An ultra-small thermal imaging core, or micro-core. The design of the micro-core may include substrates for mounting optics and electronic connectors that are thermally matched to the imaging Focal Plane Array (FPA). Test fixtures for test and adjustment that allow for operation and image acquisition of multiple cores may also be provided. Tooling may be included to position the optics to set the core focus, either by moving the lens and lens holder as one or by pushing and/or pulling the lens against a lens positioning element within the lens holder, while observing a scene. Test procedures and fixtures that allow for full temperature calibration of each individual core, as well as providing data useful for uniformity correction during operation may also be included as part of the test and manufacture of the core.