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Illumination at the output of one cylindrical optical fiber in the near and the far field. Sources are o ff axes. A high degree of azimuthal scrambling is observed. In contrast, it remains some e ff ects in the radial one.
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For fiber-fed spectrographs with a stable external wavelength source, scrambling properties of optical fibers and, homogeneity and stability of the instrument illumination are important for the accuracy of radial-velocimetry. Optical cylindric fibers are known to have good azimuthal scrambling. In contrast, the radial one is not perfect. In order t...
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Context 1
... and SOPHIE are fiber-fed spectrographs with si- multaneous reference calibrations. The stellar light collected by the telescope are lead to the instrument through a stan- dard step-index multi-mode cylindrical optical fiber. SOPHIE has two observing mode using two di ff erent fibers, the High-Resolution (HR) and the High-E ffi ciency mode (HE). In HR mode, the spectrograph is fed by a 40.5- μ m slit superimposed on the output of the 100- μ m fiber, reach- ing a spectral resolution of λ/∆λ = 75,000. In HE mode, the spectrograph is directly fed by the 100- μ m fiber with a resolution power of 40,000. Both SOPHIE fibers have an sky acceptance of 3-arcsec. HARPS has 70- μ m fiber with a sky acceptance of 1-arcsec and a spectral resolution of 110,000 (see Table 1). Non-uniform illumination of the slit or output fiber at the spectrograph entrance decreases the radial-velocity precision. Indeed, variations in seeing, focus and image shape at the fiber entrance may induced non-uniform illumination inside the pupil of the spectrograph. The optical aberrations lead to variations in the centroids of the stellar lines on the focal plane. Optical fibers are used to lead the light from the telescope to the entrance of the spectrograph. They have the properties to scrambler the atmospheric e ff ects and guiding and centering errors discussed previously. But the scrambling ability of one multimode fiber is not perfect as shown in Fig. 1. With an input o ff axes image, the azimuthal scrambling is good whereas it remains some e ff ects in the radial one. This pattern observed is bigger in far field than in near field. Near field and far field are respectively de- fined as, the brightness distribution across the output face of the fiber, and, as the angular distribution of light of fiber output beam. Therefore, changes in the input beam cause only circular symmetric changes in the fiber output pattern. In contrast, imperfect radial scrambling allows small zonal errors to remain. HARPS and the SOPHIE HR mode are equipped with optical double scramblers. It is used to increase and improve the uniformity and stabilization of the illumination of the spectrograph entrance (refs. [4] and [6]). The near field of a circular fiber is observed to be better scramble than the far field. In spite of only one fiber guiding the light from the telescope to the instrument, double scrambler is composed of two fibers coupled with two doublets. The system is designed to inverse near field and far field in order to induce a better radial scrambling, although it causes flux losses (ref. [1]). The degree of radial scrambling describes the stability of the output beam as the input image is moved from the center to the edge of the fiber. It is possible to detect in RV some variations due to the insu ffi ciency of radial scrambling. The light from a stable star is moved with the guiding / centering system from an edge to the other of the fiber. We compute the RV with the data reduction pipeline for di ff erent positions of the star. The test was done for the two modes of SOPHIE and for HARPS. The results are computed in Table 2. Because the scrambling of the fiber is not perfect, the movement of the input image corresponds to a shift on ...
Context 2
... fibers are used to lead the light from the tele- scope to the entrance of the spectrograph. They have the properties to scrambler the atmospheric effects and guiding and centering errors discussed previously. But the scram- bling ability of one multimode fiber is not perfect as shown in Fig. 1. With an input off axes image, the azimuthal scram- bling is good whereas it remains some effects in the ra- dial one. This pattern observed is bigger in far field than in near field. Near field and far field are respectively de- fined as, the brightness distribution across the output face of the fiber, and, as the angular distribution ...
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Citations
... In the absence of scrambling, the illumination errors in multimode fibres can be a significant source of RV inaccuracies. 10 To reduce the impact of illumination errors, an auto-guider was developed and installed at the prime cage of the telescope. Figure 2 shows the schematic of the guider. ...
