I have a problem with some artifacts in my brightfield images which I cannot
track down.

When the condensor is set to a low NA, there are a lot of dark spots  in
the observed image. However, these spots do not originate in the condensor
(verified by tilting the condensor -- they dont move).
When the NA of the condensor matches the NA of the objective, they
almost disappear. Problem is, in this setup, we often have to work with
very low NA illumination (for example, with the condensor well removed its
correct focus setting). Under these conditions, the spots are quite prominent.

Their location does not change when the objectives are changed.
They do not appear to be in any of the image planes.
We have cleaned all of the exposed surfaces.
They appear both in visual observations and with two different cameras,
so they are not detector artifacts.

We can acquire them as a background image and subtract, but it is certainly
better to get rid of them in the optics, before processing.

Any ideas will be much appreciated.

Thanks in advance,
--aryeh
Aryeh Weiss                          | email: [log in to unmask]
Department of Electronics            | URL:   http://optics.jct.ac.il/~aryeh
Jerusalem College of Technology      | phone: 972-2-6751146
POB 16031                            | FAX:   972-2-6751275
Jerusalem, Israel                    | ham radio: 4X1PB/KA1PB