Question Regarding Darkfield Condensers

[J_WISC]

Hello.

I’m considering a vintage darkfield condenser for a Spencer Buffalo microscope. It includes one high-magnification objective.

Question:

Does a darkfield condenser, generally speaking, work with a range of objectives? Does it produce a cone of light and the user adjusts the diameter by raising and lowering the condenser, or is it designed for one high-magnification objective.

Thank you for answering my naive questions. I try to to Google before posting here.

[Alexander]

A dark-field condenser supports a range of NA. A dry one may support NA form very low up to about 0.7 an oil condenser my support NA from about 0.4 up to 1.2. None will support every objective. They are not specific to a single objective though.

[Chas]

As Alexander says .. the oil ones do not cover much of the field when using, say, an x4 objective with a 6x eyepiece.
You might be better off trying Rolf Vossen's method/modification of an Abbe condenser, for general use :
https://microscopyofnature.com/low-budg ... lumination

But to do this you need Abbe where the top lens unscrews off (not all do).

This modification is very similar to the very early Spencer darkfield condensers, where bottom surface of the top lens of the Abbe was ground flat and blackened

[Phill Brown]

How long is the lag from posting here to when it appears on their site?

[J_WISC]

How long is the lag from posting here to when it appears on their site?

Interesting question. Apparently less than 24 hours.

Also, I should have used better search terms before posting my question here.

[J_WISC]

On closer inspection of three or four AO Spencer darkfield condensers posted for sale, it is possible to read the inscription showing they’re for NA 1.15 to 1.25. So I would require a proper objective, and insert, to use it.

Only one comes with an objective, and it doesn’t look quite right. Also, I’m not even ready to pursue normal oil immersion techniques. So a darkfield condenser is clearly beyond my league at this point.

I’m looking forward to trying some of the far lower cost DIY hacks described above. Probably more suitable for my intermittent puttering.

Thanks for tolerating me!

[Alexander]

A dark-field condenser marked 1.25 - 1.15 will work with objectives featuring a NA of 1.1 (if you are lucky) or 1.0 (for sure) down to about 0.4.

[Phill Brown]

I'm confident you will enjoy the DOF from x40 .65 far more than the resolution at the limits.
X20 .5 is great when you train yourself to see what is there.
Get some bargain diatom strew slides from eBay and just have fun with it.

[apochronaut]

On closer inspection of three or four AO Spencer darkfield condensers posted for sale, it is possible to read the inscription showing they’re for NA 1.15 to 1.25. So I would require a proper objective, and insert, to use it.

Only one comes with an objective, and it doesn’t look quite right. Also, I’m not even ready to pursue normal oil immersion techniques. So a darkfield condenser is clearly beyond my league at this point.

I’m looking forward to trying some of the far lower cost DIY hacks described above. Probably more suitable for my intermittent puttering.

Thanks for tolerating me!

The specifications marked on earlier Spencer DF condensers reference the slide thickness tolerances not N.A. The slide thickness is important because it determines the focal point of the light cone and thus whether DF will occur or not. As to their N.A. , that seems not to have been published but up until the toric DF condenser was issued Spencer systems worked at about .85 to 1.0 N.A. in the objective or below., so the oil substage DF condenser itself was likely about 1.15 - 1.35 .
They had 4 possible condenser systems for DF.
1) There were various stops made for DF between 5X and 35X objectives. These could be used in the condenser filter tray.
2) There was a stage condenser up until about W.W. II. These were placed on the stage, oiled, then the slide placed on the condenser : the condenser itself thus became the stage. They were about 2/3 the cost of a substage DF condenser but had to be manually centered. I don't think too many were sold
3) A more common type of spherical or bicentric oil DF condenser.
4) #3 with an integrated illuminator.

The one you are seeing on ebay with the objective is #3. That is a 40's vintage 95X 1.25 achromat objective fitted with a DF funnel stop with it. It will work for DF in any Spencer 160mm finite tube system but for a 95X objective, you need sufficient illumination. If using a mirror, you will need 100 watts of incandescent or halogen and about 20 watts of led.

