SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
yet to be determined. It is envisaged that a typical time per 64-point jiggle cycle will be 64 seconds, allowing
two chop cycles per jiggle position at the maximum chop frequency of 2 Hz.
POF4 involves a sequence of repetitions of POF3 with the telescope performing the steps of a raster scan
between the individual jiggle cycles in order to build up a map of a larger area.
5.3.3 Scan mapping (POF 4 and POF5)
This mode is for mapping a large region of sky by scanning the telescope to provide modulation of the
signal. It does not require the beam steering mechanism and is the preferred observing mode for deep
extragalactic surveys because it is more efficient and requires simpler data analysis. It also avoids increasing
confusion noise as happens with chopped observations if the individual signals in the two chop half-cycles
are not both transmitted to the ground. This will be done in the case of SPIRE, so chopping can be
performed if necessary without degrading the S/N in principle.
All SPIRE mechanisms are inactive. The spacecraft operates in line scan mode with the scan direction across
the sky at an angle of 14.5 o to one of the array axes so that the individual strips of the map are fully sampled
(1/2 beam spacing) in the cross-scan direction. The length of each line-scan should be such that the turnaround
time of the telescope (here assumed to be 10 seconds) does not constitute a large overhead. Each line
should therefore take at least 60 seconds. The scan rate will depend on the 1/f stability of the whole system,
but with a maximum limit of 60 arcseconds/second, determined by the spacecraft AOCS. The fastest beamcrossing
time (for a 250-µm detector) at this scan rate is 18/60 = 0.3 seconds. This is comfortably longer
than the time constant of the detectors (< 30 ms). It will be desirable to scan as slowly as possible to
minimise telescope turn-around overheads.
In the event of some 1/f noise component resulting in noise fluctuations on the timescale of a scan, the BSM
can perform chopping.
5.3.4 Photometer peak-up (POF7)
This mode is designed to allow SPIRE to peak up the pointing on a sufficiently strong point-like source. As
far as the observations are concerned, it is the same as a 7-Point Jiggle Map). It may not be necessary to use
this mode in flight. It is not needed for very bright objects (carrying out a small map is quick compared to
overheads from slewing etc.). Nor is it practical for very faint objects (poor S/N of the 7-point data would
lead to inaccurate offset calculation). It is therefore only likely to be useful for a particular band of source
strengths. POF7 may therefore be deleted for simplicity. For the moment, it is included to account for the
possibility that peaking up involving autonomous computation of the required pointing corrections by the
SPIRE DPU may be needed in some cases.
SPIRE does a standard seven-point jiggle observation. The offset of the source with respect to the
commanded pointing is computed by the DPU using the recorded data. The calculated pointing offsets are
(a) implemented by the BSM (baseline) or (b) transmitted to the spacecraft AOCS. If (b), the AOCS checks
that the required telescope movement is within the acceptable limits and executes it. SPIRE waits for a
standard period of time to elapse before flagging the data as valid.
5.3.5 Operation of photometer internal calibrator (POF8)
It is envisaged that the photometer calibrator will be operated at intervals of an hour or more. Its function is
to present a repeatable signal to the detectors. This will allow characterisation of (i) any responsivity or
system gain drifts; (ii) any variation of the detectors’ responsivity with radiant loading (e.g., non-linear
response when viewing very bright sources).
In operation, it will be powered by a pre-selected waveform, and the corresponding signals will be recorded.
A typical duration of the whole sequence will be ~ 10 sec. The BSM must be fixed at its rest position, and
the telescope pointing must also be fixed so that the only signal modulation is from the calibrator. To allow
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