The full wavelength range of Herschel is covered by six broadband (Δλ/λ=3) filters. In SPIRE, all three filters (250, 350 and 500 μm) are imaged simultaneously on three spiderweb bolometer arrays. PACS users are able to image with a "red" (130-210 μm) and a "blue" (either 60-85 or 85-130 μm) filter simultaneously on two bolometer arrays.
![[Note]](../../admonitions/note.gif) | Note |
|---|---|
| Colloquially, but inaccurately, the 130-210, 85-130 and 60-85 micron filters are often referred to as the PACS "red", "green" and "blue" bands. |
The main imaging capabilities are summarised in Table 3.1. As dust is a strong tracer of star formation, one of Herschel's greatest strengths is the possibility to study the history of star formation in the universe. By combining PACS and SPIRE data, users will be able to follow the dust emission signature of starbursts redshifted to increasing wavelengths in ever more distant galaxies; this makes Herschel an enormously powerful facility for studying the formation and evolution of galaxies.
Observations with Herschel will give a new insight into the process of star and planet formation. Herschel can study both the processes of star formation in molecular clouds and the debris disks that are the tracer of planetary system formation in young stars. To date, few debris disks are known and observations with Herschel, with its wide wavelength coverage, will allow many more to be detected and studied. Similarly, Herschel observations will be valuable in the study of the later phases of stellar evolution, particularly circumstellar shells, mass-loss in general and stellar winds.
Finally, Herschel is a powerful tool for studying the physics of the more distant and colder objects of the solar system: such as the atmospheres of the giant planets, their icy satellites, cometary nuclei and cometary atmospheres. Herschel observations are permitting the albedos and thus the surface conditions and diameters of these bodies to be measured with great precision.
Table 3.1. The main imaging capabilities of PACS and SPIRE. Please note that the wavelength range of detector sensitivity is approximate and the instrument sensitivities depend on the observing mode, so the values given are only orientative: please consult the relevant observing manual for more detailed values.
| PACS | SPIRE | |
| Wavelength range | 60-210 μm | 200-670 μm |
| Field of view | 1.75x3.5' | 4x8' |
| Pixel size | 3".2 (60-130 μm), 6".4 (130-210 μm) | 18".1 (250 μm), 24".9 (350 μm), 36".6 (500 μm) |
| Typical sensitivity (5σ/1hr, point source) | 5 mJy (70/110 μm bands), 10 mJy (160 μm band) | 6-8mJy (one ABBA cycle, point source) |
| Confusion limit (ideal case) | <0.1 mJy (70 μm), 0.1 mJy (100 μm), 0.7-0.9 mJy (160 μm) | 5.8mJy (250 μm), 6.3mJy (350 μm), 6.8mJy (500 μm) |
| Filters | 60-85 or 85-130 μm and 130-210 μm (simultaneous) | 250, 350 and 500 μm (simultaneous) |
Herschel offers a parallel mode for users who wish to carry out large-scale mapping programmes with a wide range of wavelength coverage.
Parallel mode allows observers to use both SPIRE and PACS simultaneously in a fast (60 arcsec/s) and slow-speed (20 arcsec/s)scanning mode to cover very large areas of sky quickly in all three SPIRE bands and in two of the three PACS bands, to a modest sensitivity. This mode is intended to make ambitious, multi-band, large area mapping programmes more efficient than carrying them out individually with each instrument in turn. In this mode SPIRE is the prime instrument and thus the driver in defining observations and PACS data should be treated more as a "bonus" to observers.
SPIRE and PACS point at different places on the sky separated by 21 arcminutes. This means that this mode is extremely inefficient at mapping small areas of sky. Although a minimum area of 30x30 arcminutes for a Parallel Mode map is permitted by HSpot, alternatives should certainly be considered for any area of sky smaller than one square degree and possibly even for larger areas than this.
While the SPIRE integration gets to a depth which is fairly close to the confusion limit, the depth of exposure is relatively less for PACS, thus Parallel Mode photometry should not be regarded as an adequate substitute for even moderately deep PACS scan maps.
Although very large areas of sky can be mapped quickly at the high scan speed, the speed of scan in sufficiently high that some telescope movement occurs before detector readout is complete, giving rise to a small degree of PSF smearing in the scan direction.