Spectral response of SPHEREx

TL;DR

This work tackles the challenge of delivering precise, per-pixel spectral calibration for SPHEREx across $0.75$–$5.0~\mu$m to enable accurate redshift and feature measurements in the all-sky survey. It presents a two-phase ground calibration using end-to-end optical configurations, monochromatic illumination, and a Lucy–Richardson deconvolution to recover intrinsic spectral responses, producing per-pixel $\lambda_c$ and $R$ maps and fiducial bandpass functions. Key results include quantified out-of-band leakage, comprehensive per-pixel spectral parameters for over twenty million active pixels, and in-flight verification with helium airglow and the Cat’s Eye Nebula, supporting robust data analysis in the SPHEREx pipeline. The calibration products, released via IPAC, will enable reproducible science and provide a foundation for in-flight refinements as the mission accumulates on-sky data.

Abstract

The Spectro Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx) is conducting the first all-sky near infrared spectral survey spanning 0.75 to 5.0um with resolving power R~35 to 130. Linear variable filters mounted in front of six H2RG detectors produce a position dependent spectral response across the focal plane. This paper presents the ground-based spectral calibration of SPHEREx, including the cryogenic apparatus, optical configuration, measurement strategy, analysis pipeline, and resulting calibration products. Monochromatic wavelength scans are used to derive the spectral response function, band center, and resolving power for every pixel. Band centers are measured to better than 1nm for Bands 1 through 4 (0.75 to 3.82um) and better than 10nm for Bands 5 and 6 (3.82 to 5.0um). Out-of-band leakage is negligible for detectors above 1.64um and is present at the percent level below this wavelength. The resolving power is measured to within 5% and agrees with design expectations to within 10%. An on-sky spectrum of the Cat's Eye Nebula (NGC 6543) constructed from repeated observations provides in-flight verification and shows agreement between ground calibrated response and astrophysical emission features. Calibration products, including per-pixel band center and resolving power maps, are released through IPAC to support community use of SPHEREx data. The absolute spectral calibration will continue to improve through in-flight measurements, with further reductions in uncertainty expected for the longest-wavelength bands.

Figures (26)

• Figure 1: MWIR focal plane assembly containing three H2RG detectors, each covered by a linear variable filter Korngut2024. The apparent color gradient arises from interference effects at the LVF surface. The effective wavelength increases monotonically along the vertical direction in the image. The SPHEREx instrument includes two such focal plane assemblies, one for the SWIR channel and one for the MWIR channel.
• Figure 2: Spectral channel layout for each detector. Channels are defined geometrically rather than by direct wavelength measurements to simplify survey planning. The LVF produces a monotonic wavelength gradient along the vertical axis. Curvature near the band edges causes a subset of pixels to fall outside the nominal seventeen-channel layout, and these pixels provide cross-band calibration information.
• Figure 3: Spectral calibration optical configuration. A broadband, tungsten halogen lamp feeds a monochromator on the warm optical bench to generate monochromatic light, which is collimated and directed into the cryogenic chamber. Inside the chamber, two integrating spheres and a cryogenic Winston cone produce a uniform illumination pattern over the field of view of the focal plane or telescope. This configuration is used for both focal-plane and system-level calibration.
• Figure 4: Warm optical bench setup. Left: optical bench used during focal plane assembly testing, consisting of a broadband source, monochromator, order sorting filters, and relay optics. Right: the same bench in operation during full telescope calibration in the KASI chamber, shown with the nitrogen purge system that suppresses atmospheric absorption.
• Figure 5: Cryostat used for component-level testing of the focal plane assemblies. The FPA, calibration optics, and optically blackened enclosure are mounted on 20 K cold plate, which is actively temperature-controlled to achieve $< 1$ mK stability.