GeminiFocus January 2018 | Page 20

Figure 1. MAROON-X spectrograph installed in its environmental chamber in the lab at the University of Chicago. Credit: Andreas Seifahrt for the MAROON-X team. Figure 2. Efficiency curves for the MAROON-X spectrograph from the fiber exit to the focal plane. The measured efficiency for the as-built spectrograph (arrows) are lower limits only due to limitations in resolving the blaze peak of each echelle order in a spectrophotometric setup. A model based on the measured throughput of all individual components is sho wn as the blue line. For comparison we show the theoretical predictions from an optical model with minimum and best-effort specs for the throughput and efficiency of all optical components. The delivered optics exceed the best case expectations over most of the bandpass of the blue arm and lend confidence in achieving similar efficiencies in the red arm, currently under construction at KiwiStar Optics. 18 African Large Telescope. Table 1 provides a summary of MAROON-X’s properties. We in- tend to bring MAROON-X to Gemini North as a visiting instrument beginning in 2019. Current Status In January 2017, KiwiStar Optics delivered the core spectrograph and the blue wavelength arm to the University of Chicago. This has been installed in a chamber with temperature control to better than 20 milliKelvin (Figure 1). The spectrograph is currently undergoing an intensive test and calibration campaign. All the expected characteristics of the spec- trograph (e.g., resolution, scattered light, and efficiency) have been confirmed with lab measurements. The spectrograph’s efficiency in the blue arm is particularly impressive, with peak throughputs from the exit of the fiber feed to the focal plane of over 60% (Figure 2). Ini- tial testing is being done with only the blue wavelength arm implemented and a smaller, off-the-shelf 2k x 2k e2v detector in place of GeminiFocus the final 4k x 4k custom STA science detector systems that will be used on the telescope. Orders for the red arm and the final detec- tor systems have been placed and delivery is expected for mid-2018. One of the detec- tors is a thick, deep-depletion CCD that of- fers quantum efficiencies of over 90% out to 900 nm to fully exploit the high throughput of the instrument and to suppress fringing which would otherwise limit the achievable radial velocity precision. The primary wavelength calibrator for the instrument is a stabilized Fabry-Perot etalon, traced to the hyperfine transition of rubid- ium. This device delivers a comb-like spec- trum of about 500 bright and unresolved lines per spectral order with frequencies traceable to a few cm/s (Stürmer et al., 2017). In addition, an automated solar telescope delivers solar light to the spectrograph, to test and improve the data reduction and ra- dial velocity analysis pipeline delivered with the instrument (Figure 3). First tests with the etalon calibrator demon- strated that even over the limited spectral coverage of the smaller and less stable lab detector system, the science and calibration fibers track each other to better than 20 cm/s over timescales of minutes to days (Figure 4). The high line density and exquisite stability of the etalon allows for unprecedented sta- bility vetting and calibration at a level other- wise offered only by a much more complex and expensive laser frequency comb. January 2018