Avoiding physical contact with the 40x objective

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dragonblade
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Joined: Sat Oct 18, 2014 11:16 pm

Avoiding physical contact with the 40x objective

Post by dragonblade »

I was using my compound microscope today and there were times when it looked like my 40x objective was making physical contact with the cover slip. While viewing through the eyepiece, I could see the image shift when focusing and moving the stage. I was using fine focus of course. Is there any way to adjust the microscope so that the 40x objective doesn't hit the cover slip / slide when using fine focus? It's often a bit of a worry of mine as the working distance between objective and glass is so incredibly narrow.

PeteM
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Location: West Coast, USA

Re: Avoiding physical contact with the 40x objective

Post by PeteM »

First thing you might want to do is make sure all your objectives are parfocal. That way, when you move from one position (in focus) to the next, you're not crashing into things. Parfocal shims are available in increments from about .0015" to .010" and more. They're like super thin washers with an RMS (or 25mm etc.) hole in them to move the objectives slightly out. You want the lenses to be parfocal within a fraction of a turn of the fine focus knob, to avoid contact.

You'd mount all your lenses and see which one ends up closest to the slide when in perfect focus. Then add shims to the rest to get them in focus.

I like set scopes to be parfocal across the dry objectives and then have the oil objective set slighty back (higher). Some higher end oil objectives (Leica infinity apos for example) will also have a locked up position to avoid oil contamination of the dry objectives.

If an objective is far too close when in focus (as can happen if its adapted on spacers, etc.), it's also possible to carefully shave a few thousandths from the seat of either the objective or the adapter - something that requires a lathe with a good collet system and a bit of care.

Second thing you might do, if not sure of focus is to look from the side of the objective (not through the viewing tubes) and bring high power objectives very near the surface of the slide - just before touching. Then be sure to back away as you achieve focus.

Pau
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Re: Avoiding physical contact with the 40x objective

Post by Pau »

Is there any way to adjust the microscope so that the 40x objective doesn't hit the cover slip / slide when using fine focus? It's often a bit of a worry of mine as the working distance between objective and glass is so incredibly narrow.
The parfocal advices by Pete are very important, although I'm not sure if you're referring to the objective tip touching the cover slip when changing objective or just when you are focusing.

In the second case there are two aspects:

1- Your cover glass and/or specimen are too thick. The only workable solution is to make adequate slides. Even if you change to a 40X objective with longer working distance able to avoid contact, the image will be degraded if the cover/mounting medium/specimen thickness is too big.

2- If you're able to get good focus without touching but you are annoyed by the ease of touching (or even crashing) the slide by error, some microscopes (but not all the entry level ones) have a fix or regulable stop* to avoid this issue.

* often a a bolt with a locking nut
I don't know the particularities of your microscope. The only way to give advice is with detailed pictures by your part
Pau

Olympusman
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Joined: Sun Jan 15, 2012 12:31 pm

Re: Avoiding physical contact with the 40x objective

Post by Olympusman »

You might also try moving the condenser slightly up or down, thereby providing a bit more working distance.

Mike
Michael Reese Much FRMS EMS Bethlehem, Pennsylvania, USA

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