Keeping a 60x objective from crashing

[Jerradin]

I recently replaced my 100x oil with a 60x dry objective. I'm having difficulty keeping it from crashing, even though it seems parfocal with the other objectives. As soon as I begin focusing with the fine focus knob, I see things in the wet mount start to shift around, which I assume means the objective is making contact. This is happening even when I filter the sample to remove large grit. How can I use my 60x objective without crashing? Thanks!

[DonSchaeffer]

I have the same problem with the same objective. Sometimes I continue using the lens if the cover glass shift isn't extreme. I have been viewing a lot of leaves and harder samples. Sometimes I can in fact trim them down enough that the lens doesn't hit the cover glass. Sometimes I opt not to use a cover glass for a dry subject. The cover glass itself often takes just that much space. It is a problem but tolerences with that 60x objective are very tight. It is worth it though. I usually use it for almost my whole viewing-photographing session.

[Hobbyst46]

I recently replaced my 100x oil with a 60x dry objective. I'm having difficulty keeping it from crashing, even though it seems parfocal with the other objectives. As soon as I begin focusing with the fine focus knob, I see things in the wet mount start to shift around, which I assume means the objective is making contact. This is happening even when I filter the sample to remove large grit. How can I use my 60x objective without crashing? Thanks!

First of all, make sure that the lower magnification objective is parfocal with the 60X objective. This is done with a calibration slide - best of all, with a stage micrometer.
1. Cover the calib slide with a cover slip.
2. Focus on it with a lower mag - say the 10X or 20X objective.
3. Then, instead of looking through the eyepieces, look from the side - horizontally at the slide, watch as you rotate the turret to bring the 40X objective above the coverslip, and see that the objective front lens does not hit the glass.
4. Look through the eyepieces and verify that the focus has not changed (within a small fraction of rotation of the fine focus knob).
Note: this is a verification of parfocality.
5. Repeat this process withe the 60X objective: Look from the side, horizontally at the slide to verify that the 60X front lens is above the coverslip. Adjust fine focus as above.

If objectives are not parfocal, a horizontal look at the slide, when switching to a higher mag, is the way to prevent crash.
The above is even more important for the 100X, which typically has a shallower working distance than the 60X. The working distance of a 100X is smaller than 1mm.

If the coverslip is slanted, contact with the objective is likely to happen.

Two other useful rules:
1.When focusing, try to set focus such the the fine focusing knob is not at its limit, but rather in the middle (roughly) of its travel range.
2. Never try to correct focus with the coarse focus knob when using the 40X or higher mag objectives.

[Jerradin]

Thanks for the tips Hobbyst46, but those are things that I've mostly considered and tried. And yes, I never use the coarse focus knob above 10x.

One thing I hadn't considered was the position of my fine focus knob when focusing with the coarse at 4x, so I'll try fiddling with that and see if I can get it to a better place. Thanks!

[hans]

Is it possible your cover glass is excessively thick? I have not had a problem myself but have read that the cheap off-brand ones can vary a lot with some batches much thicker than they are supposed to be.

[75RR]

I recently replaced my 100x oil with a 60x dry objective. I'm having difficulty keeping it from crashing, even though it seems parfocal with the other objectives. As soon as I begin focusing with the fine focus knob, I see things in the wet mount start to shift around, which I assume means the objective is making contact. This is happening even when I filter the sample to remove large grit. How can I use my 60x objective without crashing? Thanks!

High magnification objectives tend to have very small working distances.

Also, in high magnification objectives, the subject/object is assumed to be pressed up against the bottom of the cover slip in order to be in focus.

If a wet slide has too much water that means that the subject/object will not be pressed up against the bottom of cover slip

(there will be a layer of water between the subject and the cover slip), and attempts to focus on the deeper subject/object will inevitably mean crashing into the cover slip.

Additionally, dry high magnification objectives are particularly susceptible to cover slip thickness variation, you need to measure them and make sure that they are 0.17mm or just under.
.

[MicroBob]

Hi Jerradin,
this would be an easy test:
-Take a slide, mark it with a blue waterproof pen
-Take the cover slip you had problems with (or any other), makr the underside with a black waterproof pen
-Mount with drop of water
-Can you focus on both marks with the 60:1?
-Adjust water layer and try again

Do you happen to use a non-parfocal camera setup?

Bob

[75RR]

Hi Jerradin,
this would be an easy test:

-Take a slide, mark it with a blue waterproof pen
-Take the cover slip you had problems with (or any other), makr the underside with a black waterproof pen
-Mount with drop of water
-Can you focus on both marks with the 60:1?
-Adjust water layer and try again

Like that test! Clearly shows the importance of cover slip thickness and water level.

[Jerradin]

That's a good idea Bob, I'll probably try it tomorrow.

My camera is a little bit out of focus from the objectives, but not by a whole lot. I tend to get a focus through the eyepieces before I even think about the camera, anyway.

[apochronaut]

If thick specimens are being routinely viewed and the 60X is hitting the coverslip frequently, as identified, it is the technique that is the culprit not the equipment. Some objectives that have close tolerances have been made so that there is a very thin bumper around the front lens or the front lens is slightly recessed below the front bezel, to avoid scraping the front glass surface repeatedly but be sure repeated contact with the cover isn't damaging your front lens surface.


If maintaining the correct tolerances continues to be a problem, you can defocus the 60X a little and give it some breathing space by putting very thin shims under the other objectives. Shimming each of the other objectives equally will keep them parfocal but back the 60X away a little. You will know it needs to be focused down with the fine focus but having it contact the cover slip vertically with a water cushion underneath is much preferable to rotating it into the cover slip laterally and scraping the two together.
Perhaps using 15X eyepieces with the 40X is a better option?

[Jerradin]

Yes, that's why I asked "how." Apologies if I somehow gave the impression that I was blaming the equipment.