Background Power Module

Once the transmission of the different optical elements has been computed, the Background Power Module calculates the photon noise associated to the system from the emission of all the elements between the sky and the detector. The system is defined by four elements at different temperature stages. The primary mirror, the secondary mirror and the beam steering mirror at Ttei temperature, and the cold optics box or cryostat at T ox temperature. Both Ttei and T ox are user defined. 

The first step is the computation of the transmission to the detectors td for the different elements. Again the left and right side of the system are considered. Table 4.1 shows the list of elements and their properties, where L and R indicate the path of the system. 

The stray light contribution is calculated as the product of the blackbody emission from the Sun multiplied by the scattering coefficient (user defined), transmitted 

Once the individual transmissions and emissivities are known for each element i, the background power level on detector, Q is computed. 

The simulator also takes into account the emission from the Cosmic Microwave Background (CMB), the Cosmic Infrared Background (CIB) and Zodiacal light (Zodi), at the respective temperatures Tcmb, Tcib and Tzoi,-. From internal communication, the emlssivites for these three components are 

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At the Detector Noise Moduie, the Noise Equivalent Power (NEP) associated to the detectors and the 1 // noise are calculated. In parallel, with the physical properties of the system defined, the Background Power Module calculates the background power noise due to the instmment and the Cosmic Microwave Background (CMB), Cosmic Infrared Background (CIB) and Zodiacal Light. 

Weak spectral absorption is, however, characterised by small changes in a large background power and is therefore ill suited to superheterodyne detection. That background is also detected and overloads the sensitive intermediate frequency amplifier tuned to the beat frequency that is derived from the mixing process. Instead it is customary to modulate the absorption in some way and to observe the modulation on the detected signal rather than the absorption itself... 

Due to the binary phase modulation, the distribution of the background power is not uniform and there tend to be small peaks of intensity which may occur at fiber positions. This becomes less of a problem with large numbers of CGH pixels and careful CGH design. 

Fig. 1. Set-up of the PTB laser system. The Nd YAG laser is frequency stabilized onto a selected iodine absorption line using the phase modulation method. The probe beam is modulated at 2.05 MHz by an electro-optic modulator (EOM), the pumb beam is frequency shifted by an acousto-optical modulator (AOM). The driving AOM rf power is chopped in order to cancel frequency offsets introduced by the Doppler background using a lock-in detection scheme. The transmitted probe beam signal is detected by a photodiode (PD) and mixed with the EOM rf in a double balanced mixer (DBM)...

For a single-pass FM spectrometer working at 150 GHz the best operating condition is to minimise background, by controlling power reflection within and at each end of a long-path absorption cell, and to apply a modulation depth of 240 kHz p-p at a sample pressure 8-13 Pa. By contrast in a cavity spectrometer, the optimum modulation depth is governed by the cavity width, rather than the sample linewidth (Section 2.4). 

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