  As part of a study funded by NASA Headquarters, we are developing a
Probe-class mission concept called the Cosmic Evolution Through UV Spectroscopy
(CETUS). CETUS includes a 1.5-m aperture diameter telescope with a large
field-of-view (FOV). CETUS includes three scientific instruments: a Far
Ultraviolet (FUV) and Near Ultraviolet (NUV) imaging camera (CAM); a NUV
Multi-Object Spectrograph (MOS); and a dual-channel Point Source Spectrograph
(PSS) in the Lyman Ultraviolet (LUV), FUV, and NUV spectral regions. The large
FOV Three Mirror Anastigmatic (TMA) Optical Telescope Assembly (OTA)
simultaneously feeds the three separate scientific instruments. That is, the
instruments view separate portions of the TMA image plane, enabling parallel
operation of the three instruments. The field viewed by the MOS, whose design
is based on an Offner-type spectrographic configuration to provide wide FOV
correction, is actively configured to select and isolate numerous field sources
using a next-generation Micro-Shutter Array (MSA). The two-channel camera
design is also based on an Offner-like configuration. The Point Source
Spectrograph (PSS) performs high spectral resolution spectroscopy on unresolved
objects over the NUV region with spectral resolving power, R~ 40,000, in an
echelle mode. The PSS also performs long-slit imaging spectroscopy at R~ 20,000
in the LUV and FUV spectral regions with two aberration-corrected, blazed,
holographic gratings used in a Rowland-like configuration. The optical system
also includes two Fine Guidance Sensors (FGS), and Wavefront Sensors (WFS) that
sample numerous locations over the full OTA FOV. In-flight wavelength
calibration is performed by a Wavelength Calibration System (WCS), and
flat-fielding is also performed, both using in-flight calibration sources. This
paper will describe the current optical design and the major trade studies
leading to the design.
