Spherical aberration on the low-power Plan objective

[Naphthalene]

Good day!

Please enlighten me if there could be some spherical aberration observed on the the field of view edges when using the 10x Plan achromat objective (Considering that the condernser aperture is set to a proper NA and the field diaphragm is open no more than it is neccessary to illuminate the whole field) ? I can see this effect through the eyepieces but it is much less pronounced on the camera shots.
Or could this aberration arise from the minute misalignment of the trinocular head prism or the focusing lens below it?

I'd be sooo grateful if someone could help me with the answers!

Here is the story behind my question:
I have recently purchased a Chinese compound microscope from Leader Precision Instruments company. It is stuffed with infinity optics and a lot of other nice features, and was at least three times less expensive compared to similar models sold by Amscope and Brunel.
When it arrived I have found it had some factory dust inside the trinocular turret so I carefully dismantled it. Some particles had firmly adhered to the surface of the trinocular prism and the lens at the bottom of the turret. As it was not possible to reach the contaminated surfaces the other way, I took out these parts from their fixed positions. I have carefully marked how they were initially installed, and tried to put them back exactly the same way. But now I'm anxious that my reassembly was not sufficiently precise.

The image below illustrates my issue:
EDIT: Sorry the previous link was not working

[MicroBob]

Hi,
I'n sure whether I can see the problem. The image is a bis less sharp towards th edges than in the center. As it is only an achromat and the colour separate a bit towards the corners.
Here a blown up image of the lower left corner. To me this looks like what I would expect from a chinese plan achromat. And then just the green channel, considerably sharper.

Bob

[apochronaut]

It is hard to tell somtimes from prepared slides but this being an amphibian or reptile blood smear, it should be fairly flat, with minor undulations The lower left corner appears to be sharper than the other 3 corners, so something is off kilter. Do you see the same issue in the eyepieces, with the center and the direction of 7:30 being best focus and towards 10:30 and 4:30 poor , with 1:30 bad? Many microscopes have a stage that is not perfectly perpendicular to the optical axis due to slight casting irregularities. Chinese factories and assemblers save a lot of production cost by overlooking such precision adjustments to their stands when they are shipped. Quality makers make sure that the stage is level prior to shipment or have a qualified dealer network to make sure their instruments meet quality control standards. The main quality control issue in Chinese factories is whether the cigarette machines are well stocked.

[Naphthalene]

Thank you for your replies!
I see you both disregard the chinese optics. I knew there must have been some catch behind the low price. My previous microscope was Amscope T340B, also produced in china. It had N-117M number on it, which can be easily tracked to Xiamen Phio Scientific Instruments. Since it served me well for more than 7 years I decided to get my next one directly from China to avoid unneccessary expenses on the reseller company. Though being far from perfect, my new one afforded some significant improvement to the image quality.

MicroBob , so do you think my problem is rather explained by low quality of the objective lens than by a misalignment of the trinocular turret components? Could this imply I may get a better picture if I would equip my microscope with an objective from a reliable manufacturer?

apochronaut indeed this is a frog blood smear, a commercial slide. The uneven off-focus edges are also seen in the eyepieces, and exactly as you say the worst area is at 1:30, while the upper left and 4:30 being slightly better. I observe that focus is lost when I move the slide on the stage, that may support your assumption about the angled stage position. Confusingly, such loss of focus only occurs when I use the slide holder, but not when I detach the holder and move the slide manually.
Further, I have used the precision digital protractor to estimate the slope of the table, and have found that it is deviated from the slope of the table (on which the microscope is positioned) by 0.04 degree on the X axis (left to right on the image) and by 0.13 degree on the Y (top to bottom on the image).

[MicroBob]

I think the quality is quite ok for a chinese plan achromat and there might be other reasons why it isn't even better. You might try the slide again after turning it left to right, is the sharpness the same or does it change? Do you have another slide that is perectly flat, e.g. a stage micrometer (the chinese ones ARE good in this case )
Another quetion is whether your camera is parfocal with the eyepieces. If you have to refocus you would use the objective outside it's intended operating point.

[Naphthalene]

MicroBob , thank you for the advice! I think I'm making some progress now. I have employed the stage micrometer for this experiment. I only have the old russian one, it is only 1 mm in length. It does not cover the whole field of view but it still useful to estimate that off-focus areas. As I rotate the objective I see that the blurry area migrates from one corner to another (I had to adjust focus slightly when rotating the objective in its thread). Before doing this I tried to center the camera adapter by the screws on the camera port to achieve maximum matching of the field illumination in the eyepieces and the camera.

