Frank -

As Jim points out, temperature changes in one or several components of your
microscope setup are the most common cause for focus instability (and
that's the main reason why we recommend and sell temperature-controlled
enclosures for the whole microscope), but there are other causes also. E.g.
if you work with a closed chamber and you have some air trapped in there,
changes in outside air pressure will cause the coverslip to flex. Same
thing happens with perfusion chambers when the pressure (or flow) changes.
In all of these cases, the effect usually is oscillatory in some way, i.e.
as temperature or pressure fluctuate the plane of focus moves up and down.
But there are purely mechanical problems also. typically with the focus
mechanism of the microscope slowly slipping. This is easy to detect as the
focus drifts monotonously following gravity. That's what you seem to
observe so you should have your microscope checked and the focus mechanism
tightened if necessary. If this doesn't help, you may have to modify the
microscope with some sort of a focus clamp or exchange it for a new one -
or move to a zero-gravity environment, if you can afford some time on the
ISS :-)

Best regards,

Beat


At 16:08 26/07/01, you wrote:

>Javier,
>
>The problem almost undoubtedly has to do with the cycling of the
>temperature control for your chamber. Monitor the heating current and
>see if it cycles too. Then set the current to the average value of
>the current over time and disconnect the feedback system on the temp
>control (i.e., run it "open loop")  As there is no longer feedback
>you will avoid the oscillation but will probably pick up some longer
>term drift in temperature and focus.
>
>If this isn't it, there is always the "air-conditioning".
>
>Good luck,
>
>Jim P.
>--
>Jim Pawley (Summer address) c/o Postmaster, Egmont, BC, Canada,
>V0N-1N0 604-883-2095