My light cone does not appear to be centered

[Boomod]

I have been trying to size-out my system in order to prepare some Rheinberg filters, so i have prepared a spectrum of black disks of various diameters centered in a clear ring. The positioning is precise - I am using a crafting cutter and software (signs/scrapbooking etc device) to cut clear rings in acetate sheets, and duplicate cuts in black material. I then replace the inner spot with a black spot, assembled carefully between laminating plastic, and fuse them together. Works very well.

However, I have not been able to find the size that gives the clean expected result of dark background, bright foreground - i get close with some filters, but only parts of the sides of the foreground are bright white while the center of my view is dark to distorted yellow, yet the filters on either side of them are too dark all around, or too bright and hazy on the background.

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I think i figured out why just now, I put a smaller dot filter in place, then dropped my condenser all the way down and fiddled with the condensor iris a bit... I can clearly see the round dark filter is not centered in my viewfield, but off to a side. I placed a micrometer slide in place, and with dim light, no filter, and slowly closing my iris, i could see that the bright focused spot of the condensor is clearly off center


This hasn't had an impact on the majority of stuff i've used it for before, but, I've gotten excited about optical staining and some dark filters for interesting illimunation effects on upcoming projects, and seeing how this misalignment will be an issue - because the prinicple requires the condenser focusing the outer ring to a point, and the inner ring completely blacking out the background evenly. I have removed and placed the filter in different orientations, it does not change (again, precision centering of the filter center on disk); I have rotated my condenser unit and wiggled it while tightening, and though i can slightly modify its placement, the displacement from center s still quite large.

I need to adjust something to tweak the alignment closer to center, as the current positioning looks like this:

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This is the scope:
http://www.microscopenet.com/40x1000x-t ... 10112.html

Is there something I can adjust for the condensor housing or stage connectors or such that would let me adjust the position of it and get things lined up on center? Is this a common problem for units to have, with a solution guide?

[75RR]

Have you checked if there are any centering screws at the back of the condenser?
If there are they could be at 10 and 2 o'clock.

[Boomod]

Ah hah yes indeed, tiny tiny screws at the back of the condensor, small phillip-head ones. I can examine using them as setscrews and see if i can make adjustments.

Are there any formal procedures used to get the absolute best result, processes that give a way to be able to give best possible results? I can use a micrometer slide to crosshair the center based on measures to either side...
The only way i can see adjusting is to use my smallest black-dot disk but that requires the condensor still being the furthest away from the stage as possible, but perhaps that wouldnt matter if manages to align the normal axes

[Boomod]

Correction to that - the phillips screws are on the condensor itself where the filter unit is, likel as attachment.
I see that the holderframe does indeed have two black hex-set screws in the 10 and 2 that were impossible to see in my ambient lighting.
I will have to do some hunting for a keywrench that size before attempting

[75RR]

Mine are large knurled screws, I use them every time I change objectives if they are not exactly centered.
They are not on the condenser but on the base that the condenser sits on. see photo.

To center: I close down the field iris to center roughly, then open the iris until it is just on the edge of the field of view, when the thin border is the same width all round it is centered.

Note: The one at the front is only used when placing the condenser or taking if off.

[gekko]

Boomod, my 2-cents' worth (and please forgive me if all the following is old hat to you; also if I am wrong-- quite possible, I hope someone will correct me): your condenser has been centered at the factory and should not need re-centering unless someone has altered that. If your microscope was bought new, I would be reluctant to attempt to recenter it unless I knew exactly what I am doing.
For what it is worth, to check centering, I would raise the condenser to its top stop, focus on a slide, remove an eyepiece, and look down at the back focal plane of the objective while slowly opening and closing the condenser iris. You should see the iris in focus and reasonably centered. Try that with the 10x and 40x objectives. If it does not look too much out of center I would leave the condenser centering alone (I, myself, would be reluctant to try to center the condenser unless I'm absolutely sure that there is a problem.)
Your problem with the DF stop: what I would do is focus on a slide, raise the condenser to the top stop, place your DF stop (it should be located immediately below the condenser iris (a filter tray that is part of the condenser is usually a good place). Open the iris all the way, then remove the eyepiece, and look down the tube: you should see the DF stop completely covering the objective aperture. If not, slightly tweaking the vertical position of the condenser may get you there. If you see an annulus of light, then you need a slightly larger DF stop. You should be able to get good DF with achromatic objectives up to 20x or 25x. For higher objectives (40x) getting DF is more iffy-- you can try to size the DF stop more exactly, and you may need to oil the condenser to the slide (in that case the condenser must have a NA greater than 1.0, as otherwise oiling it won't help and may damage the condenser since it may not be sealed against oil).

