The High Frequency Instrument

The High Frequency Instrument or HFI (Figures 1-4, Table 1) will cover the high frequency part (100 - 857 GHz) of Planck.

The HFI consists of (i) the HFI focal plane unit, (ii) the readout electronics, (iii) the Data Processing Unit, (iv) the coolers, and (v) harness and tubes linking various subsystems. It is based on the use of bolometers cooled at 0.1K, that are the most sensitive detectors for wide band photometry in the HFI spectral range. Bolometers are sensitive to the heat deposited in an absorber by the incident radiation. Very low temperatures are required to obtain a low heat capacity giving a high sensitivity with a short enough thermal time constant.

Cooling the detectors at 0.1K in space is a major requirement that drives the architecture of the HFI. This is achieved, starting from the passively cooled 50K/60K stage of the payload module, by a four-stage cooling system (18K-4K-1.6K-0.1K) detailed in Cooling System. The 18K cooler is common to the HFI and the Low Frequency Instrument (LFI).

The 4K stage protects the inner stages from the thermal radiation of the 18K environment. It provides also an electromagnetic shielding (a Faraday cage) for the high impedance part of the readout electronics. It is the envelope of the HFI focal plane unit. The coupling of the telescope with the detectors is made by back-to-back horns attached on the 4K stage, the aperture of the waveguides being the only radiative coupling between the inside and the outside of the 4K box. Filters are attached on the 1.6K stage, and bolometers on the 0.1K stage, which corresponds to an optimal distribution of heat loads on the different stages.

The HFI focal plane unit has an extension to the 18K and 50/60K stages, enclosing the first stage of the preamplifiers (J-FETs at 120K). The AC bias and readout electronics performs all the electrical functions of the cold stages, including the temperature measurement and control.

**Figure 2:** An isometric view of the HFI front-end or ``cold box". At the top are the entrance horns, feeding radation towards the bolometers. The body consists of nested radiation shields maintained at various temperatures (at 4 K, 1.6 K, and 0.1 K) by the active cooling system. Only the outermost shield at 4 K is shown. The bolometers are at 0.1 K.

The heart of the HFI - the detectors - are bolometers, solid-state devices in which the incoming radiation dissipates its energy as heat that increases the temperature of a thermometer. For a bolometer with a given time constant, the temperature increase (on time scales longer than the time constant) is inversely proportional to the heat capacity of the bolometer. The cooling of these detectors to very low temperatures provides for the low heat capacity needed for high sensitivities. Models of bolometer performance indicate that in practice, allowing for non-ideal effects, to obtain a useful speed of response and the highest sensitivity, the maximum allowable physical temperature of the bolometer heat sink is T ~ $_{max} \simeq$ K. Thus, the HFI bolometers must be cooled to temperatures below 0.15 K. As an indication, if the temperature were increased to 0.3 K, the sensitivity at the most interesting frequencies would be degraded by a factor of $\sim$ 10 - a degradation which is unacceptable in terms of the expected scientific return. The goal set for the HFI bolometers is that they should operate at a temperature of 0.1 K. With this assumption, and using the current best detector technologies (e.g. spider-web bolometers, Bock et al.995, Gear & Cunningham 1995) it is possible to reach Noise Equivalent Powers (NEPs) of the order of 10^-17 W Hz^-1/2 (see Table 1). The total number of bolometers will be 48 (Figure 3), split into 6 channels at central frequencies of 100, 143, 217, 353, 545, and 857 GHz. The placement of the channels in frequency space has been optimized, not only to remove the foregrounds (mainly dust emission at these high frequencies) and recover the CMB, but also for the detection of the Sunyaev-Zeldovich effect. Filters provide the necessary frequency selectivity for each channel, and also block the thermal radiation coming from the telescope itself. Light from the telescope will enter the cold box (Figure 4) through an initial blocking stage at 4 K, proceed through a second bandpass filter at 1.6 K made from interfering cross-shaped grids embedded in a polyethylene matrix, and will finally be concentrated on the detectors in an integrating cavity. The entrance apertures of the horns define the fields of view of the detectors; they are sized to the diffraction pattern in the four lowest frequency channels, and oversized in the two highest frequency channels in order to cope with the aberrations of the telescope. The total number of detectors in each channel is limited by the available area of the focal plane and the available heat lift of the cooling system. The detector arrangement with respect to the scan direction gives a fully sampled image of the sky, and partial redundancy.