Shortly after W.W. I Spencer changed from a sleeve condenser mount to a dovetail. The early DF condensers were a 1 29/64" sleeve or just about 37mm, less than .1mm under. All Spencer DF condensers continued to be a sleeve of that size up until 1980, only with an appropriate solid chrome plated brass dovetail ring fitted over the sleeve and locked on with a single tiny set screw.
So , any Spencer oil DF condenser with a chrome dovetail mount can be used on an earlier microscope with a 1 29/64" sleeve condenser mount, simply by removing the dovetail ring.
There were 4 dovetails. The earliest was used on most stands up until the series 15/35 came out after WW. II. There was another larger dovetail used on larger stands and research stands up until 1955. The third type was used on series 15/35, 2/4 and infinity corrected scopes up until 1980. The 4th was used after 1980, is black anodized aluminum and is integrated into the condenser body.

Generally speaking the Spencer oil DF condenser can be used with objectives of 40X and higher if the eyepieces are W.F. but with earlier microscopes fitted with Huyghens,compensating or planoscopic eyepieces you usually can use a 20X objective as well, since the f.o.v. is much less.

For objectives of over .85 N.A. there were 3 methods of lowering the N.A. in order to achieve DF.
The first is using a factory funnel stop, which sets the N.A. at .85. These were available for early 58, 82 and 85X apochromats, as well as the more common 60, 90 and 115X apochromats. Additionally, they were available for 60X oil immersion, 95, 97, 113 and 115X oil achromats.
The second was a 95X achromat objective with a reduced N.A. of .85. This objective was made for 20 or so years after W.W. I and is marked " for dark field" on it.
The third is an integrated iris diaphragm. This was available for the 95 and 97X 1.25 N.A. achromats and as well briefly for the 90X 1.30 N.A. apochromat.

[J_WISC]

Okay. After reading everything I could find in the forum, carefully checking specs, and measuring my condenser holder, I decided to try this Spencer condenser …

IMG_5452.jpeg (122.09 KiB) Viewed 102783 times

I neglected one measurement. The dovetail fits perfectly, but the darkfield condenser is much simpler and much shorter than the original Spencer condenser assembly and the holder won’t lift it far enough to reach the stage!!!

It was a gamble, but not yet lost. I set up a temporary fix and was pleasantly surprised to see sand grains on a black background at 4x and 10x. So I know the condenser will work. 40x, no. But the temporary fix isn’t going to work for careful centering or oil between the condenser and slide or setting up better illumination.

I’m writing to ask about the threads on the top engraved part of the darkfield condenser. The outside diameter appears precisely an inch. Two options might be (1) a simple extension tube or (2) an adapter ring so I can thread the top part of the darkfield condenser into the RMS of the original condenser.

Has someone already solved this problem? Or can someone suggest specs for the darkfield threads? Hoping for existing easily acquired adapter parts. Thank you.

[MichaelG.]

Okay. After reading everything I could find in the forum, carefully checking specs, and measuring my condenser holder, I decided to try this Spencer condenser …

I have a nice example of that one, in a good box complete with some funnel stops

Perhaps most relevant to your difficulty … it’s assembled onto a centring mechanism [which it appears that yours is not]

Assuming that the forum is working again; let me know if you need any photos or measurements.

MichaelG.

[apochronaut]

Actually Michael, that condenser has centering screws but one is on the other side. All Spencer and AO DF condensers do except after 1985 when it was incorporated into the condenser yoke.

Regarding the height.J_WISC. The condenser section , which is the narrow, chrome, sort of nose of the condenser, is threaded into the housing and is adjustable over a range of about a cm. It looks like it is adjusted pretty low. You should be able to increase it's height sufficiently on the threads. Also check and see if there is a condenser stop that might be in the way.

[MichaelG.]

Thanks for the correction … I did wonder, but the screws on mine look much longer so I wasn’t sure.

MichaelG.

[J_WISC]

I apologize for occasional poor or absence photos. I find it difficult to resize for the forum. So there’s higher activation energy to transfer to the iMac to properly resize.

I took the photo of the condenser, the one I bought, off of eBay. Several do have longer adjustment screws, but otherwise appear identical.

I’m going to frame my question better over next few days. In mean time I carefully set the vintage Spencer darkfield condenser on a Nikon Alphaphot condenser holder, and still pleasantly surprised it worked up to 20x. I’m to call this success for $30. But now I want to secure it better so I can use the centering screws and oil.

