SPIRE Photometer Beam Profiles

Final beam profile derivation including shadow maps ( Bernhard Schulz, October 2014)

SPIRE Photometer Solid Angles

Solid angles in [arcsec^2] PSW PMW PLW
SPIRE photometer reference spectrum (nu*F_nu = const.) 470+/-1 832+/-4 1793+/-12
Measured with Neptune spectrum 454+/-1 803+/-4 1687+/-12

The values in the first line were measured from a very finely sampled observation of Neptune as outlined below. Note that these are only accurate for a source of the same spectral shape. Since SPIRE flux densities for point sources are conventionally quoted for the reference wavelengths 250, 350, and 500 microns, assuming a source spectrum rising proportionally to wavelength (nu*F_nu=const.), we can adopt the same convention for (infinitely) extended sources. In this case the values in the second line apply. For sources of other colors a correction will have to be applied. The radial model beam profile is described in Griffin et al. 2013 and the data files given below can be used to calculate such beam profiles. This analysis showed that the static part of the model came from noise due to residual background galaxies and is not needed anymore. Rather the data show a well described fall-off with radius that is well described by a power-law. This fit is used to extrapolate beyond the measured radius.

Normalized Beam Profile Products to Download

The beam profile maps are available with all backgrounds subtracted and in a normalized form to the peak.

Detector Array PSW PMW PLW
PSF as FITS file FITS file FITS file FITS file
PSF limited to 700" radius (click on the image to enlarge)

Note that the position angle keyword posAngle is not set in the FITS headers. This is the position angle of the Z-axis of the spacecraft coordinate system projected onto the sky at the time of the observation. The Z-axis runs parallel to the short side of the SPIRE photometer FOV. For these observations the position angle is 250.6 degrees.

Radial Beam Profile Model

Datasets to construct the radial beam profile model are available below. These allow to take into account the color dependent FWHM of the true beam profiles. An implementation of the beam profile model in IDL is available for download.

The radial beam profile model for SPIRE was developed and described by Griffin et al. 2013. The peak normalized monochromatic flux, depending on the distance from the peak (Theta) can be described as:

A value of -0.85 should be adopted for gamma, which controls the dependence of the FWHM of the variable part of the beam on wavelength. The reference frequency nu_eff is the isophotal frequency for the product of the spectrum of Neptune and the respective SPIRE spectral filter profile. The frequencies 1217.27, 867.75, 610.87 GHz for PSW, PMW, PLW respectively make the model beam profile consistent with the measured Neptune beam using the spectral indices 1.29, 1.42, 1.47 respectively to describe the Neptune SED in the three filter bands.

PLW PLW_Core.csv PLW_Constant.csv
Radial beam profile model Values are given for radii in 1 arcsec steps starting at 0
Detector Core (variable) part Constant part
PSW PSW_Core.csv PSW_Constant.csv
PMW PMW_Core.csv PMW_Constant.csv

Solid angles derived from integrating this radial beam profile model over area and frequency were consistent with the numbers above within 1.5%.

Analysis Details

The interested reader can find a detailed description of the data analysis that led to the data products on this page.

-- BernhardSchulz - 19 Mar 2015

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