to track down the original glasses used for lens construction from just the index of refraction and Abbe number specified in the patents in order to retrieve the Sellmeier coefficients. About half I was able to match exactly, the other half I had to find a closest fit (from Schott or Ohara Glass), with an error of a few% in the Abbe number or refractive index.

Now on to the experiment: I modified the field diaphragm to project very finely spaced lines onto the objective. I did this by placing finely spaced occluders into the field (shown below).

Image

The objective projects these lines cleanly onto the sample plane as we would hope and focuses the reflected light back up at infinity. Below we can see a very zoomed in view of the sample plane, with the bottom-most lens of the objective at the top of the image, and the objective focal plane below it. We can see the finely spaced lines coming into focus and being reflected back (click image for larger version):

Image

The tube lens refocuses the image at the sensor plane, where we can measure it. Below I'm showing the raw image measured by the virtual sensor. We can see the lines mostly in focus, but with significant amount of lateral CA (click image for larger version):

Image

Now the interesting part: I then inserted the color correcting doublet from the patent into the light path between the tube lens and image plane. I'm showing multiple images recorded by the sensor below, that correspond to different distances between the top of the tube lens and the doublet, ranging from 10mm, 50mm, 90mm and 130mm (click images for larger version):

Image
Image
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Image

As we can see, the doublet has a significant impact on the amount of CA present in the image. The correcting effect is stronger the closer the doublet is to the tube lens (or, the farther it is from the image plane). Too close and you get reversed CA; too far, and you get barely any correction. The ideal location seems to be somewhere between 50mm and 90mm.

Now, there are obvious caveats in this: This is a simulation following the patent only, but of course we don't know what was actually implemented. Second, I'm using epi-illumination which this objective was likely not designed for. Third, I'm using best-match glasses - we don't know the precise type of glass used in the original system. Fourth, the measured focal points in the virtual system are different from the patent. Is the simulation wrong or the patent? We don't know. Fifth: From the amount of spherical aberrations at the borders it is likely I am imaging a much larger field of view than intended for this objective. The relevant areas are likely the center parts. Sixth, and the most significant one: To get a useful correction, I had to swap the glass material of the two elements in the doublet. If I use it as specified in the patent, I get CA correction in the wrong direction (i.e. intensifying the CA already present). After swapping the material of the first and second element, the doublet works as intended. A typo in the patent? I do not know.

In any case, I hope this is interesting. Certainly a lot more effort than is reasonable, but it was an interesting endeavor.

BrianBurnes
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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#147 Post by BrianBurnes » Wed Oct 21, 2020 5:13 pm

I should also mention that I did some physical experiments as well, and found that I could not get enough correction with the doublet in a direct projection setup - I just could not get the doublet close enough to the tube lens.

In an afocal setup the results are better (I posted pictures of the adapter elsewhere). There are still some issues in my system, most likely because my nosepiece is misaligned. I will have to do some alignment work before things are perfect, but results are promising so far.

hans
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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#148 Post by hans » Wed Oct 21, 2020 8:44 pm

