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Help please--can't reach focus with 40x objective

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#1 Frugal Astronomer

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Posted 23 January 2023 - 09:29 AM

I'm cleaning and adjusting some student microscopes.  I have a standard student microscope (National brand) with 3 objectives (4x, 10x, and 40x).  I have a prepared slide on the stage, which I can easily focus under the 4x.  When I switch to the 10x, it focuses with just a little fine adjustment.  When I switch to the 40x, it is out of focus and as I carefully bring it all the way to the slide, it never reaches focus.  I can see that the specimen is still there, but it remains beyond focus.  I've cleaned the lens, and all 3 objectives are original to the scope (they are the kind that are "locked in" anyway).  

 

Any suggestions about a fix or adjustment that I'm missing?  I can't understand why the 40x wouldn't be reaching focus?

Thanks for any help!



#2 j.gardavsky

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Posted 23 January 2023 - 09:47 AM

Your 40x objective may be designed for the oil (or another fluid) immersion. This is as a rule engraved on the objective tube.

Another reason may be, the objective is corrected for infinity, and your microscope is the finite (tube) system.

 

More infos are here, https://zeiss-campus...indexflash.html

 

Best,

JG



#3 MisterDan

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Posted 23 January 2023 - 10:00 AM

Does the microscope utilize a "zoom" eyepiece? Likely not, but if so, ensure the zoom magnification is minimized, then try focussing.

 

Also - the unit may have both coarse and fine focus knobs. Make sure you're working with the coarse (usually larger) knob(s), to reach "rough" focus, then refine your focus with the fine adjustment knob(s).

 

Here are some of National's current models:

https://microscopece...ctions/national

Is yours one of (or similar to one of) these models?

 

Hang in there.

Best wishes.

Dan



#4 Frugal Astronomer

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Posted 23 January 2023 - 01:11 PM

Does the microscope utilize a "zoom" eyepiece? Likely not, but if so, ensure the zoom magnification is minimized, then try focussing.

 

Also - the unit may have both coarse and fine focus knobs. Make sure you're working with the coarse (usually larger) knob(s), to reach "rough" focus, then refine your focus with the fine adjustment knob(s).

 

Here are some of National's current models:

https://microscopece...ctions/national

Is yours one of (or similar to one of) these models?

 

Hang in there.

Best wishes.

Dan

It's the C1028.  No zoom eyepiece.  It's not a problem with the fine and coarse focus.  I literally put the slide in touch with the 40x objective (I know you're not supposed to do that) and it still wasn't in focus.  I could tell that it needed to continue in that direction ("through the slide") in order to reach focus. In other words, I was focusing in the right direction, but couldn't get there.



#5 Frugal Astronomer

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Posted 23 January 2023 - 01:13 PM

Your 40x objective may be designed for the oil (or another fluid) immersion. This is as a rule engraved on the objective tube.

Another reason may be, the objective is corrected for infinity, and your microscope is the finite (tube) system.

 

More infos are here, https://zeiss-campus...indexflash.html

 

Best,

JG

Nope, it's not an oil-immersion.  Just a plain old 40x.  The objective is the one that came with the microscope and is 'locked in".  There's probably a way, but I can't remove it in any obvious way, and I know it's the original objective for this microscope, so should be focusable.  



#6 MisterDan

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Posted 23 January 2023 - 05:27 PM

My only other thoughts, at this time (I apologize ahead of time if you've already checked these)...

 

- Is it possible that the objective's front lens is smeared/smudged (oil or moisture)?

 

- Does the objective include a cover slip thickness specification on the housing (likely "0.17")? See this page:

https://www.microsco...over-slips.aspx

The "working distance" of a 40x objective is very small - about half a millimeter. If the cover slip is too thick, it will act as a rack stop and prevent the specimen from getting near enough to the objective to reach focus. 

 

- If the cover slip is not too thick, you might try focusing on a bit of notebook paper (either on a slide with no cover slip; or directly on the stage with no slide). The light source should be strong enough to allow you to see the matrix of fibers if the paper reaches focus.

 

-Fingers crossed...

fingerscrossed.gif


Edited by MisterDan, 23 January 2023 - 05:27 PM.


#7 Frugal Astronomer

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Posted 23 January 2023 - 10:03 PM

My only other thoughts, at this time (I apologize ahead of time if you've already checked these)...

 

- Is it possible that the objective's front lens is smeared/smudged (oil or moisture)?

 

- Does the objective include a cover slip thickness specification on the housing (likely "0.17")? See this page:

https://www.microsco...over-slips.aspx

The "working distance" of a 40x objective is very small - about half a millimeter. If the cover slip is too thick, it will act as a rack stop and prevent the specimen from getting near enough to the objective to reach focus. 

