SPIDER (astronomy)

SPIDER (astronomy)
Name	SPIDER
Operator	Caltech, Princeton University, Cardiff University
Country	United States
Launched	2014 (first flight)
Mission type	Balloon-borne submillimeter polarimeter
Wavelength	90 GHz, 150 GHz, 280 GHz
Aperture	25 cm (each of six telescopes)
Status	Completed multiple Long Duration Balloon flights

SPIDER (astronomy) is a balloon-borne cosmic microwave background polarimeter developed to measure degree-scale polarization in the microwave sky. The instrument targets polarized signatures produced by primordial inflation-era gravitational waves and foregrounds associated with Galactic interstellar medium structure, using cryogenic receivers and a long-duration Antarctic balloon platform. SPIDER complements satellite missions such as Planck and ground-based experiments like BICEP2, Keck Array, and South Pole Telescope by providing high-fidelity measurements across multiple frequencies and angular scales.

Overview

SPIDER comprises an array of six small refracting telescopes flown on a Long Duration Balloon from Antarctica to map polarized microwave emission. Designed by teams at Caltech, Princeton University, and Cardiff University, SPIDER targets the Stokes Q and U parameters over large sky fractions to constrain tensor-to-scalar ratio r and characterize Galactic synchrotron and thermal dust polarization. The project builds on heritage from experiments including BOOMERanG, EBEX, ACT, Polarbear, and WMAP, integrating technologies developed for Planck HFI and ground-based B-mode searches. Flights have been coordinated with Antarctic logistics provided by NASA and Columbia Scientific Balloon Facility supports.

Instrumentation and Design

The payload carries six monochromatic refracting telescopes using lenses, anti-reflection coatings, and cold stop optics modeled on BICEP designs. Each telescope couples to transition-edge sensor (TES) bolometer arrays read out with time-domain multiplexing developed at NIST and JPL. Frequency bands of 90 GHz, 150 GHz, and 280 GHz are selected to separate cosmic signal from foregrounds associated with Orion Molecular Cloud, Perseus Arm, and other Galactic regions. Polarization modulation is achieved via stepped half-wave plates and a sky-rotating gondola that includes star cameras and a suite of attitude sensors derived from Hubble Space Telescope and Spitzer Space Telescope pointing systems. Cryogenic systems use liquid helium and closed-cycle refrigerators influenced by designs from Herschel Space Observatory and SOFIA instrumentation.

Observations and Survey Strategy

SPIDER uses long-duration Antarctic flights to exploit stable, dry atmosphere and continuous sun-synchronous scans near the South Celestial Pole. Scan strategies combine azimuthal scans with elevation nods to control systematics, similar to tactics used by WMAP and Planck for full-sky coverage. Target fields are chosen to overlap with deep ground-based surveys from South Pole Telescope, Atacama Cosmology Telescope, BICEP/Keck, and optical/infrared maps from Sloan Digital Sky Survey and WISE to aid cross-correlation. Multi-frequency observations enable component separation leveraging methods employed by Commander and SMICA pipelines pioneered in the Planck analysis.

Key Scientific Results

SPIDER has produced maps of polarized microwave emission that place upper limits on the tensor-to-scalar ratio r at degree angular scales, complementing constraints from Planck Collaboration and BICEP2/Keck Array joint analyses. The instrument has characterized Galactic polarized dust emission across the southern sky, informing models used by Planck and WMAP teams and improving foreground priors for inflationary searches. SPIDER results have shed light on magnetic-field structure in nearby star-forming complexes such as Taurus Molecular Cloud and the Chamaeleon Complex by mapping polarized thermal dust at 280 GHz. Systematics studies from SPIDER have influenced design choices for successor CMB polarization experiments like LiteBIRD and proposed instruments within the CMB-S4 collaboration.

Data Processing and Calibration

Data processing follows pipelines developed in collaboration with software groups at Caltech, Princeton University, and Cardiff University, adapting timestream cleaning, deconvolution, and mapmaking approaches from Planck and Polarbear. Calibration uses observations of planets (e.g., Jupiter, Mars), the Crab Nebula (Tau A), and Galactic compact sources cataloged by ATLASGAL and Herschel to set absolute gain and polarized angle. Beam characterization employs near-field and far-field measurements informed by methods used by SPT and ACT teams, with end-to-end simulations based on frameworks developed for BICEP/Keck cross-checks. Component separation utilizes maximum-likelihood and Bayesian tools used by Commander and joint-analysis pipelines with Planck Collaboration.

Collaborations and Mission Timeline

SPIDER is a collaborative effort led by principal investigators at Caltech, Princeton University, and Cardiff University, with contributions from NASA, NSF, NIST, and international university partners. The first Antarctic flight occurred in 2014, with subsequent flights and upgrades following lessons from payload recovery, balloon operations coordinated with United States Antarctic Program, and science analysis phases aligned with public data releases modeled after Planck and WMAP schedules. The SPIDER legacy informs ongoing collaborations within the CMB-S4 planning and future satellite proposals such as LiteBIRD and continues to provide value via archived maps used by the astronomy community.

Category:Cosmic microwave background experiments