We recently demonstrated sub-m/s sensitivity in measuring the radial velocity (RV) between the Earth and Sun using a simple solar telescope feeding the HARPS-N spectrograph at the Italian National Telescope, which is calibrated with a green astro-comb. We are using the solar telescope to characterize the effects of stellar (solar) RV jitter due to activity on the solar surface with the goal of detecting the solar RV signal from Venus, thereby demonstrating the sensitivity of these instruments to detect true Earth-twin exoplanets.
The GMT-Consortium Large Earth Finder (G-CLEF) is an optical-band echelle spectrograph that has been selected as
the first light instrument for the Giant Magellan Telescope (GMT). G-CLEF is a general-purpose, high dispersion
spectrograph that is fiber fed and capable of extremely precise radial velocity measurements. The G-CLEF Concept
Design (CoD) was selected in Spring 2013. Since then, G-CLEF has undergone science requirements and instrument
requirements reviews and will be the subject of a preliminary design review (PDR) in March 2015. Since CoD review
(CoDR), the overall G-CLEF design has evolved significantly as we have optimized the constituent designs of the major
subsystems, i.e. the fiber system, the telescope interface, the calibration system and the spectrograph itself. These
modifications have been made to enhance G-CLEF’s capability to address frontier science problems, as well as to
respond to the evolution of the GMT itself and developments in the technical landscape. G-CLEF has been designed by
applying rigorous systems engineering methodology to flow Level 1 Scientific Objectives to Level 2 Observational
Requirements and thence to Level 3 and Level 4. The rigorous systems approach applied to G-CLEF establishes a well
defined science requirements framework for the engineering design. By adopting this formalism, we may flexibly update
and analyze the capability of G-CLEF to respond to new scientific discoveries as we move toward first light. G-CLEF
will exploit numerous technological advances and features of the GMT itself to deliver an efficient, high performance instrument, e.g. exploiting the adaptive optics secondary system to increase both throughput and radial velocity
We report the design, installation and testing of a broadband green astro-comb on the HARPS-N spectrograph at the
TNG telescope. The astro-comb consists of over 7000 narrow lines (<10-6 nm width) spaced by 16 GHz (0.02 nm at 550
nm) with wavelengths stabilized to the Global Positioning System (GPS) and with flat power from 500 to 620 nm. The
narrow lines are used to calibrate the spectrograph and measure its line profile. The short term sensitivity of HARPS-N
is measured to be less than 2 cm/s and the long-term drift of the spectrograph is approximately 10 cm/s/day. The astrocomb
has been partially automated with future work planned to turn the astro-comb into a fully automated, push button
Searches for extrasolar planets using precision radial velocity (PRV) techniques are approaching Earth-like planet sensitivity, however require an improvement of one order of magnitude to identify earth-mass planets in the habitable zone of sun-like stars. A key limitation is spectrograph calibration. An astro-comb, an octave-spanning laser frequency comb and a Fabry-Pérot cavity, producing evenly spaced frequencies with large wavelength coverage, is a promising tool for improved wavelength calibration. We demonstrate the calibration of a high-resolution astrophysical spectrograph below the 1 m/s level in the 8000-9000 Å and 4200 Å spectral bands.
Searches for Earth-like exoplanets using the stellar radial velocity measurements require accuracy <10 cm/s over years.
To achieve such high accuracy requires a wavelength reference that provides many calibration lines with fractional
frequency accuracy of 10<sup>-10</sup> in the visible spectral range. We have developed a green astro-comb that generates ~6000 lines equally spaced by ~0.15 Å over 1000-Å bandwidth (centered at 5500 Å). The frequency of each line is directly locked to a frequency standard with fractional accuracy of 10<sup>-12</sup> over decades. We plan to bring this green astro-comb to the HARPS-north spectrograph at the TNG telescope for tests in 2012.
Wavelength calibrators are a critical component of high precision and accuracy radial velocity measurements. An order of magnitude improvement of the state-of-the-art of calibration of echelle spectrographs is amongst the requirements needed to achieve detection of earth-mass planets around sun-like stars in the habitable zone. We present studies of calibrators using a custom Fourier Transform Spectrograph (FTS) optimized for characterizing broadband, high repetition-rate laser frequency combs ("astro-combs") as well as other calibration sources including Th:Ar lamps and white-light etalons.
We describe recent work calibrating a cross-dispersed spectrograph with an "astro-comb" i.e., a high repetition rate,
octave spanning femtosecond laser frequency comb; and a filter cavity suppressing laser modes to match the resolution
of the spectrograph. Our astro-comb provides ~1500 evenly spaced (~0.6 A) calibration lines of roughly 100 nW per line
between 7800 and 8800 Angstroms. The calibration lines of the laser are stabilized to atomic clocks which can be
referenced to GPS providing intrinsic stability of the source laser below 1 cm/s in stellar radial velocity sensitivity, as
well as long term stability and reproducibility over years. We present calibration of the TRES spectrograph at the 1.5 m
telescope at the Fred L Whipple Observatory below 1 m/s radial velocity sensitivity in six orders from 7800-8800 A.
We describe recent progress toward developing optical frequency laser combs and tunable laser to the problem of more
precise calibration of high dispersion astronomical spectra, thus permitting radial velocity determinations in the few
cm/sec regime. We describe two programs in progress to calibrate both a cross dispersed echelle spectrograph with a
laser comb and to calibrate a multiobject echelle spectrograph with a tunable laser.