Evaluation and control of performance of in-house achromatic triplet collimator assembly by infinite conjugate lens test using a Fizeau interferometer

The laboratory measurement and evaluation of the performance of an apo-achromatic collimator is presented in this work. This customized collimator was optimized to utilize a high-resolution spectrograph located at the Thai National Telescope (TNT). The Center for Optics and Photonics (COP) under the National Astronomical Research Institute of Thailand (NARIT) aims to evaluate and control the performance of the achromatic triplet collimator with a Fizeau interferometer to ensure that the correspondence with optical and optomechanical tolerances. The parameters controlled include the Root- Mean-Square (RMS) and Peak-To-Valley (PTV) wavefront errors and the Modulation Transfer Function (MTF). The achromatic triplet consists of three lenses optimized for spectroscopy in the visible range at 0.1 numerical aperture (NA). Its effective focal length is 150 mm, and the lens clear aperture is 0.15 NA at 50 mm. These three customized lenses were assembled into a build-in-house opto-mechanical aluminum housing. The collimator is measured by the infinite conjugate lens test to describe wavefront errors and the MTF. The result suggests the performance of the assembled achromatic triplet collimator conforms with Zemax optical design below 0.1 NA. The RMS and PTV wavefront errors and MTF measured with the interferometer appear comparable to the Zemax optical design of the collimator within this range. Therefore, we conclude that our in-house manufacturing, assembly, and control are acceptable for astronomy applications.