10.1.2 Performance

The absorbing coatings on the HAWC detector arrays were optimized to produce about 50% efficiency across the wide (40 - 300 μm) range of bandpasses. The TESs were designed to optimize the sensor time constants and background power at which they saturate, with the goal being operation at both laboratory and stratospheric background levels. The final design included a superconducting transition temperature of ~0.3 K and a detector yield of > 50%. Measurements of detector noise show that their contribution to total measurement uncertainties is negligible such that noise levels are dominated by background photons from the atmosphere.

Measurements of the HAWC optical system in the laboratory are consistent with optical models, confirming that the system should be diffraction limited on SOFIA. Table 10-1 lists the FWHM of Gaussian beams approximating the SOFIA/HAWC+ beams, taking into account the monochromatic diffraction limit, the finite size of the HAWC detectors, and a convolution across the measured filter bandpasses. The filter transmission curves (text tables) are available as a zip file or individually from Table 10-1.

Table 10-1: HAWC+ Instrument Characteristics

HAWC+ Instrument Characteristics
Parameter Units Band A Band B Band C Band D Band E
Mean Wavelenth (λ0) μm 53 63 89 154 214
Filter Width (Δλ/λ0) - 0.17 0.15 0.19 0.22 0.20
Resolution (FWHM) arcsec 4.7 5.8 7.8 14 19
Pixel Pitchf arcsec 2.6 4.0 4.0 6.8 9.1
Field of View arcmin 2.7x1.7 4.2x2.6 4.2x2.6 7.3x4.5 8.0x6.1
NESBa (photo) MJy sr-1 h1/2 15 12 4.7 1.2 0.57
MDCFb mJy 69 85 60 47 43
Mapping Speedc See footnote c 0.016 0.059 0.39 18 110
MDCPFd % Jy 11 13 9.5 7.3 6.7
MIfPe MJy sr-1 h1/2 24,000 20,000 7700 1900 940
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Note: All photometric sensitivity estimates assume 100% observing efficiency without chopping and nodding. These values are pre-flight estimates and subject to change after the HAWC+ instrument has been commissioned. The SPT time calculator will estimate the correct overhead values for C2N.

a Noise Equivalent Surface Brightness for S/N = 1 into a single HAWC+ beam (FWHM given here).

b Minimum Detectable Continuum Flux for a point source with S/N = 4 in a 900 second integration.

c Real scan rate required to achieve a given an NESB. Units: arcmin2 h-1 (MJy sr-1)-2

d Minimum Detectable Continuum Polarized Flux for a point source with a S/N = 4 in a 900 second integration. eMinimum total Intensity required to measure Polarization (MIfP) to an uncertainty level σp ≤ 0:3%. All chop/nod and polarization overhead values have been applied to this value.

f The center-to-center spacing of the pixels; pixel sizes (the space taken up by the photon sensitive area) are smaller by 0.21 arcsec at 53 μm and 0.75 arcsec at 215 μm.

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