We currently (June 2011) have a bayonet style quick exchange objective mount. This works well up to M27 but is not suitable for mounting M32 or piezo objective positioners directly. Our current solutions for those situations result in a Z displacement of the back aperture of the objective about 40 mm. This is bad for 2 reasons:

the collection efficiency of scattered photons will be reduced
the pivot point between the mirrors comes out of focus and would require adjustment of the the scan lens w.r.t the scan mirrors.

For the second point Euler et al 2008

Euler Tet al.Pflugers Arch457p1393-414(2009 Apr)

have this to say:

Since the beam is collimated (the image at infinity) between the tube lens and the objective, a
change in the distance does not degrade the imaging performance (a fact that is used in the
so-called fixed-stage microscopes where the objective alone is moved when focusing). Changing the
distance between tube lens and objective does, however, move the objective exit pupil away from
the image of the scan-mirror pivot point (Eq. 2). This effect could become a serious concern at
the outer reaches of the travel ranges (±12.5 mm in each direction) but can be tolerated for
moderate displacements and for a limited field of view. It would be possible to eliminate the
pivot-point mismatch by moving the scan mirror assembly in synchrony with the xyz movements by an
amount that can be calculated using Eq. 2.

Expanding this a little bit, since our X (fast) and Y (slow) scan mirrors are not both in focus w.r.t the scan lens, as we move away from 0 angular displacement the effective focal plane in the sample changes resulting (ideally) in saddle shape when the point between the two mirrors is in focus. By moving both mirrors out of focus we will get a double hyperbola. This resulting non-flatness is obviously worse further away from the centre of the image and might be avoided at high zoom. I have not tried to figure out just what fraction of the image we would consider unacceptable by this metric.

Objective mount details

Alex asked the workshop to make a brass M27-M27 extension tube, which drops the objective somewhat to give more space laterally. We also have an M27-M32 adapter (double check M27), which Steve made when we were testing the Nikon 25x 1.1 NA in early 2011.

In order to mount either a 726 or 725 piezo objective positioner without significantly increasing the distance from the back aperture to the dichroic, we will need to modify the nosepiece to increase the horizontal clearance. This can be seen in this photo:

A = approximate position of main dichroic
B = bayonet objective mount in nosepiece
C = objective holder that slots and rotates into B
D = brass extension tube to ensure that piezo or wide lens is not impeded by B (or indeed F)
E = portion of piezo objective positioner connecting to nosepiece (above) and objective (below)
F = vertical mounting plate for dichroic and objective assembly (to the left) and PMT detector assembly (to the right)

The main problem is that the piezo positioner mounting assembly (E) is too wide to fit directly into the area left free by B (and F) requiring the conical brass extension tube (D) to lower it to a place with more horizontal clearance. However the final result is that the back aperture is ~ 4 additional cm from the dichroic. We are thinking of putting a 65mm diameter PI-726 piezo in place which would be even more difficult to fit. The simple option would be to see if we can get a cm or so back by improving D. The more involved but probably better performing solution is to replace B and F with something that would bring the piezo substantially closer, in the case of a 726.