The Terahertz Intensity Mapper (TIM)

The Terahertz Intensity Mapper (TIM) is a balloon-borne experiment to study the cosmic star formation history via line intensity mapping (LIM). TIM will launch from Antarctica and—over the course of a ~30 day flight—investigate the power spectrum of [CII] and other far-infrared lines across the last 4.5 billion years of cosmic history. Developed by an international collaboration of scientists, TIM is the first experiment of its kind to conduct intensity mapping in the far-infrared wavelength regime. TIM consists of a 2-meter diameter primary mirror and an imaging spectrometer operating between 240−420 microns. TIM's spectrometer contains 7200 kinetic inductance detectors (KIDs), a novel superconducting detector technology and the largest amount ever fielded in the far-infrared. It will map a volume spanning 4.5 billion years of cosmic history (0.52<z<1.67), on scales from 1−50 Mpc (0.5 to 60 arcminutes) with complete spectroscopic information. In the coming decades LIM will be a powerful cosmological tool for charting the 3D structure of the universe. There is significant discovery potential with TIM because it will be probing an under-explored wavelength range with unprecedented sensitivity using a novel astrophysical technique. TIM will be the first generation of experiments using LIM in the FIR regime, filling in a crucial wavelength regime only accessible from either space or a balloon platform. TIM serves as an important technological test-bed, and occupies a unique and vital scientific niche not filled by previous or existing instruments.

Annotated schematic of the TIM payload (Credit: Talia Saeid, Shubh Agrawal)

Science

Explaining the history of cosmic star formation through the evolution of galaxies is one of the most important challenges in modern astrophysics. The path to understanding galaxy evolution will necessarily run through observations in the far-infrared (FIR) given other radiation such as optical and ultraviolet is extincted by interstellar dust. There are a variety of un-extincted FIR diagnostic lines that can reveal the physics of galaxy evolution by tracing the star formation rate (SFR), black hole accretion rate, mass function of stars, spectrum of ionizing radiation, and metallicity of the interstellar medium (ISM).

TIM will be a vital technological, data analysis, and scientific stepping stone to future orbital missions, and will also advance our understanding of galaxy evolution through observations that cannot be replicated with current FIR instruments.

(Top) Line intensity mapping cube, ionized carbon ([CII]) detections shown as black dots. (Bottom left) Simulated data of relative intensity vs frequency and redshift; colored spikes indicate detections of bright [CII] emissions. (Bottom right) Redshift slices of LIM cube showing [CII] emitters (Credit: S. Agrawal, R. Keenan, E. Mayer)

Instrumentation

TIM combines a long-slit spectrometer operating from 240−420 μm with a 2m low-emissivity carbon-fiber telescope to provide a substantial increase in sensitivity over existing instruments.


The assembled TIM primary mirror. This segmented aluminum 2-meter mirror and its carbon fiber backup structure and tripod were designed by mtex Antenna Technology.

Telescope

The TIM telescope is a reflecting telescope with a segmented, 2.0-meter diameter, carbon fiber primary mirror, as well as a fully carbon fiber secondary and support structure, with gold metallization of the reflecting surfaces to minimize the emissivity.


Spectrometer

To efficiently perform spectroscopy over the entire 240−420 μm band, we have two independent spectrometer modules: a short wavelength (SW) module covering 240−317 µm, and a long wavelength (LW) module covering 317 − 420 µm.


The two modules follow the same basic design; each consists of a plane diffraction grating mounted between concave collimating and camera mirrors in a Czerny- Turner configuration.


Top left: overall optical system with the telescope and cold optics. The grey cylinder end caps represent the cold volume. Top right: the ray trace of the cold optics where the splitting of the bands is shown. Note that there is a second fold mirror after the slit mirrors to re-orient the optical axis into the page. Bottom: a split out view of each module to show the geometry of the Czerny-Turner spectrometers.

Kinetic-Inductance Detector (KID) Arrays

KIDs have emerged in the last decade as a straightforward approach to very large detector arrays for astrophysics. These devices use thin-film, high-Q micro-resonators that absorb incident radiation and respond by changing resonance frequency and line-width. Due to the high resonance quality factors Q~10^5  that can be obtained, large numbers of KIDs may be read out on a single RF/microwave circuit. Each circuit is simply a single RF line down to the focal plane and another line returning via the amplifier, and it carries the signals of ~10^3 detectors.

KID technology is now rivaling the performance levels of the SQUID-multiplexed bolometer systems in ground-based instruments.


An image of a fabricated main quadrant array with 864 KID pixels on a single readout line.

On September 23, 2024, from the NASA Columbia Scientific Ballooning Facility (CSBF) in Fort Sumner, New Mexico, TIM took to the sky for the first time on its engineering test flight. This was the most assembled the instrument had ever been before. The purpose of this campaign was to fully integrate and test the gondola and its subsystems, including: pointing motors, flight software, pointing sensors, telemetry system, ground station commanding, power subsystem (including a solar panel array), integration with NASA launch systems and payload communication network, and overall mechanical design.

At 7:26 am MDT, TIM took off for its 3 hour climb up to a float altitude of 120,000 ft (36.6 km). Once at float, various pointing and scanning tests were commanded from the ground, exercising motor control and communication with the reaction wheel, pivot shaft, and elevation axis motors. There were many sensors onboard, including star cameras, pinhole sun sensors, gyroscopes, inclinometers, and magnetometers. Their data were streamed to the ground and stored locally on the payload on hard drives secured inside steel pressure vessels for later recovery. Data from these sensors are combined in software to figure out where TIM is pointing, and to create a pointing solution on the sky. 

Complete list of published literature