Optics for the Columbus Project Telescope

J. M. Hill

Steward Observatory, University of Arizona
Columbus Project Technical Memo

UA-92-01

June 8, 1992
presented at the Columbus, Ohio AAS meeting

http://medusa.as.arizona.edu/lbtwww/tech/ua9201.htm

Abstract

The Columbus Project telescope is a 2 x 8.4 meter binocular instrument being built by the University of Arizona and the Osservatorio Astrofisico di Arcetri. Dual 8.4 meter borosilicate honeycomb primary mirrors provide an equivalent collecting area of 11.8 meters. With a focal ratio of F/1.14, these primaries will permit a very compact and stiff telescope structure. The combined focus provides an interferometric baseline of 22.8 meters on a common mount. The phased focus provides a coherent field-of-view up to 6 arcminutes in diameter at F/33. In addition to a central focus for interferometry, the telescope will be instrumented with dual Cassegrain focal stations at F/5 and F/15. The F/5 foci will be optimized for wide field work in the optical and near-infrared. The 1.96 m diameter secondaries provide a naked Cassegrain focus at F/5.2. A three-element refractive corrector provides a 50 arcminute field-of-view at F/5.4. This wide field will be used for multiobject spectroscopy as well as wide field imaging and long-slit spectroscopy. The F/15 foci will be optimized for work at mid- and thermal infrared wavelengths with approximately a 10 arcminute field-of-view. The infrared foci will be used for imaging and spectroscopy over a wide wavelength range. The 0.74 m diameter secondaries will have chopping and rapid guiding capabilities. Adaptive optics will also play a key role in the telescope operation. Tertiary flats will provide bent Cassegrain focal stations. Secondaries which are not in use will be stored in the center of the telescope structure allowing for rapid interchange during the night. The error budget for the optics and the telescope will attempt to match the wavefront provided by an r0 = 45 cm atmosphere.

Columbus Project --- Optics for the Second Voyage

Primary Mirrors

Focal Stations

Figure 1: Telescope Optical Layout for Wide Field Dual Cassegrain at F/5.2. A three element refractive corrector provides a 50 arcminute field of view. Two 8.4 meter mirrors provide an interferometric baseline of 22.8 meters.

Secondary Complement

Other Optics

Figure 2: Three Element Refractive Corrector for Wide Field Cassegrain. This corrector was designed by Domenico Bonaccini at the Osservatorio Astrofisico di Arcetri. It contains three all spherical fused silica elements. 80% of the energy is concentrated with 0.25 arcsecond in the inner 40 arcminutes of the field. This degrades to 0.86 arcsecond over the full 50 arcminute diameter field. This could be improved by adding one aspheric surface. The corrector works over a wavelength range of 0.33 to 1.1 microns. The corrector may be used with or without a pair of counter-rotating prisms to correct atmospheric dispersion. The focal plane has a 5 meter radius of curvature.

Instrument Mounting Clearances

Error Budget

The error budget for the total telescope is 0.225 arcsec FWHM which corresponds to an r0 = 45 cm atmosphere at 0.5 microns. This has been divided as follows:

Telescope Structure and Enclosure

Figure 3: Design of the Two-Shooter Telescope Structure. This drawing was done by the engineering firm ADS Italia. The dual ring elevation support allows a structural resonant frequency of 9--10 Hz.

Figure 4: Design of the Two-Shooter Telescope Enclosure. This drawing was done by the engineering firm ADS Italia. The enclosure is a corotating box with two portal shutters. Openings in the front, sides and rear allow good ventilation to assure thermal equilibrium. A large elevator on the ground provides access to the observing chamber for instruments and other heavy equipment. Functions which produce heat are isolated in the lowest level of the rotating section.