SPHEREx will observe the entire sky multiple times during its planned two-year mission. Its overlapping scan strategy will obtain a minimum of four 0.75--5.0 micron spectra at every point along the ecliptic, with much greater redundancy in the deep survey fields at the North and South ecliptic poles. SPHEREx will achieve point source sensitivities magnitudes deeper than 2MASS in every SPHEREx spectral element (Figure 1):

Fig 1. Point Source Sensitivity

Sensitivity of SPHEREx and current surveys (all at 5σ). The SPHEREx sensitivity is quoted in each spectral channel at current best estimate (CBE, top) and maximum expected value (MEV, bottom) performance. The sensitivity does not include the effects of astrophysical source confusion, which is significant at the deep survey depths.

Fig 2. Instrument Performance

The scientifically required point source sensitivity has large margin over the estimated instrument performance. We show above the science requirement (black dashed line) and the MEV (solid colored curve) and CBE (dashed colored curve) performance. Note the Bands 5 and 6 sensitivities is shown rebinned to R=40 spectral resolving power. Bands 5 and 5 easily meet the 9 AB mag (100σ) required for the Ices Investigation.

The innovative SPHEREx field-of-view is comprised of rectangular linear variable filters (LVFs), so sky surveys will be constructed by pointing the LVF so as to tile the sky over successive orbits (Figure 3). This will result in:

  • One all-sky survey every six months, or four all-sky surveys after the full 25-month mission
  • Two deep surveys accumulated at the North and South ecliptic poles. Since the SPHEREx orbit precesses by ~1° in RA per day while maintaining the full range of declination on each orbit, the deep surveys are naturally built up over the mission lifetime
  • A redundancy which allows the SPHEREx surveys to satisfy solar and terrestrial avoidance angles.

Fig 3. Illustration of LVF spectroscopy for Band-1 near the Andromeda galaxy

Every exposure images the sky with a central wavelength that varies over the 3.5°x3.5° Band 1 FoV. For a given object, each exposure therefore provides photometry at one wavelength. The spacecraft executes a series of maneuvers to build multiple images, each shifted by 12.3 arcmin. When complete, the ensemble of images gives a full spectrum with at least 24 samples in Band 1.

Fig 4. SPHEREx Images the sky through LVF filters

  • In one exposure, each object is measured at a different wavelength
  • On a given object, each new exposure adds a new wavelength

Fig 5. SPHEREx produces spectra from multiple pointed exposures

  • SPHEREx takes exposures separated by small and large slews
  • Succesive exposures approximately follow a great circle 90° from the Sun. The great circle rotates 360° over a year.

Fig 6. SPHEREx obtains complete spectra in Every survey

  • A given region is typically completed in a few days
  • The entire sky is completely sampled in six months

Fig 7. Illustration of Survey Strategy

SPHEREx achieves all-sky imaging and spectroscopy by observing along a region near a great circle defined by sun avoidance constraints. It slews several times per orbit to satisfy Earth avoidance constraints. The scheduling algorithm alternates between accumulating exposures on the all-sky survey and the deep fields to achieve full spatial and spectral coverage as the sun-synchronous orbit precesses throughout the year. This simulation shows the scheduling algorithm running with a realistic sun-synchronous orbit including variations in the orbial plane angle. After 190 days, the first survey completes, and it then repeats every six months.

The scan strategy used to develop the SPHEREx survey plan is described in detail here.