First generation heterodyne instrumentation concepts for the Atacama large aperture submillimeter telescope

The Atacama Large Aperture Submillimeter Telescope (AtLAST) project aims to build a 50-meter-class submm telescope with >1-degree field of view, high in the Atacama Desert, providing fast and detailed mapping of the mm/submm sky. It will thus serve as a strong complement to existing facilities such as the Atacama Large Millimeter/Submillimeter Array (ALMA). ALMA is currently the most sensitive observatory covering the atmospheric windows from centimeter through submillimeter wavelengths. It is a very powerful instrument for observing subarcminute-scale structures at high, sub-arcsecond spatial resolution. Yet its small field of view (< 15” at 350 GHz) limits its mapping speed for large surveys. In general, a single dish with a large field of view can host large multi-element instruments that can more efficiently map large portions of the sky than an interferometer, where correlator resources and the smaller fields of view of the antennas tend to limit the instantaneous number of beams any instrument can have on the sky. Small aperture survey instruments (typically much smaller than < 3 × the size of an interferometric array element) can mitigate this somewhat but lack the resolution for accurate recovery of source location and have small collecting areas. Furthermore, small aperture survey instruments do not provide sufficient overlap in the spatial scales they sample to provide a complete reconstruction of extended sources (i.e. the zero-spacing information is incomplete in u,v-space.) Heterodyne instrumentation for the AtLAST telescope will take advantage of extensive developments in the past decade improving the performance and pixel count of heterodyne focal plane arrays. The current state of the art in heterodyne arrays are the 64-pixel Supercam instrument, the 16-pixel HARP instrument, the dual band SMART receiver with 8 pixels in two bands, and the GREAT instrument on SOFIA with 21 pixels (14 at 1.9 THz and 7 at 4.7 THz). Future receivers with larger pixel counts have been under development: CHAI for CCAT (64-pixels) and SHASTA for SOFIA (64 pixels) or under study, e.g. HERO for the Origins Space Telescope (2x9 to 2x64 pixels). Instruments with higher pixel counts have begun to take advantage of integration in the focal planes to increase packaging efficiency over simply stacking modular mixer blocks in the focal plane. The authors believe that heterodyne instruments with pixel counts of approximately 1000 pixels per band could be considered for AtLAST on a decade timescale. The primary limiting factor in instrument capability (pixel count, instantaneous bandwidth, number of frequency bands, polarization capability, side-band separation etc.) is likely to be cost, rather than any fundamental technological limitation. As pixel counts increase, the cost and complexity of the IF system and spectrometer also rapidly increases, particularly if wide IF bandwidth, dual polarization and sideband separation is desired. Currently the IF and backend are limited by the cost and power consumption per unit bandwidth of the total processed science signal. While that cost is likely to decrease modestly in the next decade, no technology is likely to disrupt the scaling argument. Many of the front-end costs will also scale with pixel count, for example the size and cooling capacity of the cryostat, the complexity of the LO subsystem, and the I&T cost associated with developing, assembling and testing the focal plane units. In this presentation, we review the state of the art for millimeter/sub-millimeter heterodyne instrumentation technology that could be suitable for AtLAST and attempt to forecast how the technologies will advance over the next decade. We then present a design concept for a potential first-generation AtLAST heterodyne instrument. These considerations meet the scientific demands and atmospheric considerations for a ground-based facility in the Atacama Desert.