#35
Post
by apochronaut » Wed Mar 22, 2023 1:16 pm
I see very good results in your other recent rotifer post.
Be careful using water as a condenser immersion. As zzffnn mentioned, water condensers are few. Actually, I don't know any, except for the one I ended up with while trying to make a Reichert 1.4 condenser for a Microstar/Diastar. I didn't design it that way, it just happens to work best with water, entirely by accident.
Your working N.A. using a dry condenser is about 1.2 with that objective and depending on whether the condenser is mfg. as a dry condenser or is an oil condenser used dry, there may be some added spherical aberration, which should be avoided when trying to optimize a high end objective. As the colour correction and N.A. of an objective increases, so do the demands on the condenser, otherwise the costly specifications of the objective diminish in value.
Major defects in a condenser that fits and works and is mechanically fit are : incorrect front focal length, too low an N.A, too high an N.A., too much spherical aberration, too much chromatic aberration, inadequate field coverage, incorrect back focal length. Usually, a condenser that originated as part of a microscope's original equipment doesn't suffer from too many of those or not to a degree that detracts from it's intended function but once you use it off spec., all of a sudden some of them can become evident, as evident as condenser defects can become. They are much more subtle than those of objectives and often are only perceivable by way of comparison. Poor condenser performance is often attributed to the objective or the system as a whole. It is hard to isolate but defective condenser performance is real and sometimes solveable, sometimes not.
It's not that easy to change an oil immersion condenser to a water immersion condenser. It can be done by altering the space relationships between the internal components, at least getting close to perfect anyway, but that is not a very practicle solution. Outside of that the only way to make an oil immersion condenser adjustment, in order to function as a water immersion condenser, is by changing the spatial relationships between the condenser and object plane. As thin as possible an immersion layer coupled to an increase in slide thickness will cover the bases but unfortunately, the chromatic aberration base and spherical aberration base are in different locations, so there is no one size that fits all. Choose your poison. Those two ills are much less evident with simple achromat systems and lower grade condensers (abbe) because they are already there but with better condensers and well corrected high N.A. objectives, they aren't supposed to be, so it turns out that even an abbe condenser when correctly set up and illuminated properly can be better than an achromat aplanat that is not. An unoiled achromat aplanat could easily be worse than an oiled abbe and when both are unoiled , there might be very little difference
What the condenser is trying to do is direct the photons in order to increase contrast and resolution. This is observable very easily in phase contrast where the objectives used can be exactly the same objectives used for the lowliest of bright field microscopy. In fact, some systems use student objectives, the very same objectives used in a public school microscope with the simplest abbe condenser, yet the revolution in contrast is such that a simple .65 N.A. achromat can resolve details as though it were a .85 objective with fluorite elements inside, resolving punctae on diatoms, when without the phase component, not even striae are evident. This is all done in the condensing system. Yes, there is an annulus in the objective but that is only for convenience. It can be removed to elsewhere and the objective left as a basic bright field achromat. The diaphragm, condenser and annulus act as a combined condensing system but must be critically aligned and focused in order to perform this miracle. This is just to illustrate how much the condenser can affect the whole when accurately conformed, or just as easily not, when innacurately conformed.