I’m also using digital calipers for better measurements.

Thank everyone for helping perpetual beginner.

[Chas]

Is it possible that the wedge-shaped ring is mounted the wrong way up? ....I only say this because R&J Beck condensers have the flanged face of the wedge-ring turned towards the bottom.
(I guess it helps to keep the condenser body aligned)

[apochronaut]

The dovetail on that condenser is mounted correctly. There is one set screw which by loosening it allows the dovetail to be removed altogether or slightly raised in order to accomodate dovetail yokes that are a little thicker.
That same basic DF condenser design was catalogued for almost 50 years and was designed to be adaptable to a wide range of microscope models, from W.W. I student models fitted with a sleeve condenser to the 20+ lb. research models that they made between 1932 and 1955.

Michael. Your version with the longer centering screws, is most likely the model 214F, which is the same basic condenser but threaded into a slightly larger mount with longer screws and a wider dovetail. It was used on designs originating post W.W. II : the 15/35, 2/4, 10/20 and a few other student models.

[MichaelG.]

Thanks for that ^^^

MichaelG.

[J_WISC]

Is it possible that the wedge-shaped ring is mounted the wrong way up? ....I only say this because R&J Beck condensers have the flanged face of the wedge-ring turned towards the bottom.
(I guess it helps to keep the condenser body aligned)

I briefly wonder about this. But the condenser dovetail is correct and the holder dovetail is correct. Given the source of the microscope, an assembly error wouldn’t have surprised me. Thank you for thinking outside the box and sharing all possibilities.

[J_WISC]

Okay. Can’t upload photos for some reason. Arrrgh.

Anyway …

I moved the condenser holder as high as physically possible. The top of the darkfield condenser lens is about 20.5 mm from the bottom of a slide. Threading the top portion out of the assembly might give me few mm, but no longer firmly attached if I try for more. And there are not 20.5 mm of threads.

Option 1

Male thread OD 1.0180”. Female thread ID 1.0145”. I was wondering if this is SM1, but the first attempt to count threads was 50 tpi. Any suggestions for acquiring a 25 mm long extension tube? Should I just order an SM1 threaded tube and try it?

Option 2

An SM1 to RMS ring so I can put darkfield lens on current condenser assembly. I prefer Option 1 if possible and extension tube isn’t a problem. There are affordable, black interior, extension tubes for optics.

Option 3

Rest the darkfield condenser on a second dovetail ring. 3D print a raised circle it could sit on and the second dovetail.

I cannot — or won’t — remove the darkfield part from the current dovetail. The centering screws are sort of integrated and below the dovetail and move the lens independent of the dovetail ring.

Can someone suggest ID for thread or help me think more outside the box? I wish extension tube was easy calculation.

Sorry I couldn’t upload photos. Thank you for suggestions.

[apochronaut]

prrobably the photos are too big. I usually compress mine to no more than 1400 width or 800 height, depending on the aspect ratio or crop.
There is only 10mm of extension on the thtead and as you note, a limit to how far out you can go.

Juet to be clear. What microscope are you adapting this to?

[Chas]

There seems to be a general and ongoing problem uploading images :
viewtopic.php?f=8&t=19281

[J_WISC]

Juet to be clear. What microscope are you adapting this to?

Spencer No. 5 Research Microscope

The top part of the current condenser, which is quite nice, is attached by RMS threads.

The top part — the working part — of the darkfield condenser is close to 1” diameter.

It would be most convenient to add an extension tube to the darkfield. But I’m not able to identify the modern equivalent of the thread. It could actually be Spencer Buffalo proprietary!

I saw someone else posted the same discovery elsewhere in the forum … darkfield too short for Spencer No. 5. I think they decided to solve the problem by selling the too-short condenser.

Would it affect performance to add an extension tube? Doesn’t even have to have threads if designed right.

Thank you for your help.

