High accuracy radial velocity measurement isn’t only one of the most important methods for detecting earth-like
Exoplanets, but also one of the main developing fields of astronomical observation technologies in future. Externally
dispersed interferometry (EDI) generates a kind of particular interference spectrum through combining a fixed-delay
interferometer with a medium-resolution spectrograph. It effectively enhances radial velocity measuring accuracy by
several times. Another further study on multi-delay interferometry was gradually developed after observation success
with only a fixed-delay, and its relative instrumentation makes more impressive performance in near Infrared band.
Multi-delay is capable of giving wider coverage from low to high frequency in Fourier field so that gives a higher
accuracy in radial velocity measurement. To study on this new technology and verify its feasibility at Guo Shoujing
telescope (LAMOST), an experimental instrumentation with single fixed-delay named MESSI has been built and tested
at our lab. Another experimental study on multi-delay spectral interferometry given here is being done as well. Basically,
this multi-delay experimental system is designed in according to the similar instrument named TEDI at Palomar
observatory and the preliminary test result of MESSI. Due to existence of LAMOST spectrograph at lab, a multi-delay
interferometer design actually dominates our work. It’s generally composed of three parts, respectively science optics,
phase-stabilizing optics and delay-calibrating optics. To switch different fixed delays smoothly during observation, the
delay-calibrating optics is possibly useful to get high repeatability during switching motion through polychromatic
interferometry. Although this metrology is based on white light interferometry in theory, it’s different that integrates all
of interference signals independently obtained by different monochromatic light in order to avoid dispersion error caused
by broad band in big optical path difference (OPD).
Exoplanet detection, a highlight in the current astronomy, will be part of puzzle in astronomical and astrophysical future,
which contains dark energy, dark matter, early universe, black hole, galactic evolution and so on. At present, most of the
detected Exoplanets are confirmed through methods of radial velocity and transit. Guo shoujing Telescope well known
as LAMOST is an advanced multi-object spectral survey telescope equipped with 4000 fibers and 16 low resolution fiber
spectrographs. To explore its potential in different astronomical activities, a new radial velocity method named
Externally Dispersed Interferometry (EDI) is applied to serve Exoplanet detection through combining a fixed-delay
interferometer with the existing spectrograph in medium spectral resolution mode (R=5,000-10,000). This new
technology has an impressive feature to enhance radial velocity measuring accuracy of the existing spectrograph through
installing a fixed-delay interferometer in front of spectrograph. This way produces an interference spectrum with higher
sensitivity to Doppler Effect by interference phase and fixed delay. This relative system named Multi-object Exoplanet
Search Spectral Interferometer (MESSI) is composed of a few parts, including a pair of multi-fiber coupling sockets, a
remote control iodine subsystem, a multi-object fixed delay interferometer and the existing spectrograph. It covers from
500 to 550 nm and simultaneously observes up to 21 stars. Even if it’s an experimental instrument at present, it’s still
well demonstrated in paper that how MESSI does explore an effective way to build its own system under the existing
condition of LAMOST and get its expected performance for multi-object Exoplanet detection, especially instrument
stability and its special data reduction. As a result of test at lab, inside temperature of its instrumental chamber is stable
in a range of ±0.5degree Celsius within 12 hours, and the direct instrumental stability without further observation
correction is equivalent to be ±50m/s every 20mins.
It's a very important point that fully open up power of Gou Shoujing telescope (LAMOST) in exoplanet detection field
by developing a multi-exoplanet survey system. But it's an indisputable truth in the present astronomy that a traditional
type of multi-object high resolution spectrograph is almost impossible to be developed. External Dispersed
Interferometry is an effective way to improve the radial velocity measuring accuracy of medium resolution spectrograph.
With the using of this technique, Multi-object Exoplanet Search Spectral Interferometer (MESSI) is an exploratory
system with medium measuring accuracy based on LAMOST low resolution spectrograph works in medium-resolution
mode (R=5,000 - 10,000). And it's believed that will bring some feasible way in the future development of multi-object
medium/high resolution spectrograph. After prototype experiment in 2010, a complete configuration is under the
development, including a multi-object fixed-delay Michelson interferometer, an iodine cell with multi-fiber optical
coupling system and a multi-terminal switching system in an efficient fiber physical coupling way. By some effective
improvement, the interferometer has smaller cross section and more stable interference component. Moreover, based on
physical and optical fiber coupling technique, it's possible for the iodine cell and the switching system to simultaneously
and identically coupling 25 pairs of fibers. In paper, all of the progress is given in detail.
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