Optical design of temperature-insensitive afocal attachments

Afocal optical attachments are essentially telescopes, mostly of the Galilean type, used to alter the magnification of an existing imaging system. These afocal attachments have gained popularity as range extenders in thermal imaging. A range extender enhances the resolution of the base thermal imager (TI) by increasing the effective focal length (EFL) of the imaging optics. The optical performance of range extender optics should be sustained over the operating temperature range so that the athermalization scheme of base TI works well for the extended TI. The optical design aspects, specific to afocal optics in mid-wave infrared (3-5 micron MWIR), are discussed in this paper. A mathematical model is developed using paraxial calculations to select suitable material combinations for an athermal and achromatic afocal solution. Finally, few optical designs are presented as the verification of the model.