Alignment based on a no adjustment philosophy for the Immersion Grating Infrared Spectrometer (IGRINS)

IGRINS, the Immersion GRating INfrared Spectrometer includes an immersion grating made of silicon and observes both H-band (1.49~1.80 μm) and K-band (1.96~2.46 μm), simultaneously. In order to align such an infrared optical system, the compensator in its optical components has been adjusted within tolerances at room temperature without vacuum environment. However, such a system will ultimately operate at low temperature and vacuum with no adjustment mechanism. Therefore a reasonable relationship between different environmental variations such as room and low temperature might provide useful knowledge to align the system properly. We are attempting to develop a new process to predict the Wave Front Error (WFE), and to produce correct mechanical control values when the optical system is perturbed by moving the lens at room temperature. The purpose is to provide adequate optical performance without making changes at operating temperature. In other words, WFE was measured at operating temperature without any modification but a compensator was altered correctly at room temperature to meet target performance. The `no adjustment' philosophy was achieved by deterministic mechanical adjustment at room temperature from a simulation that we developed. In this study, an achromatic doublet lens was used to substitute for the H and K band camera of IGRINS. This novel process exhibits accuracy predictability of about 0.002 λ rms WFE and can be applied to a cooled infrared optical systems.