The near infrared camera for the Subaru Prime Focus Spectrograph

Abstract

We present the detailed design of the near infrared camera for the SuMIRe (Subaru Measurement of Images and Redshifts) Prime Focus Spectrograph (PFS) being developed for the Subaru Telescope. The PFS spectrograph is designed to collect spectra from 2394 objects simultaneously, covering wavelengths that extend from 380 nm - 1.26 mu m. The spectrograph is comprised of four identical spectrograph modules, with each module collecting roughly 600 spectra from a robotic fiber positioner at the telescope prime focus. Each spectrograph module will have two visible channels covering wavelength ranges 380 nm 640 nm and 640 nm 955 nm, and one near infrared (NIR) channel with a wavelength range 955 nm - 1.26 mu m. Dispersed light in each channel is imaged by a 300 mm focal length, f/1.07, vacuum Schmidt camera onto a 4k x 4k, 15 mu m pixel, detector format. For the NIR channel a HgCdTe substrate-removed Teledyne 1.7 mu m cutoff device is used. In the visible channels, CCDs from Hamamatsu are used. These cameras are large, having a clear aperture of 300 mm at the entrance window, and a mass of similar to 250 kg. Like the two visible channel cameras, the NIR camera contains just four optical elements: a two-element refractive corrector, a Mangin mirror, and a field flattening lens. This simple design produces very good imaging performance considering the wide field and wavelength range, and it does so in large part due to the use of a Mangin mirror (a lens with a reflecting rear surface) for the Schmidt primary. In the case of the NIR camera, the rear reflecting surface is a dichroic, which reflects in-band wavelengths and transmits wavelengths beyond 1.26 mu m. This, combined with a thermal rejection filter coating on the rear surface of the second corrector element, greatly reduces the out-of-band thermal radiation that reaches the detector. The camera optics and detector are packaged in a cryostat and cooled by two Stirling cycle cryocoolers. The first corrector element serves as the vacuum window, while the second element is thermally isolated and floats cold. An assembly constructed primarily of silicon carbide is used to mount the Mangin mirror, and to support the detector and field flattener. Thermal isolation between the cold optics and warm ambient surroundings is provided by G10 supports, multi-layer insulation, and the vacuum space within the cryostat. In this paper we describe the detailed design of the PFS NIR camera and discuss its predicted optical, thermal, and mechanical performance.