The Echelle spectrograph operating at Vainu Bappu Telescope (VBT) is a general purpose instrument designed for high resolution spectroscopy. It is being considered for precision Doppler measurements without altering the existing design and basic usage. However, the design level limitations and environmental perturbations are a major source of instability and systematic errors. As a result, a small Doppler signal in the stellar spectra is completely swamped by the large and uncontrolled instrumental drift. In this paper, we discuss some of the remedial measures we took to improve the radial velocity performance of the spectrograph. We show that a new auto-guider assembly has greatly reduced the mechanical jitter of the star image at the fibre input, making the illumination of the spectrograph slit at the other end stable. We have also installed an iodine absorption cell to track and eliminate the instrumental drifts to facilitate precision radial velocity observations. Furthermore, we have developed a generic algorithm that uses iodine exposures to extract the stellar radial velocities without the need for the complex forward modeling. Our algorithm is not accurate to the level of traditional iodine technique. However it is convenient to use on a low-cost general-purpose spectrograph targeting a moderate Radial Velocity (RV) precision at a few 10-100~$\textrm{ms}^{-1}$ level. Finally, we have demonstrated the usefulness of our approach by measuring the RV signal of a well known short-period, planet-hosting star.
... In the absence of scrambling, the illumination errors in multimode fibres can be a significant source of RV inaccuracies. 23 To reduce the impact of illumination errors, an auto-guider was developed and installed at the prime cage of the telescope. Figure 2 shows the schematic of the guider. ...
The Echelle spectrograph operating at Vainu Bappu Telescope is a general purpose instrument designed for high-resolution spectroscopy. It is being considered for precision Doppler measurements without altering the existing design and basic usage. However, the design level limitations and environmental perturbations are a major source of instability and systematic errors. As a result, a small Doppler signal in the stellar spectra is completely swamped by the large and uncontrolled instrumental drift. We discuss some of the remedial measures we took to improve the radial velocity performance of the spectrograph. We show that an autoguider assembly has greatly reduced the mechanical jitter of the star image at the fiber input, making the illumination of the spectrograph slit at the other end stable. We have also installed an iodine absorption cell to track and eliminate the instrumental drifts to facilitate precision radial velocity observations. Furthermore, we have developed a generic algorithm that uses iodine exposures to extract the stellar radial velocities without the need for the complex forward modeling. Our algorithm is not accurate to the level of traditional iodine technique. However, it is convenient to use on a low-cost general-purpose spectrograph targeting a moderate radial velocity (RV) precision at a few 10 to 100 ms −1 level. Finally, we have demonstrated the usefulness of our approach by measuring the RV signal of a well-known short-period, planet-hosting star.
... Thus variations of illumination at fiber link output, in near field or in far field, have critical consequences on spectrum stability on the CCD. For this reason, SOPHIE is more sensitive than HARPS to illumination variations in the input fiber due to seeing variations or guiding and centering errors as already described in [13]. ...
... To go further, a seeing effect had been identified ( [13]), illustrating RV variations due to seeing conditions. Figure 8 shows the RV obtained on standard stars as a function of the seeing. ...
High-precision spectrographs play a key role in exoplanet searches using the
radial velocity technique. But at the accuracy level of 1 m.s-1, required for
super-Earth characterization, stability of fiber-fed spectrograph performance
is crucial considering variable observing conditions such as seeing, guiding
and centering errors and, telescope vignetting. In fiber-fed spectrographs such
as HARPS or SOPHIE, the fiber link scrambling properties are one of the main
issues. Both the stability of the fiber near-field uniformity at the
spectrograph entrance and of the far-field illumination on the echelle grating
(pupil) are critical for high-precision radial velocity measurements due to the
spectrograph geometrical field and aperture aberrations. We conducted tests on
the SOPHIE spectrograph at the 1.93-m OHP telescope to measure the instrument
sensitivity to the fiber link light feeding conditions: star decentering,
telescope vignetting by the dome,and defocussing.
To significantly improve on current precision, we designed a fiber link
modification considering the spectrograph operational constraints. We have
developed a new link which includes a piece of octagonal-section fiber, having
good scrambling properties, lying inside the former circular-section fiber, and
we tested the concept on a bench to characterize near-field and far-field
scrambling properties.
This modification has been implemented in spring 2011 on the SOPHIE
spectrograph fibers and tested for the first time directly on the sky to
demonstrate the gain compared to the previous fiber link. Scientific validation
for exoplanet search and characterization has been conducted by observing
standard stars.
... Thus variations of illumination at fiber link output, in near field or in far field, have critical consequences on spectrum stability on the CCD. For this reason, SOPHIE is more sensitive than HARPS to illumination variations in the input fiber due to seeing variations or guiding and centering errors as already described in [13]. ...
