Field diaphragm problem

[Leitzcycler]

When centering and focusing the field diaphragm a multiple image occurs. Would the reason be a damaged mirror surface? Any suggestions?

[apochronaut]

Is this the correct orientation of the image, with the ghost images of the field diaphragm left and right of the reflected image, rather then above and below?

[Leitzcycler]

You are right: the ghost images are above and below. The image orientation is false.

[apochronaut]

Probably at some point, the mirror has been replaced with a backsurfaced or second surface mirror. You are getting reflections caused by the glass surface of the mirror causing ghost images.

[Leitzcycler]

But this does not affect the microscope performance, does it?

[apochronaut]

I would not think normally for average BF observation, or most observation where the lamp is critically centered to the condenser ,because you are using the central mirror reflected lamp image for koehler adjustment. It might be a problem for oblique, where a ghost image might cause unwanted edge flare or high magnification DF because of a slight diminution of the lamps intensity. I have an old microscope set up for phase and it has the same issue. I can't in any way see that the quality of the image is affected but then I have not done a qualitative one to one with the same microscope fitted with a first surface mirror.

[martinprieto]

Hi Leitzcycler,
Many times that effect is due to reflections produced by thick dichroic daylight filters fitted over the field lens. It occurs only with dichroic filters, not with absortion ones. To enhace its performance many scopes fit these filters slightly inclined and produce this effect.
Do you have a thick dayligh conversion filter fitted over the field lens? If so try to remove it and check if the ghost images appear.

[Hobbyst46]

Hi Leitzcycler,
Many times that effect is due to reflections produced by thick dichroic daylight filters fitted over the field lens. It occurs only with dichroic filters, not with absortion ones. To enhace its performance many scopes fit these filters slightly inclined and produce this effect.
Do you have a thick dayligh conversion filter fitted over the field lens? If so try to remove it and check if the ghost images appear.

Hello and welcome Leitzcycler and martinprieto.
1. I tried to imitate the ghost appearance on my scope and field lens with my assorted interference filters (cutoff/bandpass, thin (1mm) and thick (3-5mm) in vain. Tilting the filter shifted the image of the narrow, closed down field diagram, but not create another image. So there is perhaps another reason.

2. Independently of what I did not find, what would be the advantage of an interference filter over an absorption filter to just modify the lamp illumination into daylight ? and, moreover, of a thick interference filter AND a tilt on top of it ?

3. My experience is that the ghost images are because the mirror has been dislocated from its original mounting. Or possibly another mechanical damage. I have followed excellent guidance by apochronaut, MicroBob, 75RR and others in the past and replaced a (damaged) second surface mirror with an inexpensive first surface. This act considerably improved phase contrast and mostly darkfield.

[Hobbyst46]

Hi Leitzcycler,
Many times that effect is due to reflections produced by thick dichroic daylight filters fitted over the field lens. It occurs only with dichroic filters, not with absortion ones. To enhace its performance many scopes fit these filters slightly inclined and produce this effect.
Do you have a thick dayligh conversion filter fitted over the field lens? If so try to remove it and check if the ghost images appear.

Hello and welcome Leitzcycler and martinprieto.
1. I tried to imitate the ghost appearance on my scope and field lens with my assorted interference filters (cutoff/bandpass, thin (1mm) and thick (3-5mm) in vain. Tilting the filter shifted the image of the narrow, closed down field diagram, but not create another image. So could it be that there is an additional factor ?

2. Independently of what I did not find, what would be the advantage of an interference filter over an absorption filter to just modify the lamp illumination into daylight ? and, moreover, of a thick interference filter AND a tilt on top of it ?

3. My experience is that the ghost images are because the mirror has been dislocated from its original mounting. Or possibly another mechanical damage. I have followed excellent guidance by apochronaut, MicroBob, 75RR and others in the past and replaced a (damaged) second surface mirror with an inexpensive first surface. This act considerably improved phase contrast and mostly darkfield.

