Periscope and Optics

A more sensitive detection often requires more exact solutions in other parts of the instrument. With Aida, improvements have been made to the periscope and the optics of the interference system.

As Thomas Laue, the inventor of the interference system, has often stated, the source of the familiar "jitter", a minor vertical displacement of the entire interferogram, is due to the fact that in BeckmanCoulter's XLI, the light source and the periscope are located at different points of the heat sink. Whereas the periscope starts with the condensor lens, located in a cutout in the heat sink, the light source is attached to the monochromator arm, seated in a socket located elsewhere. Thus, as the heat sink raises and sinks periodically due to the activity of the Peltier elements, the light source and the periscope will perform different movements, resulting in a "pumping" of the interferograms. Though modern software will isolate and remove this noise, it is more convenient to avoid its formation from the start.

Periscope photograph
Figure 1: Periscope designed as one entity for eliminating jitter, mounted outside the centrifuge for inspection. The supporting clamp marks the position of the heat sink feedthrough.

The Aida optics are designed such that the light source is directly connected to the condensor lens feedthrough in the heat sink, forming one entity. If the optics shift, they shift in total, and the beam is not displaced. In addition, the feedthrough has been redesigned in a flexible manner: the periscope is firmly attached to the centrifuge's chassis, and the feedthrough allows for the heat sink to move slightly whereas the periscope maintains its position.

This design demands that the optics assembly remains in the rotor chamber and is not taken out for inserting the rotor. Though this requires some care when inserting and taking out the rotor, it is much to the benefit of alignment, to which this system is most sensitive. However, the light source assembly can be taken off when required, e.g. for exchanging the red light source by a blue one.

AIDA feedthrough construction drawing
Figure 2: Periscope feedthrough schematics.

The most sensitive adjustment within the optical system is made to the cylinder lens, which superimposes the two laser beams on the plane of projection. Its azimuth angle must be perfectly aligned to the light source's slit mask. The lens mount has been redesigned to allow for sensitive azimuth adjustment by a worm gear; also, it has an additional degree of freedom, allowing to move the lens horizontally, into or out of the beam.

Adjustable cylinderlens photograph
Figure 3: Redesigned cylinder lens mount.