[lorez]

I hate to be the" bearer of bad tidings", but I may be, none-the-less.

On your particular microscope you may find more problems than you may imagine.

Besides having no adjustment screws for centering the condenser there is also no field iris so you may have to make a reliable target to substitute.

Another thing to check is the alignment of the condenser holder. Many on this type of scope are not perpendicular to the plane of the stage and as a result what is centered in one position is not centered in another. In addition to the condenser holder you may also want to check the stage as it may not be perpendicular to the optical axis.

Finally, there is the glass used in the microscope. Several years agoI presented a couple of workshops on Rheinberg illumination and the construction of the filters. Each participant brought their own microscope so there was quite a variety to compare. What was evident was that the better scopes such as Nikon and Olympus and even Swift (USA) out performed those originating in China and India with regards to color and contrast.

I do not want to discourage anyone from trying new techniques and would certainly encourage everyone to try anything they wanted. I just think it is nice to have a little broader idea of what some of the challenges may be.

lorez

[lorez]

For what it is worth, to check centering, I would raise the condenser to its top stop, focus on a slide, remove an eyepiece, and look down at the back focal plane of the objective while slowly opening and closing the condenser iris.

Although this is true I don't think it is a reliable technique. Unless your pupil is exactly centered and looking straight down the tube the image of the iris will shift as you move your eye. It is somewhat like trying to aim a rifle with only one of the sights (front or rear).

I agree with gekko in his recommendations, but am not nearly as confident that everything from the factory is done correctly.

lorez

[gekko]

lorez, I think your comments are very wise. I didn't know that the Chinese scopes could be that poorly made.

[Boomod]

I tried the method before of the "remove the one eyepiece and look down the barrel of the gun" approach, but yeah, I couldn't make sense of it. By moving my eye even the slightest amount one way or the other, the image shifted significantly and I had no way of knowing what was really looking 'dead on'

--
I'm not sure what you mean by my system not having a field iris... Its a condenser with a swing-out filter holder attached to the bottom, and a ~160 degree slider range that opens and closes an iris in the condenser..

--
The wording "align my condenser" may be misleading - sure, the optics inside of it are all factory-placed and aligned, I dont want to tear it apart and readjust the lenses in it. I was in fact referring to some way to align the housing such that the light would pass through the center rather than off-side. It appears that in addition to the lone giant silver screw used for loosening or removing the condenser unit there are indeed two other screws in the same plane, which would be used all three in conjunction to properly shift and slide into a desired position - I doubt that the screws were set at the factory to that intense level of precision. Remember - its been very difficult for me to tell that it was focused offcenter at all for over a year, until trying out darkfield disks to size it out. Samples always diffused light and didn't have a shadow or bright spot under normal lighting or such. I will work on the belief that the mounting unit is indeed perpendicular to light axis, and it is just from screw settings that the placement of the normal plane in the condenser is normal, but needs to be translated.

As for stage alignment... I initially tested that by using high objectives focused on the edge of the microscope slide, and on the various measuring targets on my micrometer slides. If the stage was not perpendicular to my objective lenses, then the focus at one end of the slide should subtley shift by the other end of the slide, no, and require fine adjustment to make up for height differentials from the lenses. So I dont think that is a worry.

I might look into finding some threaded screws with a bigger head, or finger knurls, to replace the allen-key setscrews and have a condenser-holder resembling that of the picture above, so that over time, if subtle changes in removing and replacing it cause a shift, the other two screws can be easily accessed for realignment.

