Aryyeh Weiss wrote (in response to Harvey Karten's query):
 
>1. The Zeiss Axio* series use a friction vernier for the fine focus. This drive
>mechanism slips, and no matter how you drive it there are problems with
>reproducibility when scanning up and down. When doing a scan, slowly, in one
>direction, you can get decent results, at least with a new scope. I dont know
>how things will look in a year. However, if you try to go up  and down a few
>times, you may see differences of 1-2 microns in positioning. So you have to
>pay attention to your data, and look for landmarks.
>
>2. The Biorad drive is friction coupled to the fine focus. I have checked for
>slippage of the drive and found none, even when running up and down as quickly
>and the mechanism will turn. The main problem is the Zeiss vernier.
>
>3. I am told that Zeiss, on their confocal, do not use that manual fine focus
>knob. Instead, they  drive the coarse focus with a separate gearing attached to
>the focus motor. This was supposed to solve the z-positioning problem, and I am
>surprised to hear that Zeiss LCSMs also have this problem.
>
>4. I have suggested to Biorad that for Zeiss microscopes, they should use the
>Zeiss motor and gearing, and drive that with the Biorad control unit. This is
>not that hard to do. However, if the Zeiss z-drive slips, then that wont solve
>the problem.
 
The problem, I suspect, is not slippage but the simple fact that there is
limit to the precision with which one can cut a screw thread.  However
it is done, if you get your fine focus by a geared-down drive to your
coarse focus this is bound to happen.  That is why the classic microscopes
of the past used separate (and rather delicate) drives for the fine focus.
They also, of course, moved the tube rather than the stage and couldn't
use more than triple nose-pieces or the weight was too much .....
 
A separate Z-drive is clearly the answer for confocal.  We're currently
experimenting with the Autoscan XYZ stage (an Australian product, and
rather expensive).  More on this when we manage to get it integrated
into the Biorad system.
 
                                                        Guy Cox
 
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