[martinprieto]

Hi Hobbyst46,
I´m sure that apochronaut, MicroBob, 75RR have more experience and probably the Apochronaut's explanation it's the most likely. I´m just trying to give another easier option to try before check the mirror.
I don't understand your second point, usually high quality daylight coversion filters are dichroic filters, and they work better when the light does not hit perpendicularly its surface. Because of this in most modern microscopes (eg. Olympus BX43/53, Nikon Ci/Ni, etc) the place to fit filters over the field lens is not completely horizontal. Here you have a photo showing how the filter fits slighly tilted:

IMG_20191015_191013.jpg (38.84 KiB) Viewed 11321 times

I'm agree with you that ghost images are usually because the mirror has been dislocated, but a dichroic daylight filter is one more mirror on the optical path, and if it's not well placed it can produce optical artefacts.
I have taken a picture tilting my daylight filter:

dichroic1.jpg (14.85 KiB) Viewed 11321 times

dichroic2.jpg (41.8 KiB) Viewed 11321 times

[Hobbyst46]

I don't understand your second point, usually high quality daylight coversion filters are dichroic filters, and they work better when the light does not hit perpendicularly its surface. Because of this in most modern microscopes (eg. Olympus BX43/53, Nikon Ci/Ni, etc) the place to fit filters over the field lens is not completely horizontal. Here you have a photo showing how the filter fits slighly tilted:...

Martin, my 2nd "point" was just a question, so thanks for the answer and clarification. Always a pleasure to learn new things and become updated.

[martinprieto]

Thank you Hobbyst46,
I apologize if I didn't understand what you wanted to say, or if my answer has seemed rude, english is not my motherlenguage and sometimes it's hard to me to express correctly.
Take it into account for future posts

[Hobbyst46]

english is not my motherlenguage...

Neither is it mine!

[Leitzcycler]

Thank you so much! There are so many nice and kind people willing to help.

There are no filters. Actually the mirror was glued upside down as the original glue tends to loose when getting old. The answer was so simple, yet I didn't figured it out. I allready ordered front surface mirror from Ebay and will change it as I don't think I can remove it in one piece any more.

[apochronaut]

So, the illumination beam was passing through glass. Could you tell, if there was in fact a fourth ghost image but quite reduced in intensity when compared to the other two?

[Leitzcycler]

Yes, through the glass surface. There are two ghost images visible blow and only one above. Thanks!

[Hobbyst46]

Yes, through the glass surface. There are two ghost images visible blow and only one above. Thanks!

For the new mirror, a potential alternative to glue is a mirror mounting tape. It is a double sided tape, ~1mm thick, fairly heat resistant (say, up to 50-60C maybe). Or just any double sided tape.

[MicroBob]

Hi together,
this has developed into a quite interesting thread! At first a riddle, then funny (the mirror that was stuck back in reverse orientation) and then very informative with angled filter seats and interference filters for light correction. I know and use practically only microscopes of the pre 1985 era, mostly from the 1960s and there were no angled interference filters for light correction, only blue or green glass filters. On cause once one is willing to use an interference filter it makes sens how to angle it for best results.These flat glass surfaces can really cause problems. My Pentax Q with 8,5mm standard lens that I use for years as my main microscope camera doesn't work well with the old Phomi 1, I get a red hot spot that I relate to the flat front surface of the objective.

Bob

[MichaelG.]

[…] These flat glass surfaces can really cause problems. My Pentax Q with 8,5mm standard lens that I use for years as my main microscope camera doesn't work well with the old Phomi 1, I get a red hot spot that I relate to the flat front surface of the objective.

Somewhat disappointing that it should be a Pentax
... They probably know more than most about the problem: viewtopic.php?p=63056#p63056

http://whitemetal.com/pentax/ap_filters ... #Ghostless

MichaelG.

[apochronaut]

Yes, through the glass surface. There are two ghost images visible blow and only one above. Thanks!

So, in fact your illumination beam is travelling through a glass refractor to the reflecting surface. 3 ghost images of the iris is typical but if there is enough intensity, 4 can be seen; 3 below in diminishing intensity and the brightest above the intended brighter primary reflected beam but theoretically ghost images would be created oppositionally to the brightest uppermost ghost image( the only non refracted beam)to infinity, if the mirror was infinite. A laser would do that.

