A UV Double Pass Spectrograph for Monitoring Stellar Acitivity for the Keck Planet Finder

We present a compact, double-pass cross-dispersed echelle spectrograph that is tailored specifically to cover the 383 nm to 403 nm spectral range and record R similar to 16,000 spectra of the stellar chromospheric Ca II H&K lines. This `H&K' spectrometer was developed as a subsystem of the Keck Planet Finder (KPF), which is an extremely precise optical (440 - 870 nm) radial velocity spectrograph for Keck I, scheduled for commissioning Fall 2022, with the science objective of measuring precise masses of exoplanets. The H&K spectrometer will observe simultaneously with KPF to independently track the chromospheric activity of the host stars that KPF observes, which is expected to dominate the KPF measurement floor over long timescales. The H&K Spectrometer is fiberfed from the KPF fiber injection unit with total throughput of 4-7% (top of telescope to CCD) over its operating spectral range. Here we detail the optical design trade offs, mechanical design, and first results from alignment and integration testing.