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Natural Science Forum / Physics / Optics / May 2007Correction collar not working
Dear Group, Today I have a question regarding a mechanical-optical malfunction of a microscope objective made by a major manufacturer. The objective is a 40x, Plan Apo with correction collar. Focussing on a slide one obtains a relatively good image, not as good as one would expect, but acceptably good. What is curious is that ro- tating the correction collar throughout its range produces no change in focus. An explanation on what might be going on? Thank you G.R. Is the correction collar for compemsating for difference in coverslip thickness? Or somthing else, like different mounting media? One easy way to see the effect of correction collar is to measure the PSF. I would say it's a lens with good design since the correction collar is not shifting the focus. What are you expecting the correction collar to do?? How are you evaluating the image "quality"? My suggestion is the measure coverslip thickness and use that value to set the correction collar, then measure the PSF (assuming that the correction collar is for the compensating the coverslip thickness variation). Good luck! Satoru > Dear Group, > [quoted text clipped - 12 lines] > Thank you > G.R. Dear Satoru, The correction collar is for correcting the coverslip thickness. Putting a slide onto the stage and adjusting the correction collar for the measured thickness of the coverslip and focussing for best detail NO change is observed when rotating the correction collar. Best focus by fine adjustment is independent of chosen collar position. In other words there is no perceptible response to collar rotation. Mounting a non-collar objective of the same magnification, but slightly smaller aperture next to the objective in question and comparing images looking at the same detail, an image of somewhat better contrast and re- solution is obtained viewing through the objective without collar. Thanks for your thoughts. G.R. ----- Original Message ----- From: "Satoru Uzawa" <satoru_unospam@nospam.berkeley.edu> Newsgroups: sci.techniques.microscopy Sent: Wednesday, May 23, 2007 11:27 AM Subject: Re: Correction collar not working > Is the correction collar for compemsating for difference in coverslip thickness? > Or somthing else, like different mounting media? One easy way to see the effect [quoted text clipped - 28 lines] > > Thank you > > G.R. > Is the correction collar for compemsating for difference in coverslip thickness? > Or somthing else, like different mounting media? One easy way to see the effect [quoted text clipped - 28 lines] > > Thank you > > G.R. GR, I once had a Nikon objective with a faulty correction collar, The outer barrel had some set screws that allowed it to be removed for access to the next layer (the tiny screws did not have to be taken out completely). The pin was broken, which could happen by too-rough use, I expect. I popped out the broken stub, clipped off a short piece of a sewing needle of the right diameter, and with a tiny drop of epoxy, cemented it into the collar hole where the pin goes. The pin has to be to right diameter to avoid a sloppy fit, and just long enough to engage a slot in the next layer of the objective barrel. After curing and reassembly, it worked perfectly again. If the inside is clean it will move easily with little resistance, so the pin has little stress on it unless you apply too much muscle at the end of its movement. Good luck, Greg > Dear Satoru, > [quoted text clipped - 100 lines] > > - Show quoted text - GR, I once had a Nikon objective with a faulty correction collar, The outer barrel had some set screws that allowed it to be removed for access to the next layer (the tiny screws did not have to be taken out completely). The pin was broken, which could happen by too-rough use, I expect. I popped out the broken stub, clipped off a short piece of a sewing needle of the right diameter, and with a tiny drop of epoxy, cemented it into the collar hole where the pin goes. The pin has to be to right diameter to avoid a sloppy fit, and just long enough to engage a slot in the next layer of the objective barrel. After curing and reassembly, it worked perfectly again. If the inside is clean it will move easily with little resistance, so the pin has little stress on it unless you apply too much muscle at the end of its movement. Good luck, Greg > Dear Satoru, > [quoted text clipped - 100 lines] > > - Show quoted text - Greg, Many thanks for your info. Very logical and that is probably what is the problem and the cause. I have sent the objective to the dealer and will let you know what he finds. G.R. > GR, I once had a Nikon objective with a faulty correction collar, The > outer barrel had some set screws that allowed it to be removed for [quoted text clipped - 115 lines] > > > > - Show quoted text - Dear Groupmembers, I had promised to report further developments on the Plan Apo with correction collar, which I had found to be not satisfactory in my hands. With permission of the dealer I returned the objective. The dealer tested it and found it to work satsifactorily. He was kind enough to offer me a return and I took him up on his offer. I still wonder why a Plan Apo with correction collar (40x/.85) would give me poorer results as far as resolution and contrast is concerned than a PLFL lens (40x/.75), The comparison was done side by side on the same microscope, the same settings (except fine focus and correction collar) and on the same specimens. Interesting! G.R. Dear G.R., Thanks for the update! I have a feeling that lenses with more glass elements tend to have more variation on build quality. In addition to that, a lens with a movable part tends to have even more variation... My simple guess is that your PLFL has a better build than the PlanApo you had. Also the "true" NA sometime doesn't much what the catalog claims. I think all of these are within the tolerance the makers set for their lenses which is rather loose... Whenever it is possible, I ask the dealer to bring multiple lenses to check and pick the lens with the best build. If you need a lense with a tight specification, look into lenses build for semiconductor industry. :) Best regards, Satoru PS. Your speculation on my interest in reproducible PSF is for deconvolution is absolutely right, even though confocal is affected by a change in PSF, too! > Dear Groupmembers, > [quoted text clipped - 14 lines] > > G.R. > Dear G.R., > [quoted text clipped - 23 lines] > absolutely right, even though confocal is affected by a change in PSF, > too! Actually NA is NA. Its easy to calculate plus DIN requires complete accuracy. However NA is rarely understood by the user. NA is only involved with resolution that you can't see if the specimen doesn't have enough contrast or detail. Now, I real like your appoach to buying an objective. I recently worked with a customer who was very picky, the kind of customer I like, it took several nosepiece changes on a non-changable frame before he was satisfied. All the nosepiece met factory spec but his spec's were tighter. A good time was had by all. Thanks, Kevin Cunningham SMS >> Dear Groupmembers, >> [quoted text clipped - 19 lines] >> >> G.R. Dear GR, It is difficult to judge the correction collar effect at the focus, but it has rather obvious effect on out-of-focus Airy rings. The Airy rings will dissappear or get stronger as the collar rotates in certain direction. Opposite thing will happen to the other side of focus. But as the other answer stated, I am now convinced that you got a lens with a broken pin... Luckily, it never happened to me. Hope your lens will be fixed soon! Regards, Satoru BTW, I have a personal preference to pick a lens without correction collar, since it affetcs the PSF while it is almost impossible to set the collar position reproducibly... I glued the correction collar on our water-immersion lens with silicon rubber (so that I can remove it if I need to) and we are matching the coverslip thickness to the collar position by hand selecting them. Yes, you can call me a paranoid... :) > Dear Satoru, > [quoted text clipped - 97 lines] >> > Thank you >> > G.R. Dear Satoru, I thank you all for the very useful comments regarding my lens. It is now on its way to the dealer and I will let you know about further developments. Now to Satoru's comments regarding undesirable changes of PSF (Point Spread Function) with different settings of the cor- rection collar. If I interpret what he means correctly he is using deconvolution and for this reason needs to rely upon a stationary point spread function for his objective. I am not sure that such considerations are valid for visual or photomicrography work of a conventional nature. It is true that every different setting of the correction collar effectively creates a different objective lens, but the effect upon conventional microscopy can only be beneficial, since it improves definition and contrast in each case. In this case and particularly in cases were the sample is of different thickness below the coverglass a correction collar should be of value. Best regards G.R. > Dear GR, > [quoted text clipped - 119 lines] > >> > Thank you > >> > G.R. |
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