[gekko]

I still think it is a mistake to try to adjust the condenser because the problem may not be an off-center condenser, and if so, trying to adjust the condenser will only make things worse. Condensers on microscopes not equipped with Koehler illumination (and therefore not equipped with a field iris) are, in my experience, centered at the factory and are not meant to be centered by the user, which is why the centering screws are setscrews rather than knurled thumbscrews. (Microscopes with Koehler illumination always have centering screws for the condenser, because the illumination system is different).
I may be wrong, but from your original description, I am not completely convinced that your microscope has a significant centering problem that needs correction. The lack of centering screws for the condenser is because absolute centering of the condenser to each objective is not needed (remember that the objective turret indexing is never perfect, so even if the condenser is centered perfectly to one objective, it will be somewhat off center to the other objectives, and that is OK unless the centering is out of spec).
I did not understand the last picture in your original post with an off center DF stop. How was the microscope adjusted (focused on a slide? the condenser raised all the way up?) Where was the DF stop placed, and where were you looking (through the eyepiece or was the eyepiece removed)? Apologies for being so rudely inquisitive, but my aim is to try to be helpful to the extent I can be (or, more correctly, to the extent I think I can be). lorez and others will, I hope, chip in as necessary to correct me.

[75RR]

This is just a general comment on making adjustments and or removing a part should that ever be necessary.
Always try to find a way to mark the present position so that you can return the piece precisely to where it was.
Realigning misaligned parts, especially if there is more than one can be a pain.

[lorez]

My initial comments were to share some of the things I have seen that can be out of adjustment. I have no idea, without actually seeing the scope, what, if anything is, or may be wrong. It's a frustrating learning experience to spend a lot of time and effort correcting one perceived problem to find out that it is actually something different. Being aware of all the possibilities gives you an edge as you begin to troubleshoot.

As for stage alignment...

Setting the stage position to assure that it is perpendicular to the optical axis is done mechanically using a tool and a fixture. There are too many variables to do it optically. Again, there may be no problem, but it is just one more aspect of the process that should be considered.

lorez

[Boomod]

I did not understand the last picture in your original post with an off center DF stop. How was the microscope adjusted (focused on a slide? the condenser raised all the way up?) Where was the DF stop placed, and where were you looking (through the eyepiece or was the eyepiece removed)? Apologies for being so rudely inquisitive, but my aim is to try to be helpful to the extent I can be (or, more correctly, to the extent I think I can be). lorez and others will, I hope, chip in as necessary to correct me.

I attempted to use a variety of center-stop disks to find the sweet-point where the background was black, not greyed, and the sample was illuminated in the center core of the view - with the condenser iris opened fully, and condenser raised all the way to the slide. Subject-focused slides ive been using include simply dilute riverwater dehydrated on a slide, a slide with dilute diatoms (a mix of long large rods and some small crescents) a slide with small salt crystals, and a multi-target micrometer slide. The riverwater is very flat dry slide with microcrystals and small flat debris. I've worked with my 4x and 10x objectives only, as I am aware of the 'near impossibility' of getting a 40x to work. I've used a larger range of discs earlier, but a smaller range of sizes with my more precise disk-assembly method based on the 'promising leads' earlier, thinking it was a half millimeter difference making the difference betweeen 'being too small and too big' to work properly.

I have been using the approach of various centered black stop disks, full illumination, fully raised condenser, looking at the system through an eyepiece. As confirmed, when I tried using processes that require removing the eyepiece and looking at the bottom of the condenser, I found it very difficult to work with exactly as stated here "like looking through only one sight on a gun" and could also not discern things as instructions suggest they would.

In changing the disk sizes, the closest I could get was some consistancy between two diamters that gave a dark, unhazy background but the salt crystals, and bigger objects in the view were only 'white'/yellowish brightly illuminated when located in a crescent shape on the left and bottom side of the scope, while the equivalent placement on the upper and right side of the scopes had it in a dark, barely illuminated, and slightly different sized disks resulted in the brightest spot on the scope being off centered. I could not produce dark backgrounds with illuminated objects near the center, the only way to illuminate the center was a washed background, and still highly differential illumination of the object while moving it around in the field.

So it was from this "I wonder if things are off-center" pondering that I used a small disc (about 13 or 14mm) in the filter, and dropped the condenser all the way down. As the condenser dropped, I could see it shrinking (and expanding when reversing) a spot in a single location, off centered, as shown in that black-dot picture in the original posting. So it is from all of these off-center illumination effects of my samples, assymetric visuals, and shrinking-and-expanding about the same off-center centerpoint of the test disk projection within the field that seems to conclude that the light cone axis is not coaxial with objectives optics.

