We present the conceptual design of a new optical echelle spectrograph for the Astrophysical Research Consortium 3.5-m Telescope at Apache Point Observatory in New Mexico. The Nasmyth-mounted slit spectrograph, intended to replace the existing echelle spectrograph that is over 20 years old, will provide continuous spectra for wavelengths spanning the range 350 nm – 1000 nm with a white pupil design featuring separate blue and red arms. A real-time slit change mechanism will allow three primary modes: 1) Resolution R ~ 32,000 for the median seeing of 1.2 arcsec FWHM, 2) R ~ 40,000 when using a smaller slit with tip-tilt image stabilization, and 3) R ~ 64,000 using a two-slice image slicer. The peak instrument throughput is ~ 38% and at 350 nm the throughput is ~ 22%. High throughput, particularly in the blue, will be enabled through the use of an atmospheric dispersion corrector, high performance coatings, tip-tilt image stabilization, and careful attention to refractive material choices. Expected instrument sensitivity for magnitude 14 stars is SNR per pixel of 50 in 3600 sec at 355 nm (u-band) and about 1800 sec for the centers of griz bands for both the R ~32,000 and R~ 40,000 modes.
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