SPIRE Design Description - Research Services

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Draft SPIRE Design Description Document and sensitivity to microphonic disturbance. The detector noise is typically 10-20 nV/Hz -1/2 , and the noise of the readout amplifier must be of this order or less. With current transistor technology, this requires the use of silicon JFETs, which must operate at a temperature of around 100 K, and must also be located as close as possible to the detectors. This is why SPIRE employs JFET amplifier modules as part of its cold FPU. The essential features of the readout electronics are shown in Figure 3-26. The bolometer is biased (heated to its optimum operating temperature of around 1.3To) by a sinusoidal current bias at a frequency of at least 100 Hz, applied via the 10-MΩ load resistors. The bias excitation is much faster than the thermal time constant, so that bias itself does not produce a temperature modulation. This is preferred over DC bias as it upconverts the signal information to the bias frequency, getting well above the 1/f noise knee of the JFET readout amplifiers. With this arrangement, because of the inherently low 1/f noise of the bolometers, the1/f noise knee of the system can be very low (less than 0.1 Hz). AC bias ~ 100 Hz 300 mK 10 MΩ Bolometer ~ 5 MΩ 10 MΩ 4.4.5 Feedhorns and bolometer cavities 74 ~ 100 K 50 kΩ 50 kΩ Figure 4-18 Bolometer bias and cold readout circuit JFET power Differential output to warm amplifier The size of a composite bolometer (typically mm diameter) is small compared to the telescope diffraction spot (in the case of SPIRE, with f/5 final optics, the spot size is 2.44λF = 12.2 mm at 500 µm) and the firstpass absorption efficiency of the absorber is only ~ 50%. In order to couple the telescope beam onto the detectors in the array, conical feedhorns are used, with a short section of waveguide at the end of the horn to feed the radiation into a cavity containing the bolometer, as shown schematically in Figure 4-19. The waveguide acts as a low-pass filter as it does not propagate radiation of free space wavelength > 3.4a where a is the radius. For high efficiency, the absorber is located in the centre of the cylindrical cavity with λ/4 spacing between the front of the cavity and a reflecting back-short at the back. The feedhorns are packed in a hexagonal arrangement in the focal plane to fit as many as possible into the area available.
Draft SPIRE Design Description Document Circular waveguide λ/4 Silicon substrate at 300 mK 2Fλ 75 Conical feedhorn NTD Ge thermometer Metalised silicon nitride web Figure 4-19 - Feedhorn and cavity design Maximum feedhorn aperture efficiency of 70-80% is achieved for a horn diameter close to 2Fλ (e.g., Griffin 2000), corresponding to a beam spacing on the sky ~ 2λ/D, where D is the telescope diameter. The horn restricts the detector field of view, giving a tapered (near Gaussian) illumination of the telescope primary mirror (with an edge taper of approximately 8 dB in the case of SPIRE). Whilst the horns are close-packed in the focal plane, their beams on the sky do not fully sample the image unless the horn diameter is = 0.5Fλ. Several separate telescope pointings are therefore needed to create a fully-sampled image. For the 2Fλ horns 16 pointings are required in principle, as illustrated in Figure 4-20. In the case of SPIRE, the step size is dictated by the shortest wavelength channel (250 µm) and the number of steps is dictated by the longest wavelength channel (500 µm) so that a 64 point jiggle map is needed to achieve simultaneous full spatial sampling in all photometer bands. Feedhorns adjacent in the focal plane Beam FWHM = λ/D Beam separation on the sky = 2λ/D Figure 4-20 A 16-point “jiggle pattern” is needed to achieve a fully sampled map with 2Fλ feedhorns (for hexagonal packing the jiggle pattern is slightly different but 16 steps are still required). The main advantages of feedhorn arrays are: (i) maximum efficiency for detection of a point source with known position; (ii) well understood horn properties, allowing good control of the beam and reliable design; (iii) good stray light rejection - the bolometer field of view is restricted to the telescope; (iv) good rejection of EM interference - the horn plus integrating cavity act as a Faraday enclosure; (v) minimum number of detectors for a given total field size.
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