What other effect can be causal of the ability to expand and contract a filter stop center on a point that is about 1/3 radius off-center to the 2 oclock position. (its a bit more extreme than the fast drawn image above represents) Given that the center stop needs to be symmetrically centered in order to give a consistant darkness with equal illumination from the sides to focus on concentric, it just seems that the conical axis from the condenser is not centered on my objectives. And an off-tilt stage would have zero effect on the off-centered shrink/expand point of the small stop with no slide in place, so wether the stage is uneven or not, thats an independant matter.

[gekko]

Thank you, Boomod, for your exhaustive reply. I will try to duplicate what you have done on my scope with a centered and off-center condenser and see what I can learn, and then I'll post a reply. It will take me a couple of days before I can truly get to ti. Meanwhile, I hope someone else will chip in.

[The QCC]

Another two cents worth.

The two set screws on your condenser are set at the factory. They are really stop screws, not centering screws.
The big screw that holds the condenser in place presses the condenser against the stop screws.
Adjusting the stop screws while the condenser is being held firmly by the big screw will not centre the condenser.

If might be worth your while to purchase a set of Rheinberg stops that are precisely made to fit your filter holder. Rheinberg and darkfield stops

It is a feel good thing to make your own, but sometimes the manufactured item is better.

[Charles]

And my two cents. Maybe it is the filter holder that is mis-aligned? How is the DF stop being placed in the filter holder.? Move or rotate the stop in the filter holder and see if things change. Alter your DF stop to align properly in the filter holder.

[gekko]

I think Charles' suggestion is worth much more than 2 cents! It would help if, in the last image, you had closed the iris so its edge is visible. If the iris is centered, and only the DF stop is off center, then Charles has solved the problem. The filter tray stop sometimes allows the tray to "close" beyond the exact center. In fact, to check, remove the DF stop (it is a red herring, I think) and lower the condenser to get the iris in focus, and close the iris partially: is it centered?

[lorez]

lower the condenser to get the iris in focus

That would be the solution if this microscope had a field iris. Since there is no field iris so you may have to make a reliable target to substitute.

lorez

[gekko]

lower the condenser to get the iris in focus

That would be the solution if this microscope had a field iris. Since there is no field iris so you may have to make a reliable target to substitute.

lorez

I was thinking of lowering the condenser sufficiently to get the aperture iris in (or near) focus since Boomod could lower the condenser to get the DF stop in focus. I think the problem would be to get the closed iris opening small enough to get it into the field of view (a very low power objective may be required, maybe 2x or 1x). If Boomod cannot see it with the lowest power objective he has, then my idea would not be workable.
Having said that, your idea of a target to substitute for a field iris maybe the thing to do. Something like cutting a piece of white paper to fit exactly over the light port and drawing, with a pair of compasses, a series of concentric circles (alternatively drawing the target with an image editing program and printing it)? I think that may well be a useful and simple approach-- thank you.

[Boomod]

Experiment and Observations:
4x objective, condenser dropped all the way down, light at a medium-low intensity, no filter
Objective focused on a micrometer image, then the slide removed, and the iris closed all the way, light ramped up to about 90% intensity.

Image is fuzzy choppy, randomly shaped-edged ring of shadow around light within the view, approximately centered but impossible to tell from perimeter. (First image)

Slowly raised the condenser, the light area gets smaller, and becomes more sharply focused on the perimeter, which is really choppy and random (Second image)

Keep raising, and the light spot grows, the perimeter gets smoother, softer and rounder. (Third image)

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Raising further, the edge of the ring starts to blur into yellow edge effect bleeding into the black surrounds, and at a point short of the maximum height of the condenser, the 2oclock position of the bright spot 'kisses' the edge of the view in eyepiece, shrinking the border to near nothingness while a crescent black border area remains around the rest. (Forth image)

Raising further, the black border is pushed out of view, with just a fuzzy thin edge-boundary on most of the edging remaining, and the outer ~1/3 to 1/2 radius of the white spot takes on a 'crazed' appearance (Fifth image)

From that point, it is just but the slightest further turn that maxes out the condenser height, during which the craze effect rapidly smoothes out and disappears, a homogenous bright spot remains. The color of the homogenous circle appears to be greyer than the bright spots that were present in the first three images, though that could just be the contrast with so much darkness that made it appear so.

