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 Band 4 sensitivity easily meets the 13 AB mag (5σ) requirement, and will return hundreds of thousands of high-quality ice absorption spectra.
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 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.