The velocity sensitivity of resonant scattering spectrometers employing a piezoelastic modulator

The resonant scattering spectrometers of the IRIS ground-based network for measuring whole-disc solar velocity oscillations make use of a piezoelastic modulator. The velocity noise generated by this optical component is analysed with particular emphasis on the required stability of the amplitude of oscillation, a. The product of the absolute stability ¦ a – a _m ¦/a _m and the relative stability a _r.m.s./a _m may not be larger than 10 ^–4 to 10 ^–5 (depending on specific wishes), where a _m is the optimum amplitude. The velocity noise due to photon statistics is slightly enhanced, but other instrumental sources of velocity noise remain unaffected.