The bolometers are read out via J-FETs located very close to them, in a box which is not physically located inside the cold box of the HFI, and thermally insulated from it. The cryogenically-cooled J-FETs provide for the impedance matching with the following stages of the preamplifiers located farther from the detectors. The readout electronics are based on the principle of AC bias that has successfully demonstrated (in ground-based experiments, Wilbanks et al.990) its capability to detect signals at very low frequency without sky-chopping. The rotation of the satellite at 1 rpm will provide signals in the range 0.016 Hz to 94 Hz, though little power remains above 70 Hz; in terms of spherical harmonic orders, these frequencies correspond to the range l = 1 to 4000, very suitable to the measurement sought.

Table 1 summarizes the goal characteristics of the HFI channels and the instrumental sensitivities that will be achieved after the nominal observation period.

**Table:** Goal Characteristics and Sensitivity of the HFI
Center Frequency (GHz)	857	545	353	217	143	100
Center Wavelength (mm)	0.35	0.55	0.85	1.38	2.1	3.0
Detector Temperature (K)	0.1	0.1	0.1	0.1	0.1	0.1
Bandwidth (%)	0.25	0.25	0.25	0.25	0.25	0.25
Bandwidth (GHz)	214	136	88	54	36	36
Number of unpolarised bolometers	6	0	6	4	3	4
Number of polarised bolometers	0	8	0	8	9	0
Angular Res. (FWHM, arcmin)^#	5	5	5	5.5	8	10.7
No. pixels^* on sky ( $\times$ 10⁶)	5.94	5.94	5.94	4.88	2.30	1.30
NEP_bol (10^-17 W Hz^-1/2 )	3.80	1.51	1.16	1.04	0.9	0.82
NEP_{phot, unpol} (10^-17 W Hz^-1/2 ) $^\clubsuit$	14.6	-	2.88	1.49	1.24	1.01
NEP_{phot, pol} (10^-17 W Hz^-1/2 ) $^\clubsuit$	-	4.66	-	1.05	0.88	-
NEP_{tot, unpol} (10^-17 W Hz^-1/2 )	1.03	-	1.08	1.22	1.24	1.29
NEP_{tot, pol} (10^-17 W Hz^-1/2 )	-	1.05	-	1.41	1.43	-
Nominal mission $^\star$ (1 $\sigma$ sensitivities, per pixel^*)
Average integ. time per pixel (sec.)	6.2	6.2	6.2	7.6	16.1	28.4
$\Delta T/T$ Sensit. (Intensity) $^\dagger$ (10^-6)	6670	147	14.4	4.3	2.0	1.7
$\Delta T/T$ Sensit. (U and Q) $^\dagger$ (10^-6)	-	208	-	8.9	3.7	-
Flux Sensit.(mJy)	43	38	19.4	11.5	11.5	8.7
ySZ $^\ddagger$ (10^-6)	600	26	6.44	5.47	1.88	1.11
N(H) ^@ (10²⁰ H/cm²)	0.025	0.057	0.14	0.13	0.15	0.17
$^\star$ 12 months of observations, or two full sky coverages
^# Diffraction limited at frequencies below 350 GHz
^* A pixel is defined as a square whose side is the FWHM extent of the beam
$^\clubsuit$ Telescope temperature = 60 K; total emissivity = 0.01
⁺ Rayleigh-Jeans temperature
$^\dagger$ Thermodynamic temperature
$^\ddagger$ Sensitivity to the Sunyaev-Zeldovich y parameter
^@ Sensitivity to gas column density