Optical design of the CCD/Transit Instrument with Innovative Instrumentation (CTI-II) Telescope

We describe a 1p8m f/6 Cassegrain optical system that creates a 1.42° FOV with near diffraction limited images from 400nm to 1100nm with full-field distortion less than 0.01%. The astronomical application for this optical system is the CCD/Transit Instrument with Innovative Instrumentation (CTI-II), designed to produce a highly precise photometric and astrometric survey of a complete strip of sky in the northern hemisphere. We describe the scientific observation program and supporting optical design for the telescope. The all-spherical, five lens field corrector represents a very capable optical system that works well with many other astronomical telescopes such as SDSS, Pan-STARRS, SkyMapper, ESO's VST, the WIYN ODI, and the MMT WFC. In many cases, using a five lens corrector exceeded the optical performance of the original published system designs. Conversely, these and other optical concepts compromised the performance of the CTI-II design. The CTI-II design is similar to many other wide-field telescope and imaging camera designs, thus the design is of potential general use in astronomy.