Scientific/Technical Abstract:

HAWC (High-resolution Airborne Wideband Camera) is a far-infrared camera designed to cover the 40-300 µm spectral range at the highest possible angular resolution. Its purpose is to provide a sensitive, versatile, and reliable facility imaging capability for SOFIA's user community during its first operational years.

HAWC will utilize a 12x32 pixel array of bolometer detectors constructed using the silicon pop-up detector (SPUD) technology developed at Goddard Space Flight Center. The array will be cooled by an adiabatic demagnetization refrigerator and operated at a temperature of 0.2 K.

Many infrared sources are dusty. Absorption of starlight typically heats the dust grains to temperatures of tens or hundreds of degrees Kelvin where they radiate most of their energy in the far infrared, at wavelengths of 40-300 µm that are inaccessible from the ground. Imagery in this spectral range with the highest possible spatial resolution is the natural starting point from which to develop an understanding of source energetics and morphology. It is also a key to understanding the physics and chemistry of the interstellar medium.

HAWC Performance Summary:

The instrument sensitivity and resolution summaries are provided to permit estimating feasibility of scientific investigations. The HAWC performance summaries show the expected system performance for Full Operational Capability, which may differ from that during commissioning.

^a FWHM of a Gaussian approximating the Airy function convolved with the appropriate spectral filter and a square pixel
^b Assuming a Gaussian beam with FWHM given above
^c Minimum Detectable Continuum Flux to achieve S/N = 4 in a 900 second integration assuming 7.3 μm water vapor.
^d Assuming chopped observations
^e Noise Equivalent Flux Density; background limited

HAWC Angular Resolution

HAWC uses a 12 x 32 pixel bolometer array. The plate scale varies for each bandpass as given by the "pixel size" in the table above. The four bandpasses are centered at 53 µm, 89 µm, 155 µm, and ~216 µm. Reimaging optics provide a match to the diffraction limit in each passband. Each bandpass is observed separately. The pixel sizes and fields of view (FOV) for the four passbands are shown below. The beam sizes shown represent the FWHM for nominal operating conditions. Note that there are 2.7 pixels per Airy FWHM in each of the four passbands.

Shown below is a plot of the HAWC angular resolution (FWHM, arcsec) as a function of wavelength. The instrument resolution approaches the diffraction limit of the telescope. Final images from HAWC with angular resolution equal to the SOFIA telescope image size (diffraction limited for bands 2-4) should be possible with use of appropriate observing techniques and post-flight analysis.

Back to the Top »

HAWC Spectral Passbands and Filter Characteristics

Wavelength range: 50 - 240 µm. The HAWC filter bandpasses are given in the table ABOVE. The bandpasses are plotted in the figure below along with a representative ATRAN atmospheric transmission model. The ATRAN model is for an altitude of 41,000 ft, at a zenith angle of 50^o, and assuming an H₂O burden of 7.3 microns.

Back to the Top »

HAWC Sensitivities

Below is a plot of the continuum sensitivity per beam (point source) and per pixel (extended source), for each of the four bandpasses. The Minimum Detectable Continuum Flux (MDCF; mJy) necessary for a detection at S/N = 4 per beam in 900 seconds is plotted versus wavelength. Horizontal error bars indicate the filters' passband FWHM in λ.

The MDCF can be calculated for a desired signal to noise (SN) from the following equation: MDCF = (NEFD x SN) / sqrt(t), where NEFD is the Noise Equivalent Flux Density as given in the table ABOVE, and t is the integration time in seconds.

Atmospheric transmission will affect sensitivity, depending on water vapor overburden.

Back to the Top »

HAWC Observation Preparation and Data Handling

Once the observatory has been fully commissioned, additional information will be provided, including a full accounting of overheads associated with particular instrument set-ups and observing strategies; information on preparing observations using the SPT; and details regarding data formatting,

Back to the Top »

Disclaimer

All sensitivity and resolution data are preliminary, and based on anticipated performance of the observatory and the instrument.  Actual performance of the SOFIA telescope and instrument combination will be established after flight operations begin.  Telescope performance is expected to be upgraded during the first two years, and instrument performance may be upgraded, or additional modes or capabilities may be added.

Back to the Top »

Additional References:

Vaillancourt et al., "Far-infrared polarimetry from the Stratospheric Observatory for Infrared Astronomy," Infrared Spaceborne Remote Sensing and Instrumentation XV, Marija Strojnik-Scholl, Editor, Proc. SPIE 6678, 66780D (2007), DOI: 10.1117/12.730922 [pdf]

Voellmer et al., "A two-dimensional semiconducting bolometer array for HAWC," Millimeter and Submillimeter Detectors for Astronomy II, Jonas Zumuidzinas, Wayne S. Holland, & Stafford Withington, Editors, Proc. SPIE 5498, 428 (2004), DOI: 10.1117/12.552016 [pdf]

Harper et al., "HAWC: a far-infrared camera for SOFIA," Airborne Telescope Systems, Ramsey K. Melugin & Hans-Peter Roeser, Editors, Proc. SPIE 4014, 43 (2000), DOI: 10.1117/12.389132 [pdf]