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The Sub-mm Radiometer on Odin

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Figure 1: This block diagram shows the layout of the instrument. High frequency signals from the telescope and the signal references are fed via pre-optics such as the Dicke switch, the single sideband filters and the diplexers for local oscillator injection, to the mixers. The mixers together with the first low noise amplifiers are mounted inside a cryostat. IF signals are after further amplification routed to the backend spectrometers.

Instrument description

The receivers are arranged as five independent signal chains in two groups that are time multiplexed by a switching mirror (Dicke switch) such that one group is receiving the signal from the telescope as the other receives a reference or calibration signal. Figure 1 shows the radiometer layout and also the separation of frequencies, the ground state water lines have been included in both groups for redundancy reasons and to permit improved observing efficiency. Within each group the signals are further split according to polarisation. This arrangement avoids complete loss of all the receivers from a hypothetical mechanical failure of the Dicke switch. The mechanism operates every 1 to 10 s with the backends generating one spectrum per position. It is also possible to use position switching with the complete spacecraft and the Dicke switch halted. The backend spectrometers monitor attitude control information to blank out bad data during attitude changes. The sub-mm receivers select sideband and inject the LO signal using polarising interferometers while the mm receiver uses a fixed tuned ring filter. The Schottky mixers, the mm preamplifier with ring filter, and the low noise amplifiers (LNA) are cooled and mounted inside the cryostat. The 1.2 GHz wide intermediate frequency (IF) signals are amplified and sent in parallel to one acousto-optical (AOS) and two autocorrelation spectrometers. Basic data are shown in Table 1.

Table 1

Item

Characteristic

Antenna

~1.1 m offset Gregorian

Pointing accuracy

~10 arc sec

Pointing stability

~4 per hour

Frequency range

~ 118.75, 486-504, 541-581 GHz

Typical system noise

~ 600, 3300, 3300 K

Spectral resolution

~125 to 1000 kHz

Backend bandwidth

~100 to 1000 MHz

Cooling

Stirling cycle to 140 K

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