Fig 1. Thermal and Optical Subsystems. The SPHEREx spacecraft (photon shields cut away) showing the bus with the payload optical and thermal subsystems. The optics and detector cool passively by radiating heat to space with a 3-stage V-groove passive cooler. Lightweight deployed photon shields protect the cooler and optics from radiation from the Sun and Earth.

Fig 2. SPHEREx is based on a telescope with an effective 20 cm diameter and a 7°x3.5° field of view. The focal plane is split with a dichroic to two short-wavelength and two long-wavelength detector arrays. Each 2k x 2k focal plane array is covered with a linear variable filter, 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 implements a simple and robust design that maximizes spectral throughput and efficiency. SPHEREx achieves this with no moving parts except for one-time deployments of a sunshield and aperture cover. Design features have been proven effective on previous missions, and include:

  • 20 cm all-aluminum telescope with a wide 3.5° x 7° field of view, imaged onto four 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.
  • Four space-demonstrated linear varable filters (LVFs) to produce spectra. The spectrum of each source is obtained by moving the telescope in the dispersion 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.
  • A telescope that is radiatively cooled to ≤ 80K and two detectors that are cooled to ≤ 55K with 320% total margin on the net heat load. The thermal methods employed on SPHEREx have been previously demonstrated by Planck, Spitzer, and WISE.

Table 1.-- SPHEREx Instrument Parameters
Parameter Value
Telescope Effective Aperture 20 cm
Pixel Size 6.2" x 6.2"
Field of View 2 x (3.5° x 7.0°); dichroic
Resolving Power and Wavelength Coverage λ= 0.75- 4.18 µm; R=41.4
λ= 4.18 - 5.00 µm; R=135
Arrays 2x Hawaii-2RG 2.5 µm
2x Hawaii-2RG 5.3 µm
Point Source Sensitivity 18.4 AB mag (5σ) per frequency bin referenced at 2 µm. Sensitivity has 380% total margin to science requirement
Cooling All-Passive
2.5 µm Array and Optics Temperature 80K
5.3 µm Array Temperature 55K
Payload Mass 68.7 kg