If we imagine a single beam of light hitting the mirror from the source.

The upper edge of the mirror, or that closest to the front of the microscope reflects and is seen in the lower part of the f.o.v., the lower or rearward edge of the mirror, to the upper in the f.o.v. There is only one pure reflection that takes place, that is not the result of rays refracted through the mirror glass first. It is the reflection off of the surface of the glass, which visualizes through the eyepieces at let's say, point A. It represents only a small portion of the source beam. The reason it will show uppermost in the visual field is that the larger proportion of the source beam refracts accutely to the interface of air and glass, bending down and travelling the distance of the thickness of the glass at a diagonal, striking the true reflecting surface, travelling the thickness of the glass at a diagonal again distancing itself laterally from original reflected portion of the beam. It exits the glass, after again bending accutely at the glass to air interface and follows a more or less parallel path to that of the first reflected beam but just above it, imaging lower in the visual field at point B. It is much brighter because it has been reflected from a polished reflector and has suffered only minimal loss. Upon exiting the interface between the glass and air, a portion of it again reflects off of the glass/air interface back to the primary reflecting surface, again reflecting back out of the glass/air interface to form another slightly distanced ghost image at point C, and so forth, each one reducing in intensity, as long as the physical area of the mirror continues. The really dim ones are just hard to see.
There is always a bright image formed by a combination of refraction/reflection off of the polished reflector, with one image formed by direct reflection from the glass surface above it in the visual field, and a series of multiplely reflected/refracted images in diminishing intensity with distance below it.

Through the eyepieces we see a true spatial orientation of the mirror because the condenser inverts the image and then it is rectified by another inversion through the objective. The eyepieces do not invert the image.

In examples I have used, as the iris is opened there is a fusion of the images, with a small degree of "light bleeding" outside the fused area. I'm not sure how much actual light loss there is.

[Leitzcycler]

Thanks for the very comprehensive explanation. I think I have to read this through a couple of times to understand...

I know and use practically only microscopes of the pre 1985 era, mostly from the 1960s

Interesting. This is not the first time I have heard the microscopes before 80s are better. What actually happened after 1985?

[apochronaut]

Thanks for the very comprehensive explanation. I think I have to read this through a couple of times to understand...

I know and use practically only microscopes of the pre 1985 era, mostly from the 1960s

Interesting. This is not the first time I have heard the microscopes before 80s are better. What actually happened after 1985?

I've had the priveledge of using quality microscope optics spanning about 135 years of production and microscope stands spanning about 100 years and based on that experience , I would say that is pretty broad generalization. I don't think that there is much debate that as you go back in time that the stands themselves were heavier, with more cast metal and machined components but then the possibility of more precisely matching mechanical stresses to material strength is better now, so some of the weight in older stands was definitely overkill. Quality stands, from the reputable manufacturers since 1985 I'm sure are engineered well, with maybe some design oversights, just as would have always been the case. It is the cheaper stuff that might be suspect today.

When it comes to optics, the gains in absolute resolution, field diameters and planarity were not staggering, from the first world war up to about 1950. From then until about the mid-80's there was modest improvement. Apochromats capable of N.A.'s of up to 1.6 have been made and used , at least experimentally going way back, so the possibilities for improved resolution were pretty static and more or less market driven. Optical excellence over a wider field though, was ellusive, until computers allowed for better off axis computations and limiting of scatter. Being able to reach into wider fields with planarity and enhanced peripheral resolution is a feature of post 1985 optics , as are superior anti-reflective coatings and newer low dispersion glasses. Adding all the possible benefits of the many optical improvements, illumination improvements and contrast enhancement techniques that have been refined since the 1980's together , makes for a pretty persuasive case for computer era optics to be a bit of a mini revolution.

[Leitzcycler]

I eventually found time and inspiration to change my mirror. They are selling first surface mirror plates in Ebay at a reasonable price. I used epoxy glue which is, however, not difficult to detach if there will be a need to change it again in the future.