The Cosmic Infrared Background Experiment-2: An Intensity Mapping Optimized Sounding-rocket Payload to Understand the Near-IR Extragalactic Background Light

Abstract

The background light produced by emission from all sources over cosmic history is a powerful diagnostic of structure formation and evolution. At near-infrared wavelengths, this extragalactic background light (EBL) is comprised of emission from galaxies stretching all the way back to the first-light objects present during the Epoch of Reionization. The Cosmic Infrared Background Experiment 2 (CIBER-2) is a sounding-rocket experiment designed to measure both the absolute photometric brightness of the EBL over 0.5 - 2.0 microns and perform an intensity mapping measurement of EBL spatial fluctuations in six broad bands over the same wavelength range. CIBER-2 comprises a 28.5 cm, 80K telescope that images several square degrees to three separate cameras. Each camera is equipped with an HAWAII-2RG detector covered by an assembly that combines two broadband filters and a linear-variable filter, which perform the intensity mapping and absolute photometric measurements, respectively. CIBER-2 has flown three times: an engineering flight in 2021; a terminated launch in 2023; and a successful science flight in 2024. In this paper, we review the science case for the experiment; describe the factors motivating the instrument design; review the optical, mechanical, and electronic implementation of the instrument; present preflight laboratory characterization measurements; and finally assess the instrument's performance in flight.

Figures (28)

• Figure 1: (Left) Positions of science fields projected on the ecliptic sphere. Target fields optimized for assessing foregrounds (blue points) and those optimized for intensity mapping (orange points) are visible from WSMR between Nov. and Jul. of each year (white area). The extragalactic fields we consider (ELAIS-N1; north ecliptic pole, NEP; Bootes; Lockman; and COSMOS) have been surveyed extensively in the near-IR and have ample ancillary data for masking and cross correlation. (Right) A typical flight plan for CIBER-2 modeled on flight 36.281. The dashed green curve shows the payload's altitude versus time. Calibration lamp data is taken during ascent (orange zone), and optically dark data is taken during descent (gray zone).
• Figure 2: The overall structure of the CIBER-2 payload. The upper panel shows a partial section of the instrument section in observing configuration with the door open and pop-up baffle deployed. The rocket skin sections forward of the experiment have been suppressed. The lower panel shows the cryogenic insert in partial section, highlighting the telescope and imaging optics described in Section \ref{['sS:optics']}. The optics couple to three focal plane assemblies (Section \ref{['sS:FPAs']}), one of which is shown separated from the optics for clarity.
• Figure 3: Design of the end-to-end optical chain consisting of the telescope and the relay lens optics with the light beams (blue, green, and red lines). Each lens optics for the three photometric bands, arm S, arm M and arm L, is shown separately. The G1 lens is common to all photometric bands. The light from G1 is split into two beams for arm S/arm M and arm L by a dichroic beam splitter BS1. The shorter wavelength light is collimated by the lens unit consisting of G2, G3, and G4 and split into two beams for the arm S and arm M focusing by another dichroic beam splitter BS2. The final lens system for each wavelength band focuses the beam onto the detector. The order-sorting filter to block out-of-band light is placed in front of the focusing optics for each band. The window pane filter and the linear-variable filter are placed close to the detector surface.
• Figure 4: Transmittance of the CIBER-2 filters. The solid lines represent the transmittance of the window pane filters, while the dashed lines show the transmittance of the LVFs, multiplied by the transmittance of the beam splitters and order sort filters, which determine the bandpass wavelengths. The LVF transmittance shown corresponds to the wavelength of peak transmittance of each detector column. The transmittance values presented are at room temperature.
• Figure 5: Typical CIBER-2 thermal cycle showing four of two dozen thermometers that are monitored by ground equipment during laboratory testing. The cooling cycle is computer controlled so that $dT / dt < 1 \,$K minute$^{-1}$ to account for the differential thermal contraction of the metal and glass optical assembly. After 24 hrs the nitrogen tank can be filled, and it must be refilled every 6 hr as the optical assembly cools. The optics reach a steady-state temperature, and the instrument can be tested 54 hr after the first fill. Finally, the warming cycle takes about 48 hr from fully cold to ambient temperature. In this thermal cycle, a small amount of nitrogen gas was injected into the vacuum section to improve conduction to room temperature at 120 hr.