SPHEREx Cutaway

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.

SPHEREx Raytrace

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 viarable 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 λ= 0.75- 2.42 µm; R=41
λ= 2.42 - 3.82 µm; R=35
λ= 3.82 - 4.42 µm; R=110
λ= 4.42 - 5.00 µm; R=130
Arrays 3x Hawaii-2RG 2.5 µm
3x Hawaii-2RG 5.3 µm
Point Source Sensitivity >19.2 AB mag (5σ) per frequency bin referenced at 2 µm (MEV).
Cooling All-Passive
2.5 µm Array and Optics Temperature <80K
5.3 µm Array Temperature <55K