Thorium-Argon hollow cathode lamps are commonly used as wavelength calibration lamps for high-resolution astronomical spectrographs (e.g. the European Southern Observatory’s (ESO) High Accuracy Radial velocity Planet Searcher (HARPS) and Ultraviolet and Visual Echelle Spectrograph (UVES)). They have been instrumental in supporting high precision work such as the search for extra-solar planets using the radial-velocity method. However, several years ago astronomers found that the quality of commercial Th-Ar lamps had deteriorated, when new lamps showed a “forest” of lines at low intensity levels obscuring faint atomic thorium lines rendering them useless for wavelength calibration in some regions. Based on information provided by the manufacturers the presence of molecular emission from thorium oxides has been suspected as the likely cause of this problem. We have now conclusively identified the observed emission bands as being due to strong molecular bands of ThO, confirming the source of the contamination of the hollow cathode lamps.
We have recorded spectra of new Th-Ar lamps showing contamination using the high-resolution echelle spectrograph at the University of Wisconsin and ESO’s new Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) spectrograph in order to determine the positions of the ThO lines. We shall present our initial analyses of these spectra, describing the wavelength regions most affected, their dependence on the lamp current, and measurements of the ThO line positions.
We report on commissioning the iodine absorption cell in the High Resolution Spectrograph (HRS) on the Southern African Large Telescope (SALT). The low-, medium- and high-resolution (LR, MR and HR) modes of this fibre-fed, dual-channel, white-pupil vacuum échelle spectrograph have been in use by the SALT consortium since 2014, but the high-stability (HS) mode requires exoplanet expertise not available in our community. The original commercial HRS iodine cell was unsuitable due to an excess of iodine so it was replaced with a suitable custom-built cell. This cell was characterised at high signal-to-noise, at a resolution of 106, using the Fourier Transform Spectrometer at the National Institute of Standards and Technology before incorporation into the HRS HS bench. A combination of calibration frames and on-sky data were then used to produce an HRS-specific version of an IDL software package that derives precision radial velocities (PRVs) from spectra taken through an iodine cell. Bright stars with highly stable RVs observed during a short engineering campaign in May 2018 demonstrate that SALT HRS is currently capable of delivering Doppler precision of 4-7m/s.
We present performance reports of a new near-infrared (NIR) Fiber-Fabry Perot Interferometer (FFP) as a precise Doppler radial velocity (RV) wavelength reference.
FFPs are monolithic single-mode fiber devices that create emission spectra by interfering light traversing separate delay paths. The resulting interference spectrum provides a rich distribution of narrow lines, ideal for use as a precise spectrograph reference. The FFP has the advantages that the uniform density of emission lines gives a much wider bandwidth over which RV measurements are possible, and the finesse and bandwidth can be optimized for the specific application.
We present work done to prepare two new near-infrared calibration sources for use on high-precision astrophysical
spectrographs. Uranium-neon is an atomic calibration source, commercially available as a hollow-cathode lamp,
with over 10 000 known emission lines between 0.85 and 4 μm. Four gas cells — containing C2H2, H13CN, 12CO, and 13CO, respectively—are available as National Institute of Standards and Technology (nist) Standard Reference Materials (SRMs), and provide narrow absorption lines between 1.5 and 1.65 μm. These calibration sources may prove useful for wavelength-calibrating the future near-infrared high-precision radial-velocity spectrometers, including the Calar Alto high-Resolution search for M dwarfs with Exo-earths with a Near-infrared Echelle Spectrograph (CARMENES),1 the SpectroPolarimetre InfraROUge (SPIRou)∗, and the Habitable-Zone Planet Finder (HPF).2
The search for extrasolar planets is an exciting new field of astronomy. Since detection of a planet orbiting the
sun-like star 51 Peg,1 the field of planet finding has pushed the limits of sensitivity and accuracy in astronomical
photometry and spectroscopy. To date 455 exoplanets have been detected*, of which the radial velocity technique
is responsible for nearly 80%.2 Radial velocity measurements are also an important complement to photometric
missions such as Kepler and CoRoT, which survey vast numbers of stars simultaneously but which require follow
up measurements for positive identification of planets.
