Abstract
The two micron all sky survey (2MASS) project, currently underway, requires a camera mated to a 1.3 meter Cassegrain telescope that will simultaneously image the J, H and Ks spectral bands onto three separate NICMOS3 HgCdTe detector arrays. Dichroic beamsplitters allow the three FPAs to view the same field simultaneously. This paper reviews the development of this camera, presents a detailed analysis of the optical design, and discusses the techniques used to align the instrument and evaluate its performance. The low irradiance of astronomical sources, combined with background limited performance of the detectors, mandated a reimaging configuration and the enclosure of the camera optics within the cold volume. Correcting chromatic aberrations, and minimizing ghost images were additional challenges imposed by program requirements. The key to achieving high performance was found to be the selection of lens materials. The final F/3.5 optical design uses fifteen CaF2 elements and four Infrasil elements in an all-spherical configuration. The design is diffraction limited over the full format in the Ks spectral band, and is nearly so in the J and H bands. The tolerance environment is benign, making the design well suited for use in a cryogenic environment. A single channel prototype has been built and extensively field tested at the 50-inch telescope at Kitt Peak National Observatory. Each channel in the multi-spectral camera uses the same optical design successfully demonstrated in the prototype. Currently, the first of two three-channel cameras is undergoing lab tests for image quality and channel registration.