Here is the initial position of the objective

And here is how it looks after I have rotated the objective:

Can I make a conclusion that this "spherical aberration" is not about the incorrect assembly of the trinocular turret?

Of course I've checked multiple times that the objective lens is clean and I wiped it using cotton swabs wetted with ethanol, making sure the off-focus area is still there

[MichaelG.]

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To me this looks like what I would expect from a chinese plan achromat.

.

I would probably go further, and replace the word ‘chinese’ with ‘typical’

MichaelG.

[MichaelG.]

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As I rotate the objective I see that the blurry area migrates from one corner to another (I had to adjust focus slightly when rotating the objective in its thread).

.
An interesting exercise ... but do be cautious when you interpret the results

If the objective is loosened, and only supported by the threads, it will no longer be ‘registered’

MichaelG.

[Hobbyst46]

I copied these two images of the micrometer into Irfanview and enlarged. I only see that they are both not sharp. Out of focus. And the center is less sharp than the edges.
I do not see significant spherical abberation.
Although I fully agree with Apo's judgement of the blood smear image.
Does the microscope have a condenser and a field aperture ? perhaps the illumination can be optimized to give more contrast in the view of the stage micrometer. That in turn will enable better focusing and a sharper image.

[Naphthalene]

Thank you for your replies MichaelG. and Hobbyst46!

I would probably go further, and replace the word ‘chinese’ with ‘typical’

Did you also have the unsatisfactory results with other inexpensive Plan achro objectives? Did they have a similar problem?
And could you please explain what do you mean by "not being registered"?

I only see that they are both not sharp. Out of focus. And the center is less sharp than the edges.

I'm really sorry, I'll try to make a better images and post them ASAP. My microscope has a condenser diaphragm and the field aperture, these were adjusted as I indicated in my first post. The condenser NA was set to 0.2 to match the NA of the 10x objective. And the field diaphragm was open no wider than it was neccessary to get the even illumination of the whole field of view. Perhaps I used too bright light or I was too impatient in setting the fine focus.

Maybe it's something wrong with my perception but the central areas on these rulers look sharper for me.

I'm really thankful to all of you helping me to puzzle out this issue!

[MichaelG.]

Thank you for your replies MichaelG. and Hobbyst46!

I would probably go further, and replace the word ‘chinese’ with ‘typical’

Did you also have the unsatisfactory results with other inexpensive Plan achro objectives? Did they have a similar problem?
And could you please explain what do you mean by "not being registered"?

1.Yes ... what you are seeing is typical of the planachromat design: To do significantly better, you would need to be looking at planapochromats.

2. The screw thread on an objective is intended for attachment, but it inevitably has too much clearance to precisely locate the optical axis ... there should therefore be a ‘register’ of some sort [usually just a flat surface] which more positively locates the objective. Loosening the objective, as you have done, means you are relying on the screw thread for axis location. [which may cause more aberration than you had originally] ... A packing washer would probably help.

MichaelG.

[BramHuntingNematodes]

2. The screw thread on an objective is intended for attachment, but it inevitably has too much clearance to precisely locate the optical axis ... there should therefore be a ‘register’ of some sort [usually just a flat surface] which more positively locates the objective. Loosening the objective, as you have done, means you are relying on the screw thread for axis location. [which may cause more aberration than you had originally] ... A packing washer would probably help.

MichaelG.

in other words you can jiggle it around if it's not screwed in tight

[MicroBob]

Did you actually turn the objective or did you turn the object micrometer?

Maybe you have not adjusted the parts in the trino head right, but before fiddling at these adjusments a proper test setup is needed, e.g. all other errors reduced and understood. So I would suggest to continue

In practice these little errors might not actually be so important. When a flat object (like botanic section) is photographed it is often best do a stack of a couple of shots anyway. Most objects are uneven and little errors won't show.

Bob

[apochronaut]

.
To me this looks like what I would expect from a chinese plan achromat.

.

I would probably go further, and replace the word ‘chinese’ with ‘typical’

MichaelG.

Plan is plan. The image should be uniformly focused across the field , whether the objective be made in China or not. That is the design . Objectives made in a high throughput industrial situation, such as is the case in modern Chinese industry would be engineered to meet the plan specification but might easily fall short of achieving other specifications not necessarily associated with planarity. The lenses might not be ground to 1/2 lambda uniformly. This would not affect the planarity or the N.A. but could result in an objective that lacked a refined degree of sharpness or contrast. Secondly, the lenses might be mounted in a mount that had sufficient machining variation or material imperfection that the tolerances required to meet the engineering specifications cannot be met. This could also result in the optical axis being non linear, resulting in tipped image planes.

