Problem with Azimuth Mechanism

you probably will have heard about the difficulties we have with the azimuthal scan mechanism. It is losing steps during rastering when operated with the nominal motor current. We have not yet reached a conclusive result of our on-going investigations, but it is unavoidable to restrict the movements of this mechanism. It is now pointing near the centre meridian and will only be moved over short distances in order to counteract a potential cold welding and to maintain the presently available pointing capability. This mechanism has up to now executed less than 1/100 of the revolutions of the qualification test unit and, consequently, we did not expect that it would lose steps at this stage of its life. It is, however, rather certain that it is a mechanical and not an electronic problem.

We have selected the centre position, in line with the pre-launch failure mode analysis, as it offers many advantages over other options. In particular, the solar rotation will provide the scan motion for rastering and will bring, at least in principle, all areas of the Sun into the field-of-view. Disk and off limb observations are possible with the help of the elevation drive, although east and west regimes of the corona will not be accessible with SOHO's nominal orientation.

It is important to note that SUMER is fully operational as a spectrometer and should be used as such. It is unfortunate that we have to accept the pointing and raster limitations, but we should now concentrate on revising the SUMER sequences to match the hardware constraints.

At a later stage, other pointing options could also be considered.

With best regards,
Klaus Wilhelm
Lindau, 29 October 1996

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