Herschel is a versatile observatory with a wide range of capabilities that cover point-source photometry, imaging, large area mapping and spectroscopy at both intermediate and high resolution. Despite the relatively small size of far-IR detectors compared to their visible and near-IR equivalents, it will be able to map large areas of sky efficiently to faint limits with diffraction-limited images - resolution 6 arcseconds - at 90 microns, far superior to the image quality of IRAS.
The Herschel Space Observatory will cover the wavelength range from 60 - 670 microns. This corresponds to black bodies with temperatures in the range from 5-50K approximately. Hence Herschel will be best suited to observing icy outer solar system objects and cool and cold dust in the universe, both in the rest frame and redshifted. The Herschel range is also the one at which cool and cold gases emit their strongest lines, meaning that Herschel will also be a superb laboratory for examining the chemistry of planetary atmospheres and of the interstellar medium.
The full wavelength range of Herschel will be covered by six broadband (Δλ/λ=3) filters. In SPIRE, all three filters (250, 360 and 520μm) will be imaged simultaneous on three spiderweb bolometer arrays. PACS users will be able to image with a "red" (170μm) and "blue" (either 60-85 or 85-130μm) filter simultaneously on two bolometer arrays. This makes Herschel a superb instrument for multicolour surveys. It is anticipated that both PACS and SPIRE will be able to image approximately half a square degree per day to the extragalactic confusion limit and, of course, much larger areas of the sky to a lesser sensitivity.
The main imaging capabilities are summarised in Table 5. As dust is a strong tracer of star formation, one of Herschel's greatest strengths will be the possibility of studying 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 will make Herschel an enormously powerful facility for studying the formation and evolution of galaxies.
Herschel will also be a powerful tool for studying the physics of the most distant and cold objects of the solar system: icy satellites; inactive cometary nuclei; and Kuiper Belt objects. Herschel observations will permit the albedos and thus the surface conditions and diameters of small outer solar system bodies to be measured with great precision.
Table 5. The main imaging capabilities of PACS and SPIRE.
| PACS | SPIRE | |
| Wavelength range | 60-210μm | 200-670μm |
| Field of view | 1.75x3.5' | 4x8' |
| Sensitivity (5σ/1hr, point source) | 3mJy | 3.5mJy |
| Confusion limit (approx) | 1mJy (60μm), 5mJy (210μm) | 10mJy (200μm), 20mJy (670μm) |
| Filters | 60-85 or 85-130μm and 130-210μm (simultaneous) | 250, 360 and 520μm (simultaneous) |
Herschel offers a parallel mode for users that wish to carry out large-scale mapping programmes with a wide range of wavelength coverage.
Parallel mode allows observers to use both PACS and SPIRE simultaneously in a fast scanning mode (30"/s) to cover very large areas of sky quickly in two PACS and all three SPIRE bands, to a modest sensitivity. This mode is intended to make ambitious large area mapping programmes more efficient than carrying them out individually with each instrument in turn.
PACS and SPIRE point at different places on the sky separated by 17 arcminutes. This means that this mode is extremely inefficient at mapping small areas of sky. Alternatives should certainly be considered for any area of sky smaller than one square degree and even for slightly larger areas than this. A second potential drawback is that parallel mode data may require a special calibration regime to be used both to obtain precise absolute spatial coordinates of detected sources and to obtain the highest quality of flux calibration.
Herschel offers two types of spectroscopic capability. PACS and SPIRE offer low to intermediate resolution spectroscopy covering the full Herschel wavelength range. HIFI offers high-resolution spectroscopy over the range from 240-600μm (500-1250GHz) using heterodyne techniques. Users will thus be able to select a wide range of resolutions from Δλ/λ=100 to Δλ/λ=1 000 000 according to the brightness of their source and the science that is required. The main spectroscopic capabilities are summarised in Table 6.
In its highest resolution mode Herschel will be a chemical laboratory of unprecedented capabilities, offering a velocity resolution as high as 0.3km/s. The wavelength range covered by Herschel has many thousands of lines of water, atomic transitions and organic molecules. This will allow Herschel to study the chemistry of the interstellar medium, tracing water and organic molecules in molecular clouds. Herschel will also be able to study the chemistry of solar system bodies such as atmosphere of Mars and the comas of comets.
All three instruments have a mapping capability in spectroscopic mode, although HIFI's is somewhat limited, although by no means negated, by the fact that its detector has only a single pixel. The instruments can scan the detectors across the sky, accumulating spectroscopic data along the length of the scan, or as a raster map. This allows a spectroscopic survey to be made either of a region that has been mapped in imaging mode, such as a cluster of galaxies, or across a known extended source such as a molecular cloud.