Through the microscope

Posts tagged ‘condenser’

Centring a lomo or meopta condenser

Scenario: You’re  trying to set up Kohler illumination on an old Lomo biolam or  Meopta microscope which has no centring mechanism on the condenser. You have the lamp centred and the field diaphragm pretty well centred and you’re ready to get the condenser centred. You pop your phase telescope in the eye tube, you look through it and horror of horrors you see something like this!

iris aperture squiffy


Your iris aperture (condenser aperture) is way off centre. When you enlarge the aperture part of it disappears out of view. Kohler is impossible. You’ve wiggled everything that can be wiggled and you’re about to start smacking things with a hammer in frustration. STOP! There is a solution and no microscopes have to be hit with anything.

Step 1) Remove the condenser and the stage – this shouldn’t be a problem, they come off fairly easily, especially if you remove the mirror so you can get a screw driver in more easily.


Removing Lomo stage

Removing Lomo stage

Step 2) Observe the condenser carrier. You will see a knurled locking screw and three tiny grub screws  – one at the front and two at the back on either side. These tiny grub screws are your centring mechanism but don’t bother trying to alter them yet, nothing will happen until you have taken off the locking ring.

Locking ring

Locking ring


Step 3)  Unscrew the locking ring -I used pointy ended pliers to get it started. Now you can  adjust the grub screws. Pop the condenser back while you do the adjustment. Rack the condenser almost completely up and look through the phase telescope using a low power objective. Make sure the objective is down near where the stage should be. It’s no good leaving it 20 cms up in the air.  It doesn’t matter that you have no stage and no slide to view, you’re only looking at the condenser aperture through a phase telescope.

Approximate position of grub screws shown by pink arrows

Locking ring removed. Approximate position of grub screws shown by pink arrows.


Step 4)  Adjust the grub screws until you see something resembling this through the phase telescope.

iris aperture central


Step 5)  Cry “Hallelujah!” then throw the locking ring in the bin. Well, put it somewhere safe, I have found everything to be perfectly secure without it and it’s much easier to adjust without it there.

Step 6) Put the stage back on. Good luck with that, it’s fiddly. Put the mirror back on, put on anything else you removed.

Now you can set up Kohler in the usual way.

NB. you may have to adjust the head  if that is out of whack rather than the condenser. The head has a dove tail slider which is obvious and easy to adjust. You have probably wiggled it several times during your attempts to get the condenser centred.




Fixing a rusty iris diaphragm on a Baker Series IV microscope

I recently bought a Baker Series IV, I wasn’t planning on buying one but my friend, Merv, convinced me that I should. I don’t take a great deal of convincing because I’m a sucker for any microscope. I think the persuasion went something like this.

Merv said:

“There’s a Baker Series IV for sale, they’re nice microscopes.”

– I bought it.

It was cheap but it’s not in a great state.  It has rust patches and bubbles in the paintwork. Bits that should move freely are stiff and bits that shouldn’t move at all wiggle. The usual second-hand microscope problems.

This time though there was a more serious problem. the iris aperture was rusted, badly rusted;  it’s also a fiddly horrible design, the edges of the iris blades are bent up and the whole thing is quite fragile. I don’t like taking iris diaphragms apart at the best of times and I definitely didn’t want to take this one apart so I did what any sensible human being would do. I called my friend and asked him how to fix it.

“It’s simple” he said (he says that a lot) “isopropanol, de-ruster, isopropanol,  WD40 and a soft toothbrush”

Guess what? he was right, it was simple, so I thought I’d post it here, in case anyone else finds it useful.


STEP 1 – Observe the horrible rustiness of my aperture iris. I attempted to remove the iris diaphragm carrier from the microscope but I failed because I couldn’t turn the screw I needed to turn. I have hypermobile joints and a rubbish grip, my husband was at work so I couldn’t get him to do it. I had to  work with it in situ. I popped some absorbent cloths underneath the carrier to protect the field iris/condenser lens beneath and continued.



STEP 2 – cover the rusty iris with isopropanol and give it a gentle scrub. I didn’t have a small enough toothbrush so I used a foam camera sensor cleaning widget. I prefer them to cotton buds because they don’t leave fluff behind. They’re more expensive than cotton buds but I use them quite a lot in delicate areas.  Quite a lot of surface rust has come off already, see?



STEP 3 – after the isopropanol has evaporated cover the iris in de-ruster. I use Renaissance Metal De-Corroder.