 

- If the cover slip is not too thick, you might try focusing on a bit of notebook paper (either on a slide with no cover slip; or directly on the stage with no slide). The light source should be strong enough to allow you to see the matrix of fibers if the paper reaches focus.

 

-Fingers crossed...

fingerscrossed.gif

Objective lens is clear, as far as I can tell.  I cleaned it, but I can't remove it to get a really good look.  I checked the cover slip thickness spec on the objective and it's 0.17, which I think is pretty standard.  I was using prepared slides (2 different ones) and neither would come to focus under the 40x.  I would assume prepared slides would be at or below 0.17?  I don't have any way of knowing what their thickness is, but they're not very old slides (year or two).  

 

Just did the paper test--thanks for the idea.  Still no focus with 40x.  

 

I think something is wrong with the 40x.  It looks in good condition, but there has to be something going on in there.  Any ideas for how to remove a "locked" objective?  (It's a high school model microscope with objectives that can't just be unscrewed...because students would probably do that, if they could. wink.gif )  There's no obvious way to remove it that I can see.



#8 j.gardavsky

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Posted 24 January 2023 - 10:00 AM

Can you take a closeup picture of the 40x objective on the microscope, so we can see how it is mounted?

 

Best,

JG



#9 Frugal Astronomer

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Posted 24 January 2023 - 05:10 PM

Here are two pictures:IMG_20230124_113545806.jpg

IMG_20230124_113558337.jpg


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#10 Frugal Astronomer

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Posted 24 January 2023 - 05:11 PM

It's a little hard to see but there's a screw in the middle of the underside of the nosepiece, between the three objectives.  I'm wondering if I need to unscrew that, remove the whole nosepiece, and then maybe somehow I can remove an objective??



#11 Couder

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Posted 24 January 2023 - 05:19 PM

The eyepieces sure look like mine, which all unscrew.

#12 MisterDan

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Posted 24 January 2023 - 06:11 PM

Okay, one more thought...

 

Have you exclusively tried to unscrew the knurled ring from the turret? - or have you tried to unscrew the barrel from the knurled ring (i.e. grasping the upper barrel of the objective)?

My guess is that you've tried both, but I wanted to make sure.



#13 Frugal Astronomer

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Posted 24 January 2023 - 06:32 PM

Okay, one more thought...

 

Have you exclusively tried to unscrew the knurled ring from the turret? - or have you tried to unscrew the barrel from the knurled ring (i.e. grasping the upper barrel of the objective)?

My guess is that you've tried both, but I wanted to make sure.

Yes, have tried both on all 3 objectives.  Nothing budges.  I didn't take a pair of pliers to them, but maybe that's next?  Part of the reason that I stopped trying too hard was that I read somewhere that student microscopes often have "locked" objectives--in other words, they're not supposed to be "unscrewable".  That said, I figure there has to be SOME way to change out a bad objective. 



#14 Frugal Astronomer

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Posted 24 January 2023 - 06:36 PM

The eyepieces sure look like mine, which all unscrew.

I have a "university"-grade microscope at home, probably about the same age as the ones at school, and all the objectives easily unscrew.  But most of the microscopes that I use for class with my students are like the one pictured above and they don't budge (the objectives).  



#15 j.gardavsky

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Posted 25 January 2023 - 09:55 AM

During the assembly, the objectives have been screwed into the turret, and highly likely somehow locked.

The next step would be to take the turret with the objectives out of the tube, and to look inside the turret, if the objectives' threads are locked, if found, then get the 40x unlocked.

 

Happy doing,

JG



#16 mikerepp

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Posted 25 January 2023 - 03:07 PM

The objectives are meant to unscrew.  The knurling on the objectives clearly shows these will unscrew.  They were probably just put in too tight at the factory.   Never heard of any objectives that were locked in, I could be wrong but I doubt it.  The only way they could lock them would be with a screw and it would be on the front of the turret.  They would not design a microscope that would be throw away.   If an objective was damaged they would have a way of replacing it.

 

Get a firm grip on the turret and use some piece of rubber or something that will let you grip the objective firmly, by the knurling, and turn.  Remember Lefty Loosey so CCW looking at the objective from the bottom.



#17 Frugal Astronomer

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Posted 25 January 2023 - 07:43 PM

The objectives are meant to unscrew.  The knurling on the objectives clearly shows these will unscrew.  They were probably just put in too tight at the factory.   Never heard of any objectives that were locked in, I could be wrong but I doubt it.  The only way they could lock them would be with a screw and it would be on the front of the turret.  They would not design a microscope that would be throw away.   If an objective was damaged they would have a way of replacing it.