[apochronaut]

Usually in these instances it is a question of adjustment or lack of. Both condensers are on adjustsble mounts. The 1.30 achromat further has a lock ring. Both the 1.30 N.A. achromat condenser DF condenser, were widrly used on the same instruments.
I have used both on a # 5 many times but my 5 is not right handy today.
The first place to look for a solution to condenser height is your condenser carrier. If if doesn't go high enough , the 1.30 BF. condenser can be extended enough on it's thread to reach the slide but the DF condenser might not, since the 1.30 at minimum extension is about the same height as the DF condenser at 2/3 of it's maximum.
Some time ago , you posted a picture of your condenser carrier, with possibly a seized fine focus?. If seized at it's lowest point, it will take a little height away but the more likely culprit is the upper stop. In order for the DF condenser to reach the slide when it is mounted in one of the elevated floating slide carriers that the reeearch scopes could be fitted with, the stop needs to be almost fully in, with only a slight protrusion above the locknut. However, the achromat condenser can still be adjusted to reach the slide with stop extended out quite a bit.

The 1.30 and DF condensers can be made the same height and thus use the minimum stop height.,To do this, the 1.30 condenser section needs to be threaded right in to it's lock ring and the DF condenser needs to be threaded out about 9 threads. You needn't worry too much about overextending the DF condenser, since there are a total of 15 threads.

[Chas]

It seems that Spencer had to put a darkfield condenser onto an extension tube at some point:

Spencer darkfield long.jpg (40.08 KiB) Viewed 101980 times

I have a Leitz ' helldunkenfield' (whose head has pretty much the same thread measurements as you measure on your condenser) and fitted that into a Watson centering mount with a push-fit (ie. no threads) collar that was 3d printed with PLA.
This adaptor works very well, but it wasnt an extension tube, whose length might amplify any defects in the tightness of the fit.
You need to visit someone who has a 3d printer and take some sandpaper with you to 'fine tune' the fit of the 3D printed tube.

[apochronaut]

It is important to have an overview of the different Spencer microscope series/models in order to understand how the systems were configured and what cat.# accessories were designed to fit the various models, otherwise red herrings can get in the net.
The model # 5 microscope was introduced as a research stand in 1932 and was discontinued in 1955. It used a dovetail common to other Spencer models that first appeared just after W.W.I and last appeared on pre- W.W. II instruments that lingered into the 1950's. As the flagship research stand for many years, the # 5 may have been the last of these to be discontinued.
These were all mirror illuminated instruments with horseshoe bases and there was a common group of condensers that could be fitted to any of them. Substage components were designed to be vertically adjustable in order to accomodate the different condoensrrs and as well work on a broad range of instruments.

After world war II, the company embarked on a development stage that evolved a whole new group of instruments with larger substages and built in illuminators. These instruments used a larger dovetail and a different set of condensers, including phase contrast. This era of dovetail/condenser systems lasted until 1980.The image of a DF condenser that you have posted Chas, is from this system and era. It is a model 214F and was used from the late 40's until 1980, mostly on series 15/35, 2/4 and 10/20 instruments.

In 1980, there was yet another evolution and a different group of condensers on a different dovetail were introduced. There was one further condenser or rather a refinement of an existing one in 1985 and that era continued until 2002, when production ceased. This last dovetail design persisted into early instruments branded, Leica made both in Buffalo and Wetzlar.

[Chas]

Maybe the first thing to do is to try out the DF condenser head?

... You could attach the DF head directly to the RMS condenser mount disc with some hot melt glue if the RMS disc is flat enough in the right area and the threading on the head is long enough to raise the darkfield condenser head to to touch the underside of the slide ... if not, maybe use a (bored-out) bottle top to raise the diameter of the contact area and/or raise the head . [I notice that the bottom of a 50ml sample tube that I have, happens to be a really excellent fit for the same sized Leitz head ...not that you are likely to have the same diameter sample tube ].

b] It might be that the (top notch) achromatic condenser that you have will make a pretty good oil DF condenser ( something that Watson tried to highlight about theirs ) ... so in a similar 'suck it and see' spirit and to save the tedium of cutting out various stops; get a tin of black silicone 'magic' putty and squash a blob of it on the back of the condenser* , the material lifts off glass nicely. I have done this with a couple of achro RMS condensers and they worked pretty well.

c]There were some quite to very good RMS threaded oil DF condensers, made in the past, maybe look out for one.

d]The other thing to bear in the back of your mind is that you now have a spare ring so you could use this to mount any other condensers.


