Design, Assembly and Test of G-CLEF's Exposure Meter II: Optical Design and Prototype Test

This project aims to formulate, design, build and test a versatile, high-efficiency, low-resolution spectrograph to function as the G-CLEF exposure meter. G-CLEF, the first-generation Giant Magellan Telescope's (GMT) instrument, is a state-of-the-art, high-resolution, echelle spectrograph for the GMT, expected to be completed for the telescope's first light. The exposure meter plays a vital role for adjusting barycentric corrections of Doppler radial velocity (RV) by accounting for Earth's chromatic atmospheric influences. Its significance becomes pronounced in Extreme Precision RV (EPRV) measurements, where the atmosphere's wavelength dependency contributes to errors at the scale of tens of centimeters per second, the same level of precision required for detecting Earth-analog planets orbiting stars similar to the Sun, aligning with one of the primary scientific objectives of G-CLEF. This paper details the optical and mechanical designs, grounded in the principal requirements that have been previously validated through design trade-off analysis and performance simulations. Additionally, assembly and test phase of the exposure meter prototype are described together with the results that led to the validation of the design.