 

Get a firm grip on the turret and use some piece of rubber or something that will let you grip the objective firmly, by the knurling, and turn.  Remember Lefty Loosey so CCW looking at the objective from the bottom.

I might be inclined to think that also if it wasn't that ALL the objectives on all the microscopes of this style that I have don't budge.  I have a couple nicer ones where the objectives do unscrew.  And I've seen various websites describing their student microscopes as being "student proof" because of "locked-in" eyepiece and objectives.  

 

I'm not a wimp and I know which way to turn.  They aren't budging.  But I'm going to try removing the screw in the center of the turret, between the eyepieces, and see what happens.  When I get time for it...


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#18 MisterDan

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Posted 25 January 2023 - 08:38 PM

I don't see any grub screws, so - if the objectives are indeed locked - it's likely that a thread-locking compound was used.  Before going all out with a torch or strong solvent, try something much less volatile:  ispropyl alcohol (70% or higher).  Let a drop or two seep into the turret/knurled ring interface. Capillary action will draw the alcohol where it can. Then check things out 15-20 minutes later - try to unscrew/loosen the objective.  If it's still no go, soak "out" any excess alcohol with the edge of a paper towel, and try the process again (waiting the same 15-20 minutes - or even longer).

 

It won't hurt, and it's certainly worth a try.

 

Best wishes.

Dan


Edited by MisterDan, 25 January 2023 - 08:39 PM.

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#19 EJN

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Posted 25 January 2023 - 09:26 PM

But I'm going to try removing the screw in the center of the turret...


Some nosepieces (turrets as you call them) run on ball bearings, lots of tiny ones, not caged.
If they fall out getting it back together again is a royal pain.

#20 j.gardavsky

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Posted 26 January 2023 - 04:52 AM

I don't see any grub screws, so - if the objectives are indeed locked - it's likely that a thread-locking compound was used.  Before going all out with a torch or strong solvent, try something much less volatile:  ispropyl alcohol (70% or higher).  Let a drop or two seep into the turret/knurled ring interface. Capillary action will draw the alcohol where it can. Then check things out 15-20 minutes later - try to unscrew/loosen the objective.  If it's still no go, soak "out" any excess alcohol with the edge of a paper towel, and try the process again (waiting the same 15-20 minutes - or even longer).

 

It won't hurt, and it's certainly worth a try.

 

Best wishes.

Dan

When refurbishing the fine mechanics on the old optical instruments, I take in the first step a defroster fluid (Sonax as available at the gas stations). It has a high capillary penetration, it solves some dirt, and it lubricates.

 

Happy doing,

JG



#21 mikerepp

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Posted 26 January 2023 - 10:01 AM

I might be inclined to think that also if it wasn't that ALL the objectives on all the microscopes of this style that I have don't budge.  I have a couple nicer ones where the objectives do unscrew.  And I've seen various websites describing their student microscopes as being "student proof" because of "locked-in" eyepiece and objectives.  

 

I'm not a wimp and I know which way to turn.  They aren't budging.  But I'm going to try removing the screw in the center of the turret, between the eyepieces, and see what happens.  When I get time for it...

Sorry wasn't trying to imply anything.   Just trying to cover the bases.  One thing to note, at this magnification the focus distance is an extremely small window of distance.  It is quite easy to go right thru the point of focus.  I'm sure you know this, I just point this out to others who may read this thread.

 

If they are locked I would have to agree with others above that they may have used a locking compound since there are no set screws.

 

Have you tried looking at the objective with a loupe?  The objectives are probably cemented and the cement may have failed.  Of course this is a lot easier to do from the backside of the objective.

 

I have close to 10 student microscopes that came from colleges, never once have I found a locked objective.   That's why I was so surprised that others have run across this, quite interesting.  And yes I really need to thin the herd.  (I say this after buying another AO model 10 this week)lol.gif lol.gif

 

In my defense the student microscopes are all AO and I use these for objectives and any other parts I may need to support my research grade scopes.  Three that I'm rebuilding currently.

scopes1.jpg

this one I bought for the condenser and the stage.

scope2.jpg


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#22 Frugal Astronomer

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Posted 26 January 2023 - 02:11 PM

Yes, college microscopes are a different category from high school.  The couple "nicer" ones that I have are old college microscopes and I have no problem removing objectives when/if needed. 

 

The microscopes that I'm currently tangling with are high school microscopes, meant to be "safe" from the foolishness of high schoolers--thus the "locked" objectives.  


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