SPIRE Astrometry Corrected Maps - Herschel
SPIRE Astrometry Corrected Maps
The Herschel absolute pointing error is of the order of 2 arcsec (68% c.i.), which is relatively good, compared with the 250 µm beam FWHM of 18 arcsec. Some observations, however, suffered more important pointing offsets.
In order to derive the absolute pointing of the SPIRE maps we use the WISE all sky catalogue (Wright et al 2010, Cutri et al. 2012) and stack on the WISE 22 µm sources. We cut a region of 11x11 pixels (66x66 arcsec) from the SPIRE 250 µm map, centred on each WISE 22 µm source and then co-add all the images (see Figure 1). We fit a 2D circular Gaussian to the stack signal, keeping the FWHM fixed. If the median stack signal is a good representation of the beam, then the offset of the centroid of the fitted 2-D Gaussian, with respect to the stack central pixel, is the astrometric shift we need to apply to the SPIRE map.
Maps at level-2 or level-2.5 were regenerated with the astrometry shift applied to the timelines of the affected observation. The stacked images for all SPIRE 250 µm level-2 maps are also provided for reference.
The products are available in the Herschel Legacy repository area.