*As a start, a bit that covers half the back lens's diameter seem to work pretty well , throw the bit you have used away after using it as it will collect skin oils et al.

I hope I haven't introduced a whole new shoal of red-herrings but I was working on the presumption that your condenser mount might look like this one (i.e quite tall) :

No.5 substage from AO blue book pg-058.jpg (147.66 KiB) Viewed 101703 times

(Source https://user.xmission.com/~psneeley/Per ... oscope.htm No. 14a Page 58)

[J_WISC]

Maybe the first thing to do is to try out the DF condenser head?

Hello Chas. A shoal of red herring are fine here. I often get bogged down by an idea, endless Googling, and miss more efficient solutions using “parts” on hand.

The DF condenser works up to 20x on a Spencer No. 5 and on a Nikon Alphaphot. I carefully set it on a stack of incompatible adapter rings and partially disassembled condenser parts to assess whether I wasted $30. I was surprised it actually worked! The current goal is stability for proper centering and use of oil under the slide. Or just stability. I’m not looking for perfection. This is just a casual hobby.

The No. 5 condenser is the nice but complicated one in the photo you posted. It was a simple test to set the DF lens part on top of it in place of the original lens part. I might try the hot glue idea. I was actually wondering if there’s something a person can pour around the threaded end of a part, but doesn’t stick once solidified … instant plastic adapter!

Regarding 50-ml tubes, I just happen to have a couple 50-ml conical-bottom tubes. And the tube fits tightly over the outside of the lower threaded part of the DF condenser. I suspect I could cut the conical end just right to hold the upper threaded part of the DF condenser. Or cut the conical end off and drill a proper hole in the cap. Thank you for this suggestion. And probably zero $$ solution.

[apochronaut]

The inability to get the correct height for the DF oil condenser is strange, especially given the fact that I have found no such difficulty. I have been around the Spencer # 5 for some time and have used one with all possibilities. They are quite modular.
There is another way however, which doesn't involve tape , putty or spacers. Since you seem to have no problems getting the 1.30 achromat to work, you can just remove the DF condenser from it's mount and swap the DF condenser into the achromat condenser mount. Keep the DF condenser out of it's own mount permanently, then. Each condenser would use the same xy mount in the condenser fork and the swap takes only seconds. The achromat condenser lifts out and the DF condenser sets in and visa versa. No threading involved, except to set the DF condenser height for the first use. In this set up the DF condenser will sit a little higher than the achromat condenser, so the condenser fork has to be adjusted a little lower on the condenser focusing track.

Picture 1) These are identical condensers showing the range of travel to accomodate different substage depths. I don't know whether you have taken advantage of the range, in your pursuits.
Picture 2) To swap the DF condenser into the 1.30 achromat condenser mount, begin by finding the 3 small screws in the DF condenser mount bottom plate. You can see them in the picture.
Picture 3) Removing the 3 small screws will reveal the two xy thumbscrews and the return spring.
Picture 4) The condenser body just drops out the bottom of the mount as a stand alone unit. The adjustable centering part of the condenser body is a circular dovetail, just about 1 3/8" across at the bottom of the unit.
Picture 5) The 1.30 achromat condenser is threaded into a bezel, ring or flange that press fits into a recess in it's centering condenser mount and is secured by a light spring split collar. The ring is just about 1 3/8" across. See where this is going?

[apochronaut]

6) The achromat condenser just lifts out of the recessed well that it press fits into. You can see the split collar.
7) The DF condenser has exactly the same diameter base as the achromat collar and rests into the well in the achromat mount perfectly. The split collar might need to be slightly depressed to grip the DF dovetail but it fits snugly nonetheless.
8) The DF condenser mounted in the achromat centering mount. Note that the DF condenser now sits higher than the 1.30 achromat. Lowering it by threading it in to it's minimum height brings it close but the condenser fork will be adjusted a little lower on it's rack for the DF condenser in this scenario.

[Chas]

The inability to get the correct height for the DF oil condenser is strange, especially given the fact that I have found no such difficulty.

Would I be right in thinking that when you used the darkfield condenser on the No.5 that it came pre-mounted on a push-in disc similar to the disc with the RMS thread in the middle (that holds the achro condenser) ?