Instrument

Fig 1. SPHEREx Observatory Overview

The SPHEREx observatory is based on a wide-field aluminium telescope. The optics are passively cooled by radiating heat to space using a 3-stage V-groove system. A series of three nested conical photon shields (shown in the Fig. 1 cutaway) protect the cooler and optics from Sun and Earth radiation.

SPHEREx is based on a telescope with an effective 20 cm diameter and a 11°x3.5° field of view. The focal plane is split with a dichroic to three short-wavelength and three long-wavelength detector arrays. Two focal plane assemblies (FPAs) simultaneously image the sky through a dichroic beam splitter. This specialized optic behaves as a mirror to short wavelengths and a window to long. Each FPA contains three 2K x 2K detector arrays placed behind a set of linear variable filters, providing narrow-band response with a band center that varies along one axis of the array. SPHEREx obtains spectra through multiple exposures, placing a given source at multiple positions in the field of view, where it is measured at multiple wavelengths by repointing the spacecraft.

Fig 2. SPHEREx is based on a telescope with an effective 20 cm diameter and a 11°x3.5° field of view.

SPHEREx implements a simple and robust design that maximizes spectral throughput and efficiency. This is done in the absence of any moving parts, with the exception of a one-time ejection of an aperture cover.

Design features employed on SPHEREx have proven effective on previous missions, and include:

A 20 cm all-aluminum telescope with a wide 3.5° x 11° wide field of view doubly-imaged through a dichroic beam splitter.
Six 2K x 2K HgCdTe detector arrays. These H2RG arrays have been qualified for space observations by the James Webb Space Telescope, and are built on a long successful history of space instruments using arrays in smaller formats.
Six linear variable filters (LVFs) to produce spectra. The spectrum of each source is obtained by moving the telescope in the wavelength-varying direction of the LVF in discrete steps. This method was demonstrated by LEISA on New Horizons to obtain excellent spectral images of Jupiter and Pluto.

Table 1. SPHEREx Instrument Parameters (Updated September 2021)
Parameter	Value
Telescope Effective Aperture	20 cm
Pixel Size	6.2" x 6.2"
Field of View	2 x (3.5° x 11.3°); dichroic
Resolving Power and Wavelength Coverage	Band 1: λ= 0.75 - 1.09 µm; R=41 Band 2: λ= 1.10 - 1.62 µm; R=41 Band 3: λ= 1.63 - 2.41 µm; R=41 Band 4: λ= 2.42 - 3.82 µm; R=35 Band 5: λ= 3.83 - 4.41 µm; R=110 Band 6: λ= 4.42 - 5.00 µm; R=130
Arrays	3x Hawaii-2RG 2.5 µm 3x Hawaii-2RG 5.3 µm
Point Source Sensitivity	>19.4 AB mag (5σ) per frequency bin referenced at 2 µm (Pre-launch MEV).
Cooling	All-Passive
2.5 µm Array and Optics Temperature	<80K
5.3 µm Array Temperature	<55K