Ghosts of NEID's Past

TL;DR

This paper discusses the identification, simulation, and mitigation of optical ghosts in the NEID spectrograph to enhance its radial velocity measurement precision, providing strategies applicable to future spectrograph designs.

Contribution

It presents the characterization, simulation, and mitigation strategies for optical ghosts in NEID, improving spectrograph performance and guiding future instrument development.

Findings

Optical ghosts identified and characterized in NEID

Simulations predict ghost features and their impact on RV measurements

Mitigation strategies successfully reduce ghost effects

Abstract

The NEID spectrograph is a R $\sim$ 120,000 resolution fiber-fed and highly stabilized spectrograph for extreme radial velocity (RV) precision. It is being commissioned at the 3.5 m WIYN telescope in Kitt Peak National Observatory with a desired instrumental precision of better than 30 \cms{}. NEID's bandpass of 380 -- 930 nm enables the simultaneous wavelength coverage of activity indicators from the Ca HK lines in the blue to the Ca IR triplet in the IR. In this paper we will present our efforts to characterize and mitigate optical ghosts in the NEID spectrograph during assembly, integration and testing, and highlight several of the dominant optical element contributors such as the cross dispersion prism and input optics. We shall present simulations of the 2-D spectrum and discuss the predicted ghost features on the focal plane, and how they may impact the RV performance for NEID. We also present the mitigation strategy adopted for each ghost which may be applied to future instrument designs. This work will enable other instrument builders to potentially avoid some of these issues, as well as outline mitigation strategies.