The chief objective in the search for exoplanets is the identification of habitable Earth-like planets in close
proximity to our solar system. Of the currently detected exoplanets, only a few are Earth-like,3 the vast majority
being giants in close orbits. While it is possible that these planets are the most common type, it is likely that
an inherent selection bias in planet finding techniques is the cause. Simply, large radial velocity shifts and
high contrast occultations are the most detectable by radial velocity spectroscopy and photometry, and so we
primarily observe planets capable of inducing them.
We report accelerated aging tests on three Pt/Ne lamps from the same manufacturing run as lamps installed on
the Cosmic Origins Spectrograph (COS). Initial radiometrically calibrated spectra were taken for each lamp at
the National Institute of Standards and Technology (NIST). One lamp was aged in air at NIST at a current of
10 mA and 50% duty cycle (30 s on, 30 s off) until failure. Calibrated spectra were taken after 206 h, 500 h,
778 h, 783 h and 897 h of operation. Two other lamps were aged by the COS instrument development team in
a thermal vacuum chamber, with calibrated spectra taken at NIST after 500 h of operation. In all three lamps,
total output dropped by less than 15 % over 500 h. We conclude that the lamps will satisfy the requirements of
COS in both lifetime and spectral stability.
We report accelerated vacuum aging tests on two Pt-Ne lamps identical and/or similar to those installed on
the Cosmic Origins Spectrograph (COS) to be installed in the Hubble Space Telescope (HST) in the fall of
2008. One additional lamp was aged in air at the National Institute of Standards and Technology (NIST). All
lamps were tested at a 50% duty cycle (30 s on/off) at flight nominal (10 mA) constant current until failure.
Calibrated spectra of all lamps were taken at NIST using the 10.7-m normal incidence vacuum spectrograph at
various points in the life of the lamps. In this paper we report the results of the photometric, electrical, and
thermal monitoring of the vacuum tested lamps, while the spectroscopic and air aging results are given in a
companion paper (Nave et al., 2008, SPIE 7011-134). We conclude that the lamps will satisfy the requirements
of the HST/COS mission in terms of lifetime, cycles, and thermal and spectral stability.
Extremely large telescopes (ELTs) and most of their instruments will be optimized for operating in the near
infrared (IR) because of the wavelength dependent performance of adaptive optics. Few established sources for
wavelength calibration exist in this wavelength domain. This project aims to provide the basic data to select
the best calibration sources for instruments at the European ELT (E-ELT) as a function of wavelength range
and spectral resolution. We have made use of the existing databases such as the NIST Atomic Spectra Database
as well as first principle physics and practical considerations to select about 20 hollow cathode lamps for study.
We are investigating their spectral and operational properties through laboratory measurements using a Fourier
Transform spectrometer at the European Southern Observatory (ESO). The most interesting sources will then be
studied at atomic physics laboratories, such as the US National Institute of Standards and Technology (NIST), in
order to produce accurate wavelength standards and calibration reference data directly applicable to operations
of E-ELT instruments. The resulting data archive will be used to recommend the best suited sources for the
instruments proposed for the E-ELT.
The European Southern Observatory (ESO), the Space Telescope European Co-ordinating Facility (ST-ECF),
and the US National Institute of Standards and Technology (NIST) are collaborating to study Th-Ar hollow
cathode lamps as used for the calibration of VLT (Very Large Telescope) spectrographs. In the near IR only
a limited number of wavelength standards are available. The density and distribution of lines in Ne or Kr
lamps, for example, are inadequate for high-resolution spectroscopy. Th-Ar hollow cathode lamps provide a
rich spectrum in the UV-visible region and have been used in astronomy for a long time; current examples
at ESO include the spectrographs UVES and FLAMES. The Th spectrum from 278 nm to about 1000 nm
was studied at high resolution about 20 years ago (Palmer and Engleman, 1983). Two studies of the Th-Ar
spectrum in the near IR have recently been published, but neither work is directly applicable to the calibration
of IR astronomical spectrographs at ESO. We report new measurements using the 2-m UV/visible/IR Fourier
transform spectrometer (FTS) at NIST that establish more than 2000 lines as wavelength standards in the range
900 nm to 4500 nm. This line list is used as input for a physical model that provides the wavelength calibration
for the Cryogenic High-Resolution IR Echelle Spectrometer (CRIRES), ESO's new high resolution (R~100,000)
IR spectrograph at the VLT. We also present first calibration results from laboratory testing of CRIRES. The
newly established wavelength standards will also be available for use by X-shooter and other spectrographs in the
future. Measurements of the variation of the spectrum of Th-Ar lamps as a function of operating current allow
us to optimise the spectral output in terms of relative intensity and line density for operation on the telescope.
