The Size, Structure, and Variability of Late-Type Stars Measured ...

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the Pacific Ocean nearby. A more complete description of the telescopes can be found in
Hale et al. (2000) [37].
The interferometer employs heterodyne detection whereby the starlight is mixed
with a CO 2 laser (local oscillator) beam by matching wavefronts and focussing both beams
onto a cooled photodiode sensitive to 11 µm radiation. The beat signal between the laser
and starlight has frequency equal to the difference in their frequencies and contains all of
the phase information from the stellar signal. The detector and electronics have a passband
up to about 3 GHz, so the net bandpass used to observe the star lies within the range
[ν LO −3 GHz,ν LO +3 GHz], where ν LO can be chosen to be any of a set of discrete CO 2 lines
between 9.5 and 11.5 µm. The two lasers in the telescopes are locked to each other having
a controlled phase difference. After delaying one of the beat signals to compensate for the
extra path length to the farthest telescope, the fringe is formed electronically by multiplying
the two beat signals from each telescope. The fringe, modulated by the (known) frequency
difference between the lasers, is digitized and recorded to disk. A simplified schematic of
the ISI is shown in Figure 1.4. The ISI system is much more complex than described above
and a thorough description can be found in Hale et al. (2000) [37].
Heterodyne detection, used by the ISI, can have a better signal to noise ratio than
direct detection (forming fringes directly from starlight) when very narrow bandwidths are
employed. Additionally, in an interferometric array of many telescopes, stellar signals need
to be split into enough beams to form fringes with every other telescope. Using direct
detection, this entails a loss of signal to noise. With heterodyne detection, the signals are
electronic radio frequencies and can be amplified freely before splitting without losing signal
to noise. A more thorough discussion of signal to noise in the ISI compared with direct
detection is found in Monnier (1999) [72] or Hale et al. (2000) [37].
Before 1999, the longest baseline used by the ISI was 30 m. This baseline implies a
resolution typical of the circumstellar shells of nearby AGB stars. Almost all of the science
done with the ISI before 1999 pertained to the structure of the dust shells and molecular
regions in the circumstellar environment surrounding AGB stars. Much of the dust shell
work can be found in Danchi et al. (1994) [22] and the interferometry on molecular lines in
Monnier (1999) [72].