One of the first light images that didn’t get posted on Friday is attached below. It’s from PACS and shows a rather different imaging mode that PACS is capable of: spectral imaging.
This is an image of the Catseye Nebula in the far-infrared emission line NIII – or, for those who aren’t astronomers, Nitrogen 2+ – which lies at a wavelength of 57 microns, and is thus completely inaccessible from the ground.
This image is taken with PACS’s line imaging mode. This uses a different part of the instrument which is set up to be able to take spectra in a 5×5 grid. The result is not a 2D image but a 3D data cube where there is a spectrum for each of the 25 pixels – you can see these plotted out in the image. This approach is very powerful for looking at the spatial distribution of emission lines. The Catseye Nebula is a so-called planetary nebula. This class of objects represent a late stage in stellar evolution, where a star expels the outer layers of its atmosphere and have nothing to do with planets. The PACS image clearly shows that the NIII line emission has a hole in the middle, and is thus coming form the shell of material ejected by the star. This is just what is expected, so there’s no new science here, but it’s an ideal test of the capabilities of this aspect of PACS that has not been demonstrated before.