[Hobbyst46]

1. here is previous experience report:
viewtopic.php?f=5&t=7779&p=68463&hilit=unbranded#p68463

2.

...I'm really sorry, I'll try to make a better images and post them ASAP. My microscope has a condenser diaphragm and the field aperture, these were adjusted as I indicated in my first post. The condenser NA was set to 0.2 to match the NA of the 10x objective. And the field diaphragm was open no wider than it was neccessary to get the even illumination of the whole field of view. Perhaps I used too bright light or I was too impatient in setting the fine focus.

Perhaps the condenser setting was not optimized.
Setting Kohler illumination gives the best contrast:
1. Choose the 10X objective
2. Fully open the field aperture
3. Open/close the condenser iris to roughly the NA of the objective
2. Raise the condenser as far as it goes
3. Focus on a slide (the stage micrometer, or the blood smear, or another one)
4. Fully open the condenser iris
5. Fully close the field aperture (a small peephole will remain)
6. Lower the condenser until the field aperture is in focus as accurately as possible
7. Open the field aperture until the field of view is bright (not beyond).
8. Close the condenser iris to set it at about the NA of the objective
This process should yield a uniformly bright field of view, with good contrast and resolution.

[MichaelG.]

Plan is plan.

.

Must admit, I’m surprised to read that from you ...
Almost certain that I have seen you recently assert that there was no clear definition of Plan.

MichaelG.

[MichaelG.]

.
in other words you can jiggle it around if it's not screwed in tight

.
True

But it also means that, if not screwed in tight, the axis can tilt and shift under the effect of gravity.
... You don’t actually need to jiggle it

MichaelG.

[apochronaut]

Plan is plan.

.

Must admit, I’m surprised to read that from you ...
Almost certain that I have seen you recently assert that there was no clear definition of Plan.

MichaelG.

g
That is true because plan is determined by the f.o.v. An objective rated as plan with an 18mm f.o.v., when used with eyepieces that yield a 24mm f.o.v. will no longer be plan. That's why plan is relative. The very first AO planachro 10X was the cat. #1019. In the series 10/20 system it was plan( barely so) with the recommended 19mm f.o.v. eyepieces. However, when used in the subsequent 110/120 system, which was configured to use 20mm f.o.v. eyepieces, it's planarity was challenged. The difference wasn' great and you still see lots of them on series 100 stands but it really was no longer plan anymore in that applicaton. The cat. # 1021 objective solved that problem, which is fully plan across 20mm and beyond.
My comment in this thread doesn't contradict that. If a microscope objective is deemed plan, that spec. has only to do with it's planarity across that manufacturer's chosen f.o.v., so it is plan in that case, whether it be Chinese or not. Other issues related to it's limited conception or shoddy construction will only affect it's planarity by altering the accuracy of the optical path, thus tipping or decentering it's image circle.

[MichaelG.]

Apologies if I mis-interpreted your remark.

I only mentioned Plan in my post to stay consistent with with the opening post ... I nearly wrote simply ‘achromat’ and ‘apochromat’ but didn’t want to cause confusion by omitting ‘plan’.

... can’t do right for doing wrong, as they say

MichaelG.

[Naphthalene]

I wish to express my gratitude again to all who have spared their precious time for my question!

MicroBob , you say:

- Did you actually turn the objective or did you turn the object micrometer?-

I did the first - rotated the objective in its nosepiece thread . Thank you, I understand the consequences of loosening the mount. But I attempted to do this very slightly, trying not to shift the objective's vertical axis.

- When a flat object (like botanic section) is photographed it is often best do a stack of a couple of shots anyway.

I have forgot to mention one significant detail. I can't get the focus on the outer parts of the field of view. They are always blurry, independently of the position of the focusing knob. So it is not the focusing problem, and looks more like the dirt on this area of the objective (though it could not be, as I performed the cleaning many times).

Hobbyst46 thank you for the link! While you have concluded that Olympus and unbranded objectives are equal, I see the green halos around the numbers "1" and "3" with the unbranded objective, which are almost absent with the Olympus one.
Thank you for illumination setting protocol. I followed these directions to set up the Kohler illumination.