Excellent, after you posted the simulation of the illuminator in the other thread I was considering trying to goad you into doing the doublet, but it is already done.
BrianBurnes wrote:
Wed Oct 21, 2020 5:06 pm
For the objective, I used the five-component objective from US Patent 4417787A, which is claimed to be a 50x 0.85NA semi-apochromatic objective.
Had not seen this patent, a couple interesting things: That is the first time I have seen the 183 mm effective focal length of the telescope given explicitly. Wayne Butter has mentioned 183 mm reference focal length a couple times. The specified residual lateral CA does not match the 400-series body patent: "It is an object of this invention to provide objectives well corrected for all aberrations except lateral chromatic aberration. The residual lateral chromatic aberration between 486.1 nm and 656.3 nm is 1% of the angular magnification at 589.3 nm across usable field." Those wavelengths are the same Fraunhofer F and C lines used in the "0.6% lateral color" specification in the body patent. This is probably the difference in lateral color between the "Reichert Buffalo" and "Reichert Vienna" 45 mm parfocal infinity systems that Wayne Butter was referring to here and here. It probably also explains the noticeably worse lateral CA (vs. the Buffalo objectives) in Zuul's 25X Vienna objective test image without the doublet posted earlier in this thread:
Zuul wrote:
Mon Aug 03, 2020 10:07 pm
...used the Reichert Austria 25x... Each image is a composite of 2 captures; one with, and one without, correction. ... https://drive.google.com/file/d/1MOMIsX ... sp=sharing
BrianBurnes wrote:
Wed Oct 21, 2020 5:06 pm
Similarly, the focal point of the tube lens seemed to be a 172mm from the top of the tube lens vs the 193.725 claimed in the patent.
I think this is good agreement if the "plane-parallel plate" effect of glass plug IX is included, which should displace the focal point back by approximately 1/3rd of the 58.4 mm thickness of the plug?
BrianBurnes wrote:
Wed Oct 21, 2020 5:06 pm
The correcting effect is stronger the closer the doublet is to the tube lens (or, the farther it is from the image plane). Too close and you get reversed CA; too far, and you get barely any correction. ... Sixth, and the most significant one: To get a useful correction, I had to swap the glass material of the two elements in the doublet. If I use it as specified in the patent, I get CA correction in the wrong direction (i.e. intensifying the CA already present). After swapping the material of the first and second element, the doublet works as intended. A typo in the patent? I do not know.
This agrees qualitatively with that hand calculation I did, but I did not have to swap the glasses. I spent quite a long time trying to make sure I was not making any sign errors but apparently failed.
BrianBurnes wrote:
Wed Oct 21, 2020 5:06 pm
In any case, I hope this is interesting. Certainly a lot more effort than is reasonable, but it was an interesting endeavor.
Yes, very interesting, thank you for putting in all the work.

PeteM
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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#149 Post by PeteM » Thu Oct 22, 2020 12:13 am

I wonder if we're closer to answering the question - what's the best setup to get good images out of a Reichert Microstar IV trinocular head and into a camera?

Is it still something like an AO 145 objective and afocal?? The direct projection that looked sort of OK? Or, is there a currently-available (seems the OEM version is near impossible to find) correction lens available that can do better?

hans
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Re: Reichert 410 "Microstar IV" head -- corrective element present in binocular path but not trinocular camera port

#150 Post by hans » Thu Oct 22, 2020 7:40 pm

Yeah a summary is a good idea, Pete. My (and I think Brian's also) original question was basically, why so much lateral CA in various setups over the camera port, and how to at least get results similar to afocal through a binocular eyepiece while using the camera port. In practical terms I was considering that question pretty much answered already in the first post of the thread -- the three "ports" of the trinocular head are closely-matched optically up until the point the light leaves the last flat glass element, which is the splitter in the case of the binocular ports or glass plug IX (the thick cylinder) in the case of the camera port. So if you use the complete eye tube and eyepiece with color-correcting doublet over the camera port you get an image equivalent what you get from the normal binocular path. The rest of the thread has mainly been about demonstrating more conclusively, both experimentally and theoretically, that the eye tube window actually is a doublet providing the corrective effect claimed in the first post.

The aluminum eye tube mount I posted photos of is intended to be used afocally, replacing the bamboo skewers in the first post, but equivalent optically. (Although I did try to make it flexible enough to experiment with some other approaches, mainly out of curiosity.) Brian's mount is also using the same afocal setup as in the first post, if I understand correctly.

There has also been some discussion of approaches other than afocal:
PeteM wrote:
Thu Oct 22, 2020 12:13 am
Is it still something like an AO 145 objective and afocal??
I still have not bought or experimented with a 145 eyepiece, I think apo is the only one who has, not sure if there are test/example shots posted anywhere. If I remember correctly apo's setup with the 145 was projective, not afocal.
PeteM wrote:
Thu Oct 22, 2020 12:13 am
The direct projection that looked sort of OK?
I had not been considering direct projection from the camera port through the color-correcting doublet a very promising approach, at least in terms of field curvature, as explained in this thread: Reichert 181 eyepiece field curvature

However note the caveat mentioned in #8. In the photo the limiting aperture stop in the system is the smartphone camera which the EXIF data shows as 2.5 mm. When used in the microscope the limiting aperture stop is in the objective and the exit pupil is somewhat smaller, more like 1-2 mm if I remember correctly. So that photo could be exaggerating the field curvature.

I don't think there have been any really conclusive tests of direct projection through the doublet. Brian mentioned he was unable to get the doublet in the right position. Zuul posted some test photos but I think those are confounded by the "Vienna" objective which may have more residual lateral CA than the "Buffalo" objectives designed for the 400 series. I have not tried it myself.

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