The image of our Amray SEM has a horizontal band across the center
third of the image which is slightly out of focus. The top and bottom
thirds are fine. This machine, while being fairly old, is new to our
institution and I believe some of the maintenance practices have
lapsed. I was wondering if it was possible that a weak filament or
contaminated optical column or dirty apertures could cause a problem
like this.

Any advice would be appreciated.

Thanks.
Gary