Since Th and Ar line intensities show a different response with respect to operating current, such measurements
can be used as a diagnostic tool for distinguishing the gas and metal lines. Our findings show that Th-Ar lamps
hold the promise of becoming a standard source for wavelength calibration in near IR astronomy.
The Space Telescope European Co-ordinating Facility (ST-ECF) and National Institute of Standards and Technology (NIST) are collaborating to study hollow cathode calibration lamps as used onboard the Hubble Space Telescope (HST). As part of the STIS Calibration Enhancement (STIS-CE) Project we are trying to improve our understanding of the performance of hollow cathode lamps and the physical processes involved in their long term operation. The original flight lamps from the Faint Object Spectrograph (FOS) and the Goddard High Resolution Spectrograph (GHRS) are the only lamps that have ever been returned to Earth after extended operation in space. We have taken spectra of all four lamps using NIST's 10.7-m normal-incidence spectrograph and Fourier transform spectrometer (FTS) optimized for use in the ultraviolet (UV). These spectra, together with spectra archived from six years of on-orbit operations and pre-launch spectra, provide a unique data set--covering a period of about 20 years--for studying aging effects in these lamps. Our findings represent important lessons for the choice and design of calibration sources and their operation in future UV and optical spectrographs in space. Our results will be directly used for planning science operations of the Cosmic Origins Spectrograph
(COS) which is going to be installed on the HST during the next servicing mission.
The Space Telescope European Coordinating Facility's (ST-ECF) lamp project, funded directly by the European Space Agency (ESA), is dedicated to the study of hollow cathode calibration lamps as they are used onboard the Hubble Space Telescope (HST). There are two main objectives: First, we have measured the spectra of Pt/Cr-Ne lamps in order to obtain accurate and reliable wavelengths for all emission lines between 115 and 320 nm. This wavelength range corresponds to the coverage provided by the Space Telescope Imaging Spectrograph (STIS) Echelle modes. Extensive laboratory measurements were performed at the National Institute of Standard and Technology (NIST) using their 10.7 m normal incidence spectrograph and a Fourier Transform Spectrograph. Until now no good laboratory wavelengths for Cr had been available and their addition has a major impact on the wavelength calibration, in particular in the near UV. The new line list is being used in conjunction with the physical instrument model of STIS which is employed to derive an improved wavelength calibration as part of the STIS Calibration Enhancement (STIS-CE) effort. Second, we attempt to gain a better understanding of the performance of such lamps and the physical processes involved in their long term operations. Among the issues studied are the change of the spectrum as a function of current, its change as a function of time and the tolerances of alignment. The bulk of the measurements were performed on flight spares from STIS and on new space qualified lamps for the accelerated aging test. The original flight lamps from the Faint Object Spectrograph (FOS) and the Goddard High Resolution Spectrograph (GHRS) are the only lamps ever to be measured after their return from space. Together with the spectra archived from six years of on-orbit operations they provide a unique data set for studying ageing effects in these lamps. The new Pt/Cr-Ne line list has been successfully applied in the STIS-CE effort. Thereby the ST-ECF's lamp project directly leads to an improvement in the quality of scientific observations of existing HST spectrographs. Our findings also constitute important lessons for the design and operations of future UV and optical spectrographs in space.
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