The CCAT Heterodyne Array Instrument (CHAI)

Figure 1: Millimeter to near-IR dust continuum emission of the inner part of the Milky Way. APEX 0.87 mm (red), Spitzer near-IR (blue), and Planck extended faint emission (red). This is a part of the Galactic Plane that will be covered in spectral line observations with CHAI in the GEco survey.

CHAI Science

The University of Cologne is building the CCAT Heterodyne Array Instrument (CHAI), a 2 x 64 pixel, high resolution spectrometer to study the cycle of interstellar matter in the Milky Way and nearby galaxies.

The Galactic Ecology project (GEco) gathers several science cases to improve our understanding of the assembly of interstellar gas clouds, the formation of filaments and cores within them, and ultimately the formation of stars, which in turn provide stellar feedback to the cycle of matter. All these processes shape and determine the evolution of galaxies in the nearby and distant Universe.

The large number of pixels, high sensitivity, and choice of spectral lines available with CHAI will allow us for the first time to scan large areas of the sky in key diagnostic spectral lines of atomic Carbon (C) and Carbon monoxide (CO). This will provide a rich new data set to study the evolution, dynamics, and chemistry of interstellar gas clouds at high angular and spectral resolution, highly complementary to similar, already existing and ongoing studies of different spectral lines and continuum emission (Figure 1).

The CHAI Instrument

Figure 2: (Left) CHAI mounting inside FYST. (Right) CAD model of CHAI’s focal plane unit

CHAI is a dual-color heterodyne array receiver with a spectral resolving power in excess of 10⁶. Each of two separate cryostats houses a square 8×8 pixel array of balanced SIS mixers. The Low Frequency Array (LFA) covers most of the 650 µm atmospheric window, primarily targeting the CO J=4→3 and the [CI] ³P₁→³P₀ transitions. The High Frequency Array (HFA) is tuned to complementary spectral lines in the 350 µm window: CO J=7→6 and the [CI] ³P₂→³P₁.

A retractable optics unit diverts the light from FYST’s main optics path and sends the signal to Instrument Space 2, where CHAI is located (left panel Figure 2). The signal is split by polarization and distributed to the two cryostats. Each cryostat uses two closed cycle refrigerators to cool the detectors down to 4 K. A compact focal plane unit (FPU, See right panel Figure 2) contains the mixers together with the low noise amplifiers (LNA) and the LO distribution network.

After preamplification, the received signals are further processed by warm electronics and spectrally analyzed by digital Fourier transform spectrometers.

CHAI Detector Technology

Figure 3: *CHAI’s balanced mixer chip. The dimensions give the widths of the signal waveguides for the two frequency bands*

CHAI uses on-chip balanced Superconductor-Isolator-Superconductor (SIS) mixers to detect the signal and to down-convert it to a frequency that can be accessed by conventional high-frequency electronics.

Balanced mixers make optimal use of the weak local oscillator (LO) reference signal and also reject noise on the LO signal, improving the stability of the receiver. Our mixer chips are developed at the University of Cologne / Germany with all the required circuitry on a single silicon membrane to make them small enough for a multi-pixel instrument.

Since balanced mixers have an LO input port separate from the signal input, the LO signal is introduced through a waveguide and distributed by a cascade of waveguide power dividers.