New SPIRE empirical beams

     The flux calibration of SPIRE is strictly valid only for point sources, and the brightness unit of the
     maps (Jy/beam) is appropriate only for point sources. Since an accurate flux calibration for
     extended sources depends on the knowledge of the beam areas, it is crucial to derive these
     with care.

     The official empirical beams were built from an observation of Neptune in bright-source mode.
     They are therefore not as deep as if they had been derived from a nominal observation.
     In particular, the secondary lobes are very noisy and the diffraction spikes are below the noise level
     in these images.

     Consequently, I have derived a new set of empirical beams from a nominal observation of Uranus,
     with normal gain settings and also twice as many scans as the observation of Neptune (8 instead of 4).
     At the time of the observation, Uranus had an angular diameter of 3.53", slightly larger than that
     of Neptune (2.26").

     Images of Uranus have been made with Scanamorphos, which uses a different projection scheme
     than Hipe. This means that they should be used in combination with Scanamorphos maps.
     Their orientation, along the direction of the first scan, is designed to match the point response
     functions (PRFs) of maps whose orientation is also constrained along the direction of the first scan.
     Faint background sources have been masked or subtracted. The result is shown below.

     Here are the beam areas computed from these images, corrected for the finite size of Uranus.
     By default, beam widths are slightly larger in maps made with Scanamorphos, by about 1.5%.
     A small correction has to be applied for beam areas in Hipe maps.
        PSW (250 microns):      468 arcsec2
        PMW (350 microns):     850 arcsec2
        PLW (500 microns):      1763 arcsec2

     The beams are available in IDL save format:          prf_psw.xdr          prf_pmw.xdr          prf_plw.xdr

     They have pixels sizes of 4.5", 6.25", and 9", respectively (approximately one fourth of the FWHM).

posted March 15, 2010