When aligning the laser path of a system in which mirrors are translated, like for instance for the x/y/z adjustments on a 2-photon microscope, the laser path needs to be kept in parallel with each of the translation axes. Also, for almost any system, it is important to keep the beam well centered in the axis of optical elements. It is therefore common practice (in systems where the mirror mounts are placed precisely in line) to align beam paths by centering the beam on each mirror in the system, by using an alignment target that is put in the mirror mount in place of the mirror.
Because the final alignment on a 2p scope can often not be done in a visible wavelength (either none is available, or the laser angle varies too much for the visible tuning range to be useful for alignment), IR viewer cards are needed to check if the beam hits the target in the center. This is cumbersome, requiring at least 2 hands, and error prone.
By making a mirror mounted IR viewing card that sits at the same plane and x/y position in the mount as the mirror surface, this process can be made much faster. There are a few existing options that look promising (Thorlabs, 3d printed cap with target for laser cutter), but none of these seem to provide the same precision and repeatability as a machined mirror target that is well seated in the mirror mount.
Here’s an easy recipe for adding an IR viewer card to a mirror alignment target, requiring only a target (Mirror mounted holder + aperture plate), a good IR viewer card, or a 1/2″ IR viewer disk (I’m not 100% about the quality of these though) and some very common tools:
Cut part of an IR viewer card to the same size as the aperture in the mirror mount adapter (1/2″ in this case)
Insert viewer card into the adapter and and make a small mark in the center. Then hold the card in place with the 1/2″ aperture plate and secure it with the set screw. Make sure that the adapter you’re using ends up placing the IR card at the same plane as the mirror surface, or there will be a small position offset.