Phase Centering Telescopes for Olympus BH2 Microscopes
The Phase Telescope (sometimes referred to as a “Bertrand Lens”) is a device used to align the various optical components of a light microscope. The phase telescope allows observation of the back focal plane of the objective lens and its conjugate focal planes, and has multiple uses in microscopy (some of which are listed below). Every serious microscopist should own a phase telescope.
PHASE CONTRAST: The phase telescope can be used in phase contrast microscopy to align the phase annuli within the condenser with the phase rings within the phase contrast objectives. This alignment is critical for optimal phase contrast performance. In order to perform this alignment, the phase telescope is inserted in place of one of the eyepieces and the helical focus of the phase telescope is adjusted to bring the bright ring of light from the phase annulus into sharp focus. Next, the centering of the annulus is adjusted so as to superimpose this ring of light with the phase ring within the objective. This alignment should be repeated for all phase contrast objectives, making sure to match the objective magnification with the proper phase annulus as you do so. After this alignment has been performed, the phase telescope is removed and replaced with the standard eyepiece.
DARKFIELD: The phase telescope can be used in darkfield microscopy to verify that the alignment of the darkfield patch stop is such that all zero-order (i.e., direct) light is excluded by the patch stop. When the condenser / patch stop are properly aligned, the back focal plane of the objective should appear completely dark. Any bright areas indicate misalignment of the patch stop, which will prevent adequate darkfield.
KÖHLER ILLUMINATION: The phase telescope can be used for setting up Köhler illumination, since the light source and condenser diaphragm should appear in focus at the back focal plane of the objective.
NOMARSKI: The phase telescope can be used in Nomarski/DIC to ensure that the Wollaston/Nomarski prisms, polariziers, and analyzers are oriented and aligned so as to achieve the proper optical interference and background coloration.
Olympus made 23mm and 30mm versions for the BH2, as described below.
CT-5
CT-5 Phase Contrast Centering Telescope
The CT-5 phase centering telescope has a 23mm diameter, and is compatible with the various 23mm viewing heads. The CT-5 is marked as “CT”.
CT-30
CT-30 Phase Contrast Centering Telescope
The CT-30 phase centering telescope has a 30mm diameter, and is compatible with the super-wide binocular and trinocular heads. The CT-30 is marked as “CT”.
Third-Party Options
Just about any phase centering telescope from other manufacturers will work with the Olympus BH2, so long as the barrel diameter matches the diameter of the ocular tubes on the BH2, and so long as the physical length is such that it will not reach too far into the ocular tube, thereby posing a hazard to the glass prisms withing.