[gekko]

Thank you, Boomod, for trying this. It does appear that your condenser is misaligned to your objective. Please accept my apologies for doubting that, my aim was to play it safe. Before trying to center it using the set screws, I would try the experiment suggested by lorez: make a "target" and place it centrally over the light port/lens on the base, focus the microscope on a slide, then adjust the condenser height to get the target in focus. Switch from 4x, to 10x, to 40x and see if the direction of out-of-center is much the same with all objectives. Centering becomes less critical for lower objective powers. If it is fairly well centered with the 40x objective, I would not try to correct it.

As lorez pointed out, it may well be that the condenser is centered at the top of its travel where it would be when using the microscope, but may drift off center as you lower it because the rack-&-pinion on which it travels is not aligned vertically as it should be. In any case, it would be best to center the condenser (or at least test the centering) when it is close to its normal operating position, which is why I think lorez's "target" idea is a good one. A slight shift when you change objectives is, I think, inevitable, so it would be best to get the centering optimal with the high power objective.

I hope lorez and others will comment on this before you center the condenser in case I am missing something important.

[Charles]

It would be interesting to see what the 10X looks like and the 40X as Gekko suggests. The 4X is a poor objective to center most scopes. I find on most of my scopes each objective is different. That is why most upper end scopes have condenser centering capabilities. If the 10x and 40x show similar views, then I would center the condenser. If they are different, then center the condenser with which objective you will use most for your DF projects. Centering for DF for lower powers are less critical than with higher powers.

[Boomod]

10x objective, focused on micrometer target, closed iris far as possible, dropped condenser down to bottom as far as possible, light source at maximum.

The field is awashed with a dim off-white gray, with a thin edge on the left side. As it raises, the edge thickens slightly, and condenses in darkness, but is rather translucent-ish. A little higher, and suddenly the edge takes definition as a craggy crescent, but still very light and the darkness of it coming from a gradient of shades towards the craggy edge. A bit further still, and then the craggy edge comes into focus, and the cresent has nearly uniform density/color, as appears in image A. Rising further, the cresent diffuses out rapidly, barely shifting while the craggy edge becomes smoother and softer with the smallest increase in height (B), then it starts to diffuse into a vaporous edge. Just before losing the last bit of visible definition, a blobby, diffuse bright spot appears with it off center to the 2oclock position (Image C). This brighter spot doesn't have a pinprick origin, but first appears with an approx diameter of 1/8th view diameter, with undefined edging, just a blobby gradient.

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The bright spot grows rapidly while the cresent continues to diffuse and recede to the outer edge, then the cresent becomes very sharply dark with a bright red edge diffusing to yellow. (Image D) Raising further, the field becomes awash in bright light with no edge pushing the crescent out of view. Just as it is is no longer visible, a diffuse small grey blob appears at the same spot where the bright light blob appeared earlier (Image E) As with the bright blob before, it is small and very diffused, no clean edges visible, and hard to observe existing before it suddenly appears at 1/8th diameter of field. This small diffuse grey cloud expands very thinly over the entire field as it continues rising, thinning out and being barely different from bright white. (Like RGB(254,254,254)) (Image F). As soon as the entire field is homogenously colored and bright, the next travelling position results in extremely course crazing of the entire field almost uniformly (Image G), then quickly beyond that, clear homogenous brightness as the condensor is raised into its maximum height position.

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====================================
40x objective, focused on micrometer target, closed iris far as possible, dropped condenser down to bottom as far as possible, light source at maximum.

Initial view is totally homogenous bright but off white field. As condenser is raised, field becomes brighter and brighter uniformly (no visible edge from iris, or hot-spots with gradients outwards) and continues to increase in brightness until briefly growing slightly dimmer immediately before the entire field crazes, then beyond that, uniform brightness returns, but the is slightly less bright/whiter than the brightest field which occured just before dipping to craze.

