4 Comparison of the ALMA and Herschel - ESO

Figure 3: The Hubble Deep Field (angular resolution 0.05”) is shown on the right. On the left are
the Full Width to Half Power beamsizes of the PACS (three highest) and SPIRE instruments (three
lowest).
Table 5: Some Characteristics of the PACS Spectrometer
Central
Wavelength (μm)
Angular
Resolution (”)
Number of pixels Field of View (”) Spectral
Resolution
(approximate)
72 9.4 16 x 25 47 by 47 1800
105 9.4 16 x 25 47 by 47 1000
210 9.4 16 x 25 47 by 47 2000
Since Herschel is above the atmosphere, signals are not blocked by terrestrial spectral
features. This location is ideal for the measurement of water vapour lines that are usually
absorbed by the atmosphere even at the ALMA site. Since the Herschel HIFI receivers
are double sideband mixers, the receiver noise for spectral line measurements is twice the
double sideband (DSB) system noise. To relate the measured temperatures to flux densities,
there is a correction for antenna efficiency but no correction for the Earth’s atmosphere (see
caption of Fig. 4). Thus, the Herschel HIFI receiver noise temperatures, shown in Fig. 4,
are all important for sensitivity. As with ALMA, the velocity resolution of Herschel HIFI
can be extremely high. The finest spectrometer resolution is 140 kHz, corresponding to 0.1
kms -1 at 480 GHz with finer values at higher frequencies.
10