||The History of the Fourier Tachometer
||478, Fifty Years of Seismology of the Sun and Stars
||Beckers, J. M.; Brown, T. M.
||Following a suggestion by one of us (T. Brown) we developed in 1978 at the Sacramento Peak
Observatory the first version of what we called a Fourier Tachometer which measured the phase
of a single frequency component of the Fourier transform of the solar spectrum associated with
a specific solar spectrum line (Beckers & Brown 1978). This phase is a direct measure of the
wavelength of that Line, its Doppler shift and by using polarization optics, its Zeeman splitting.
This first version based on a Michelson interferometer (FT I) was later (Evans 1081) greatly
improved by J.W. Evans by using a Solid Polarizing Interferometer (version FT II). The latest
version stands out by its ability to: (i) get wavelength measurements over a large 2D field-of-view
without the cumbersome use of a high-resolution spectrograph, (ii) have a wide angular
field-of-view and étendue, (iii) be mechanically stable and use much real-time digital
The FT II was selected for use in Global Oscillation Network Group (GONG) in about 1985 and has since
then also been used in the space based
helioseismometers — Micheson Doppler Imager (MDI) onboard Solar and Heliospheric Observatory (Scherrer et al. 1995) and
Helioseismic Magnetic Imager (HMI) onboard Solar Dynamics Observatory (Scherrer et al. 2012).
The FT II performance has increased over the years with the current HMI version having 4096 ×
4096 pixels, or 0.5 × 0.5 arcsec for the HMI full disk facility, and a cadence of 45 seconds.
However, except for some early observations at the Sacramento Peak, the Fourier Tachometer
has not appeared to have been applied to non-helioseismology ground-based observations. In
ground-based telescopes science full precise line profiles are generally desired making the FT II
undesirable since it only measures something close to their center-of-gravity. For future very
large diameter (1.5 – 8 m) ground-based solar telescopes that will also be the case. But
complimentary FT II observations, for example from the spectrograph reflecting slit-jaws, would
provide valuable, high time and spatial resolution complimentary observations. The HMI version
would have pixel sizes of about 0.03 × 0.03 arcsec, closely matching the telescope resolution
over a 2 × 2 arcmin field-of-view provided by its Multi-Conjugate Adaptive Optics system.