Again today I had the distinct horror of knowing that – should I follow “established procedure” while working on a scope, I would damage the prime lens of the objective in use. In this case the cause was simple:
Many microscopes use an “in focus” and “escape” positioning method to focus objectives. This is used so that, in theory, objectives don’t smash into the underside of the microscope components when they are switched from one mag to another, while focused so closely to the specimen (especially in oil). Also the oil doesn’t get dragged off the slide or specimen container, and instead beads up making it last longer and keeping things a bit cleaner.
In this case though, the reasons for not using the objective escape became obviously clear.
I had two inserts available for the microscope. One, insert A, was designed by the stage manufacturer, and as such was made to properly fit the scope/ stage. The second, insert B, was designed by a third party environmental chamber manufacturer. This insert had a lower resting position for the specimen than the original had. I’ve attached a nice cartoon depicting this:
So one can imagine the following, normally recommended, use case to change specimen holders:
- press “escape” button on scope – objective drops.
- change from insert A to insert B
- Press “return” on scope -objective raises to it’s original z height!
Obviously this will cause the objective to smash into the bottom side of the new specimen. While this usually doesn’t cause damage to the lens, it can be a pain, and it could cause damage depending on the scenario.
The simple fix, as with so many of the automated functions we now live with, is to remember the scope is dumber than you think. Simply manually drop the focus by a few spins of the focus knob, then press escape. This will provide you with a nice working margin for different specimen or holder manufactureing offsets.
As a secondary point if you look at the Z focus number on the scope, it will tell you what the current focal plane is, making it easy to manually return to a focus position if you need to drop the lens using the knob, vs. using the buttons on the scope.
Hopefully this change in normal useage of the escape / return buttons will save some of our precious oil lenses from untimely demise!
-Austin