SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
change in the OPD of ~ 12 cm or 3 cm in actual mirror movement. In order to Nyquist sample the
interferogram at the highest frequency present in the optical band (50 cm -1 ) , the optical path difference must
be sampled every 20 microns, or every 5 microns of actual mirror movement.
The most efficient method of operating an FTS is the so-called rapid scanning method. Here, the mirror is
kept in constant motion while the signal and mirror position are sampled. This method of operation
effectively transforms the optical frequency to audio frequencies by the relationship f = vopdσ. Where f is the
detection frequency, vopd the rate of change of the optical path difference and σ the optical frequency. The
maximum OPD velocity required will be determined by the frequency response of the detectors. For an
assumed detector response of 20 Hz the maximum vopd will be 0.4 cm/s or 0.1 cm/s in actual mirror speed. In
fact some “head room” may required in case of problems with microphonic interference so the maximum
velocity requirement on the mirror mechanism has been set at 0.2 cm/s.
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Figure 4-37 - Views of the engineering model of the SMEC.
In order to further improve efficiency, the interferograms can be taken “single sided”. That is only a short
movement is needed on one side of the zero path difference (ZPD) to enable the position of the ZPD to be
established and as long a distance as necessary is scanned beyond that to achieve the required resolution.
As in the photometer, the signal in the recorded interferograms will be dominated by the telescope emission.
The shape of the optical pass band means that the rate of change of the signal is greatest over the scan region
close to the ZPD (Figure 4-39); that is at low resolution. In order to fulfil the requirement on the low
resolution performance of the spectrometer it is necessary to have good control of the variation in the
velocity and knowledge of the mirror position over the scan range close to the ZPD. Further away from the
ZPD the requirement is less strict as the rate of change of the signal is much reduced.
These scientific and operational requirements impose the following baseline performance on the FTS mirror
mechanism or SMEC:
(i) maximum mirror travel required (w.r.t. the ZPD position): – 0.32 to +3.2 cm;
(ii) minimum measurement interval of 5 µm is required;
(iii) required mirror position measurement accuracy 0.1 µm over +- 0.32 scan range and 0.3 µm
thereafter;
(iv) the mirror velocity shall be within 0.001 cm/s r.m.s. within a band width of 0.03 to 25 Hz over the
entire scan range;
(v) additionally movement of the mirrors must not impose any translation or rotation on the optical
beam such as to affect the fringe visibility in the recorded interferogram.
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