The Scientific Payload

Phase A Study (5 May 1995)

Since the Assessment Study carried out on the COBRAS/SAMBA mission, it has progressed well into the detailed design stage of a Phase A study, the industrial part of which is being performed by Matra Marconi Space. As a result of tradeoffs carried out at the beginning of this year, several major changes in the spacecraft and payload have occurred. At the configuration level, the scan angle (i.e. the angle between the spacecraft spin axis and the telescope viewing direction has been changed from 30 degrees to 70 degrees. In the new configuration, the spacecraft does not have to tilt by large angles in and out of the ecliptic plane in order to achieve large sky coverage. Rather, it remains pointing in the anti-Sun direction permanently, resulting in the payload being permanently shadowed, and in no temperature modulation being induced by the spin. As a consequence, the thermal design of the spacecraft is now much simpler.

A major change has been the decision to change the baseline orbit from the L4 (Earth-Moon) Lagrangian point to the L2 (Earth-Sun) Lagrangian point. While the decision was based on technical problems at L4 (mainly related to the telemetry link and residual thermal modulation due to the Earth and Moon), the L2 orbit present major advantages to the payload: the problem of straylight due to Earth and Moon is very much reduced, and the observing efficiency is consequently very much enhanced; possible systematic effects due to interference by the RF telemetry signal will be much reduced as well, since the TM antenna will be always pointing at a large angle away from the telescope.

At the instrument level, the low-frequency (HEMT-based) receiver design has shifted from a total-power-receiver concept to a continuous-comparison design, because of the present poor 1/f noise performance of MMIC HEMT amplifiers. The sensitivity of these channels is somewhat reduced as a consequence. The horns will be redesigned to achieve higher angular resolution than originally anticipated. On the high-frequency (bolometer) instrument side, the highest channel has been split into two in order to be able to deal appropriately with the possible presence of cold dust. The (Low-Frequency and High-Frequency) instruments are now very well defined, and detailed engineering designs are underway.

On the theoretical side, many astronomers and physicists from several countries have formed a COBRAS/SAMBA Theory Working Group, whose aim is to work on the many issues which are critical to define the best possible CMB anisotropy space mission. The TWG has already produced very useful input regarding the observing strategy and definition of channel frequencies. One of the main objectives of the TWG is to develop full simulations of the microwave sky, and use them as testing ground for developing the techniques required to deconvolve foregrounds and systematic effects from the true anisotropy signals. Such a tool will be extremely useful to assess the importance of various systematic effects and will be a valuable guide in the detailed design of the instruments and spacecraft. Preliminary results already confirm the need for large sky coverage, wide frequency coverage, and high angular resolution.

This document was last updated 5 May 1995.

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