Survey
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 µm spectra at every point along the ecliptic, with much greater redundancy in the deep survey fields near the North and South ecliptic poles. SPHEREx will achieve point source sensitivities magnitudes deeper than 2MASS in every SPHEREx spectral element. This is illustrated in Figure 1 below. The SPHEREx team is also making these files defining the performance specifications publicly available to the community. These forecasts will be updated as the instrument is assembled and tested.
Fig 1: SPHEREx all-sky and deep fields point source sensitivity in magnitude. Sensitivity of SPHEREx and current surveys (all at 5σ). The SPHEREx sensitivity is quoted in each spectral channel as an optimistic (current best estimate (CBE), top) and a pessimistic (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: SPHEREx all-sky and deep fields point source sensitivity in Jy. 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 pessimistic (MEV, solid colored curve) and optimistic (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). The mission surveys the sky by pointing the LVF to tile the sky over successive orbits (Figure 3). This yields:
- Two deep surveys near the North and South ecliptic poles.
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Four independent all-sky during the nominal 25-month mission
Every exposure images the sky with a central wavelength that varies across the field of view. For a given object, each exposure therefore provides photometry at one wavelength. The spacecraft executes a series of maneuvers to take multiple images. When complete, this ensemble of images gives a full spectrum of the object of interest.
Fig 3. Illustration of LVF spectroscopy toward the Andromeda Galaxy.
Every exposure images the sky with a central wavelength that varies across the 3.5°x3.5° field of view. For a given object, each exposure therefore provides photometry at one wavelength. The spacecraft executes a series of maneuvers to take multiple images. When complete, this ensemble of images gives a full spectrum of the object of interest.
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 exposure separated by small and large slews. Successive 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 twice every year.
SPHEREx images the entire sky in over 100 bands across a key part of the infrared spectrum. Linear-Variable filter technology lets the telescope efficiently collect large quantities of both spectral and imaging data simultaneously. Since each pixel in the SPHEREx detector arrays measures a different infrared color, combing data taken across several exposures yields full imaging and spectral data for any galaxy or other target of interest on the sky.
Linear-Variable Filter technology enables SPHEREx to collect images of the entire sky in 102 colors.
This movie uses a very simple model of our instrument to illustrate how we take small steps in our pointing to move a source along our spectral filters and thus collect different wavelengths of data. The rainbow stripes represent the spectral elements into which we divide each linear variable filter. The small steps move a source onto different stripes of the rainbow. By moving a source onto each stripe, we collect a full set of spectral data for the object you see. In reality, our instrument is more complex than this, with multiple linear variable filters and detectors.
Animation illustrating the principle of LVF spectroscopy. It shows how the spectral filter moves across the sky in small steps, capturing each celestial object.
SPHEREx takes spectral imaging data across the entire sky while operating from low-earty orbit. The data gradually accumulates over time to cover the entire sky in 102 near-infrared colors. After 25 months of science operations, SPHEREx will deliver all-sky coverage in four independent surveys.
After 25 months of science operations, SPHEREx will image the entire sky four times in over 100 near-infrared colors.