The Renaissance blurb says:

Treatment selectively ruptures the bond between base metal and corrosion layer, reducing rust to a sludge which is easily wiped or brushed away. Clean-water rinse stops the process.

Even relatively prolonged immersion over several days has no significant effect on sound metal, thus giving the conservator complete control over the process – and freedom from it.

The totally benign nature of the product eliminates work and health hazards associated with common de-rusting systems such as those based on phosphoric and hydrochloric acids.

I can’t fault it, but it is expensive, so use an alternative if you wish to. In the next picture you can see the Renaissance De-corroder doing its thing. I put it on with another foam camera sensor cleaning tip and left it to work for an hour or so. I gave it a gentle scrub every now and then. It’s all very scientific.




Step 4 –  Rinse off the metal de-corroder with water, allow to dry (use isopropanol to help it on its way if necessary) then give the iris diaphragm a squirt of WD40. Give it a wiggle and smile contentedly as you watch the clean, de-rusted iris moving freely. Have a cup of tea then contemplate how you’re going to tackle the rest of the microscope.

EDIT: Please see comments section for helpful advice regarding WD40 and oil on iris diaphragms.




Thanks to Merv Hobden for his advice and guidance.



Cooke Troughton and Simms M2000 dark ground condensers


You see this little baby?
This is a Cooke Troughton and Simms M2000 with not one but two different dark ground condensers!
One dark ground condenser, the long skinny one, is an M1396 which has a collar that allows you to adjust it for slides of different thicknesses.
The other condenser is the more simple  M1399 which is adjusted for slides of 1.25mm thickness.

The Cooke Troughton and Simms dark ground condensers

M1396 above, M1399 below

M1396 above, M1399 below

Dark ground condensers can be used with objectives that have numerical apertures up to 0.95. To create the dark background of dark field illumination the centre of the cone of light that enters the objective is blocked out, either by a patch stop or by a special dark ground condenser. Only light from around the edge of the cone of light passes through the specimen. This means that if the numerical aperture of the objective is higher than that of the condenser, the light will  miss the objective.

CTS suggest fluorite oil immersion objectives as the best choice in their catalogue but almost any 2mm oil immersion objective can be used in combination with a funnel stop which cuts down the numerical aperture.

Patch stops can produce excellent dark ground if they are optimised, but only with lower power objectives. it gets tricky using a patch stop with objectives over about 20x magnification and near impossible over 40x. You really need a funnel stop (or an objective with an iris)  and a nice dark ground condenser for the higher magnifications.

I’m off to have a fight with my Meopta now. Wish me luck. I desire pretty pictures.


Spock’s sister does not see

Spock’s sister is cross. She has had an argument with a microscope. The argument lasted all day and so far the microscope is winning.

I have a very groovy Meopta anoptral phase contrast condenser and I bought a cute little Meopta microscope to put it in; today I thought I would get everything set up and take some pretty diatom pictures for you to see. It didn’t happen.

Anoptral contrast works in a similar way to phase contrast but the phase rings have sooty phase plates, this means you get less of a halo than with normal phase contrast. It also means you get some really pretty pictures with a beautiful brown background. At least that’s what should happen.

What actually happened was I set up my external illuminator (with condenser lens and iris), adjusted my mirror. Did all the usual Kohler type stuff but no matter what I did the condenser iris was so far off to one side that it was crazy. I can only just see the edge of it. I managed to get some nice dark ground images but that’s not what I was aiming for. No point attempting photomicrography.

The condenser fits the carrier perfectly, so why is it so badly misaligned?

Either the condenser is misaligned or the objective turret is misaligned. There are no centring screws on the condenser or on the carrier. Only phase wrenches, oh to be aligning phase rings!

I haven’t the faintest idea how to alter the objective turret. I twizzled the monocular tube but that didn’t seem to have any effect. I’m sure the problem is with the condenser. I really hope there are no prisms out of whack. I don’t know what images are produced by an out of alignment prism but I’m absolutely certain I would not be able to fix such a problem. I don’t have the tools or the skill set.

I’m cross and i need to consult the oracle (my friend Merv). He’ll know what to do. I’m sure he’ll know what to do. If he doesn’t I shall cry.

I shall get it sorted out and I shall post pretty pictures. watch this space….

PS saw the quack, no obvious signs of intracranial hypertension. Optic nerves look healthy (thank goodness). Blood pressure a reasonable 120/78. She took bloods. Headache from hell won’t budge. It’s been weeks but at least we still have the NHS. I do love the NHS.

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