[Boomod]

Im not clear on the "target" for the base concept.

Do you mean to put something like a quarter or a dime on my actual light lens? How to position it? Assume 100.0% precision focusing of the light source cone axis with the objective conical, and make a precisely centered object to cover the lens?

Is a dime/quarter way too big, way too small? Too thin, too thick? I would suspect whatever shadow it cast onto the condenser would be similar effect to the small diameter filter stop i first tested, though i guess if the spot is on the light source rather than in-line with the movable condenser, it would be offside in the opposite direction.

Either with a spot on the light source, or a precision spot in the filter holder (my holder has a good stop position, and i can rotate my filters fully around with no shift in the shadow position earlier) I'm guessing that the results of my tests say that indeed, planar translation of the condenser normal to the light cone axis is required in order to center the dark stop filters and to center the iris effects seen in 4x and 10x, and will have no impact on the 40x.

And given that my expectations coming in were to make a set of filters sized for the 4x and a set sized for the 10x and to not even try the 40x and higher... the conclusion is that the difficulty i had sizing those filter stops is due to the discrepancies, the 4x being much more difficult to find a non-grey spot with uniform lighting, while the 10x wasnt as far off -- due to the light rings being dramatically and mildly off-center as shown by the iris positions.

[gekko]

Target:
(1) focus microscope on a slide, then move the slide to an empty area
(2) use a pair of compasses (or a computer program) to draw concentric circles increasing in size from a small (dime-size or smaller) inner circle going out. I would use thin paper (or drafting paper if available) so the paper is not opaque. Cut the paper so it just fits the light port at the microscope base, put it on the light port/lens on base (not in the condenser's filter holder) and center it.
(3) without changing the microscope focus, adjust the condenser height to get the image of the concentric circles of the target in focus, and check how off center the circles are relative to the field of view.
(4) repeat this for all objectives. The lower the objective power/NA, the less sensitive it is to decentration.
I hope this helps.

Regarding the DF stops, you can make one stop for both the 4x and the 10x (make it slightly larger than needed for the 10x, and it will be quite a bit larger than needed for the 4x, hence will "cover up" the decentering that you see). Only when the NA of the objective and condenser become close do you need to size the DF stop very precisely.

Good luck. And, again, I hope other members will offer their suggestions.

[lorez]

Everything gekko said above is exactly what you need to do.

lorez

[Boomod]

I still dont really understand the instructions - I sense putting something onto my light source, yes, and trying to make it fit the size of it exactly.

But what does the concentric-circles part mean -- Black line drawings? Or making a couple or dozen of big disks with holes cut out? Colored in on the paper or just outlines. What spacing difference - how many should I try to generate?
Paper or would printouts on laser-acetate be better or worse?

"Center it" on the light..... meaning that the circles should be cocentric with the cutout fit on the round surface, or meaning cut the target smaller, and do an alignment of the center by looking through the scope? Otherwise, if the circles are bigger and bigger from one point then i would assume the center is already-there based on cutting the outer part to fit

Since I have 0 idea what is being described, I can't envision enough of this process to say "ah, all the information was presented, I just needed to figure it out carefully"

[gekko]

My apologies. I should have indicated the purpose of the concentric circles. They could be drawn in pencil or ink on paper, or they could be printed on acetate film. Either way, the will be imaged by the condenser on a slide. Only one circle is needed for a particular objective, and will serve to show you whether the circle is centered in the field of view or not. However, if you draw multiple concentric circles, then one "target" will work for all objectives, for the lower magnification objectives you will be using a large circle that is within the filed of view, and for the 40x, for example, you will be using a smaller circle. You could of course just have a "dot" or a single very small circle that is within the flield of view of the highest power objective and use it for all objectives, but it is harder to judge if the dot or very small circle is centered in a large field of view. I hope I have muddied the waters further.

lorez, thank you for checking and confirming my "instructions".

[75RR]

I sense putting something onto my light source, yes, and trying to make it fit the size of it exactly.

I think the idea is to place something on your light source (to substitute for a Field Iris) that would help you centre the Condenser. If you make it the exact size of the light source housing it should be centered and also stay in place.

These were made to help choose the correct size of darkfield stop. Place something similar on your light source.