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.
The Discovery Channel Telescope (DCT) is planned to have a state-of-the-art prime focus corrector which was described previously. The initial design contained I-line glasses which had long procurement times. Goodrich Corp. undertook a study funded by Lowell Observatory to determine whether significant savings in cost and schedule would be possible with an acceptable reduction in the performance of the telescope.
This paper reports on changes in the optical design of the wide-field optical corrector (WFOC) with a view to eliminating the long-lead materials. The consequent changes in performance are also discussed. The required FWHM of the telescope was relaxed somewhat and the imaging requirements of the ultraviolet (U) band were eliminated. The new design meets the two-degree field of view requirement and recovers most of the performance in the ultraviolet.
The Discovery Channel Telescope (DCT) is a joint venture between Discovery Communications and Lowell Observatory. The telescope will have a 4.2-meter clear aperture, active primary mirror working at F/1.9. Two observing stations are presently planned; a Ritchey-Chretien focus some two meters behind the vertex of the primary mirror and a prime focus featuring a wide-field optical corrector (WFOC) with a two-degree field of view. The Ritchey-Chretien focus will be used for a variety of optical and near infrared imaging and spectroscopic instrumentation while the prime focus will be largely used as a survey tool to search for near-earth and Kuiper belt objects, for example.
In order to take advantage of sub-arc second seeing at the DCT site, a stringent set of requirements has been placed on the two foci. The requirements are for the full-width, half-maximum (FWHM) image of a point source to be less than 0.20 arc second at the Ritchey-Chretien focus over a 21 arc minute field and less than 0.27 arc second at prime focus in each of six filter bands including a very broad band for survey purposes.
This paper describes the optical design of the field correctors at the two foci. Particular attention is paid to the WFOC. This state of the art device poses a number of optical challenges which are discussed here, as well as mechanical challenges which are discussed elsewhere.
An overview of the Lockheed Martin Team's NGST Reference Architecture is discussed. Our f/1 NGST concept includes a lightweight 8-meter primary mirror consisting of eight deployed petals. Alignment and figure control employs wavefront-sensing techniques. Infrared observations are enabled by using a tennis court size multi-layer deployed sunshield permitting the primary mirror to be passively cooled to < 40 K. Candidate Science Instruments cover the spectral range from 0.6 microns to greater than 20 microns. The Integrated Science Instrument Module (ISIM) is passively cooled to approximately 30 K. The Observatory is launched on an AtlasV-531M in 2008 and operates at the L2 LaGrange Point. Science Planning and Mission Operations are the responsibility of the Space Telescope Science Institute in Baltimore Maryland. The ISIM is the responsibility of Goddard Space Flight Center (GSFC). The Lockheed Martin Team, including Raytheon, Honeywell, and Jackson and Tull, is an NGST Phase 1 Prime Contractor. The GSFC manages the NGST Project in Greenbelt Maryland.
A 1.3 meter aperture Cassegrain telescope with a very wide flat field has been completed and is now producing images. The field of view is a stunning 1.7 degrees while the focal ratio is a very fast F/4. The telescope is located at the United States Naval Observatory Flagstaff Station in Arizona, USA.