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A separate small spectrometer is used to monitor the Ca H & K lines for stellar activity that could mimic Doppler shifts. A major advantage of this approach is that the KPF spectrometer does not need to cover an increased bandpass blueward to 390 nm, and can stop at 443 nm (the Doppler information is reduced for G and K stars at bluer wavelengths owing to the significantly lower flux). This approach has important advantages with respect to optical coating bandpasses and efficiency, as well as the required detector size.

The Calcium H & K spectrometer is a simple system (VPH-based with doublet lenses for collimator and camera), and is fiber fed via a UV-enhanced fiber (such as CeramOptec UVNS). Light will be injected into this fiber by the FIU, to captrue this light before major UV absorption losses occur within the long fiber run.