Here I have attempted to produce the sharper images. No post-processing software was used.
The normal objective position shows stronger blurring on the upper right edge

while it is much lower in this area when the objective is set at the slightly rotated position


Obviously, the ruler shown on your image viewtopic.php?f=5&t=7779&p=68463&hilit=unbranded#p68463 is much sharper, but it must be noted that we have a different slides. Mine is a 1 mm line with 5 micron divisions carved (not painted on the glass), while your seems to be painted by the ink and the divisions are apparently larger.

From what I have learned here, the quiality of modern massively produced objectives may be not so high, and there is no guarantee that they will ensure the perfect sharpness along the whole FOV.

Maybe you have not adjusted the parts in the trino head right, but before fiddling at these adjusments a proper test setup is needed, e.g. all other errors reduced and understood. So I would suggest to continue

And I will patiently do that!
Since I still can not draw a conclusion, if I'm facing a factory defect, or I may have decreased the image quality by inexperienced reassembly.
What I really regret that I have made only a few shots with the 10X objective before starting to dismantle the trinocular turret.

Here is one picture I have made before touching the turret. It is quite unsparp as whole, so it is hard to tell if the edges are out of focus.

[MicroBob]

They are always blurry, independently of the position of the focusing knob.

Is it possible that you expect too much of your achromat? As I showed with my blown up corner area the different colours are not projected on the same point by an achromat. This leads to a lower sharpness towars the corners. A plan apochromat wouldn't have this problem.

Is your camera set parfocal with the eyepieces or do you have to refocus? Do you have to refocus a lot when changeing objectives?

Bob

[Naphthalene]

Good day, Bob!

Is it possible that you expect too much of your achromat?

This is quite possible! I will surely look to get an apochromat objective in future.
Please tell, will the Plan Fluorite objectives also show better sharpness in the edges of the FOV?

I intended to use my 10x Plan achromat for two tasks: to make panorama images of large samples and visualize the fluorescent stained samples.
The current objective is not suitable for either of these. It is no good to make panoramas because of it's unsharp edges of FOV. It is more logical to take shots with 20X than to crop the 10X images. As for the fluorescence, the 10X objectives appeared attractive to me because they produce less photobleaching than the higher power ones. My microscope has no grey filter or fluorescence attanuator to decrease the bleaching. However, my current 10X gives very poor illumination (I believe that's because of its low NA) so the fluorescence is too faint to be imaged in most of my samples.
I purchased this microscope to perform the fluorescent staining at home, and in case of success, carry the samples to my lab where I work at. This is a great solution for the hollidays, summer vactions and especially in the current quarantine situation when it is difficult to do my job in the lab.

Is your camera set parfocal with the eyepieces or do you have to refocus? Do you have to refocus a lot when changeing objectives?

The camera is not parfocal with the eyepieces and it wasn't before the reassembly of the trinocluar head. At the same time, there is very little refocusing when I swicth the objectives.

[MicroBob]

The camera is not parfocal with the eyepieces

This is an important thing. The optics are calculated for certain distance settings. When you don't have to refocus when changeing objectives the tube length for the eyepieces is right. So when you have to refocus to get a sharp photo you move away from the correct front lens - object distance. This can lead to strong image defects, easily much stronger than visible in your images. I would suggest to try to get the camera parfocal and test the image quality again.

Stitching: My impression is that it even improves the image quality when it shouldn't show, like in web resolution. I haven't found the reason so far.

Fluorescence: Are you using transmitted light or incident light? My 18W incident light white LED runs full power to get a bright image so this really takes a lot of light.

Bob

[Naphthalene]

I would suggest to try to get the camera parfocal and test the image quality again

I would gladly do that, but my camera adapter has no special mechanism for adjustment of camera focus/height

My impression is that it even improves the image quality when it shouldn't show

Unfortunately, I experienced the opposite. I'm using photomerge function of Photoshop CS5 and a PTGui software for making panoramas. Stitching one image with off-focus outer regions often causes distortion in the finely focused areas of the adjacent images.

For fluorescence I'm using 103W mercury arc lamp as an excitation light source.

[Scarodactyl]

Please post a picture of how your camera is attached.

[Naphthalene]

The camera is mounted this way:

The male screw facing us serves to fix the the camera rotation. The bottom male screws (barely seen) are needed to mount the adapter and center it against the phototube
The camera C-mount thread is directly installed on the adapter, there is no adjustment ring.
And this is how the dismounted adapter looks like:

[MichaelG.]

IF you need to increase the camera tube length to improve parfocality, then C-mount extension rings are readily available.

MichaelG.