Mark Ackermann (Sandia National Laboratories), John T. McGraw (University of New Mexico), Peter C. Zimmer (University of New Mexico), Tom Williams, (University of New Mexico)
Keywords: Telescopes, Instrumentation
Abstract:
The NESSI Survey telescope will be the second incarnation of the CCD/Transit Instrument. It is being designed to accomplish precision astronomical measurements, thus requiring excellent image quality and virtually no distortion over an inscribed 1° x 1° scientific field of view. Project constraints such as re-use of an existing unperforated parabolic f/2.2 primary mirror, and the desire to re-use much of the existing CTI structure, have forced the design in one direction. Scientific constraints such as the 1.42° field, 60?m/arcsec plate scale, zero focus shift with wavelength, zero distortion and 80% encircled energy within 0.25arcsec spot diameters have further limited remaining design options. After exploring nearly every optical telescope configuration known to man, and several never before imagined, the NESSI Project Team as arrived at a unique optical design that produces a field and images meeting or exceeding all these constraints. The baseline configuration is that of a bent Cassegrain, employing a convex hyperbolic secondary, a 45° folding flat and a four lens refractive field group. One unique feature of this design is that all four lenses lie outside the primary aperture, thus introduce no obscuration. A second unique aspect of the design is that the largest lens is only slightly larger than the focal plane array.
The field corrector lenses are not large by todays standards but still large enough to make the availability of glass a serious concern. A number of high performing designs were abandoned when it was learned the glass was either not available or would require a special production. With a little luck, a little insight and a lot of work, we followed the rugged ways to the stars, and were able to arrive at a relatively simple Cassegrain design where only one corrector lens had an aspheric surface, a simple parabola, and all four lenses were made of BK7 glass. This design appears to be manufactureable and essentially meets all of the stringent requirements placed upon it. In this paper we present the baseline optical design and describe its associated performance.
Date of Conference: September 10-14, 2006
Track: Telescopes and Instrumentation