... To go further, a seeing effect had been identified ( [13]), illustrating RV variations due to seeing conditions. Figure 8 shows the RV obtained on standard stars as a function of the seeing. ...
High-precision spectrographs play a key role in exoplanet searches using the radial velocity technique. But at the accuracy level of 1 m.s(-1), required for super-Earth characterization, stability of fiber-fed spectrograph performance is crucial considering variable observing conditions such as seeing, guiding and centering errors and, telescope vignetting. In fiber-fed spectrographs such as HARPS or SOPHIE, the fiber link scrambling properties are one of the main issues. Both the stability of the fiber near-field uniformity at the spectrograph entrance and of the far-field illumination on the echelle grating (pupil) are critical for high-precision radial velocity measurements due to the spectrograph geometrical field and aperture aberrations. We conducted tests on the SOPHIE spectrograph at the 1.93-m OHP telescope to measure the instrument sensitivity to the fiber link light feeding conditions: star decentering, telescope vignetting by the dome, and defocussing. To significantly improve on current precision, we designed a fiber link modification considering the spectrograph operational constraints. We have developed a new link which includes a piece of octagonal-section fiber, having good scrambling properties, lying inside the former circular-section fiber, and we tested the concept on a bench to characterize near-field and far-field scrambling properties. This modification has been implemented in spring 2011 on the SOPHIE spectrograph fibers and tested for the first time directly on the sky to demonstrate the gain compared to the previous fiber link. Scientific validation for exoplanet search and characterization has been conducted by observing standard stars.
Periodic radial velocity variations in the nearby M-dwarf star Gl 411 are reported, based on measurements with the SOPHIE spectrograph. Current data do not allow us to distinguish between a 12.95-day period and its one-day alias at 1.08 days, but favour the former slightly. The velocity variation has an amplitude of 1.6 m s ⁻¹ , making this the lowest-amplitude signal detected with SOPHIE up to now. We have performed a detailed analysis of the significance of the signal and its origin, including extensive simulations with both uncorrelated and correlated noise, representing the signal induced by stellar activity. The signal is significantly detected, and the results from all tests point to its planetary origin. Additionally, the presence of an additional acceleration in the velocity time series is suggested by the current data. On the other hand, a previously reported signal with a period of 9.9 days, detected in HIRES velocities of this star, is not recovered in the SOPHIE data. An independent analysis of the HIRES dataset also fails to unveil the 9.9-day signal. If the 12.95-day period is the real one, the amplitude of the signal detected with SOPHIE implies the presence of a planet, called Gl 411 b, with a minimum mass of around three Earth masses, orbiting its star at a distance of 0.079 AU. The planet receives about 3.5 times the insolation received by Earth, which implies an equilibrium temperature between 256 and 350 K, and makes it too hot to be in the habitable zone. At a distance of only 2.5 pc, Gl 411 b, is the third closest low-mass planet detected to date. Its proximity to Earth will permit probing its atmosphere with a combination of high-contrast imaging and high-dispersion spectroscopy in the next decade.
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With about 2000 extrasolar planets confirmed, the results show that planetary systems have a whole range of unexpected properties. We present a full investigation of the HD219828 system, a bright metal-rich star for which a hot neptune has previously been detected. We used a set of HARPS, SOPHIE, and ELODIE radial velocities to search for the existence of orbiting companions to HD219828. A dynamical analysis is also performed to study the stability of the system and to constrain the orbital parameters and planet masses. We announce the discovery of a long period (P=13.1years) massive (msini=15.1MJup) companion (HD219828c) in a very eccentric orbit (e=0.81). The same data confirms the existence of a hot-neptune, HD219828b, with a minimum mass of 21 MEarth and a period of 3.83days. The dynamical analysis shows that the system is stable. The HD219828 system is extreme and unique in several aspects. First, among all known exoplanet systems it presents an unusually high mass ratio. We also show that systems like HD219828, with a hot neptune and a long-period massive companion are more frequent than similar systems with a hot jupiter instead. This suggests that the formation of hot neptunes follows a different path than the formation of their hot jovian counterparts. The high mass, long period, and eccentricity of HD219828c also make it a good target for Gaia astrometry as well as a potential target for atmospheric characterisation, using direct imaging or high-resolution spectroscopy. Astrometric observations will allow us to derive its real mass and orbital configuration. If a transit of HD219828b is detected, we will be able to fully characterise the system, including the relative orbital inclinations. With a clearly known mass, HD219828c may become a benchmark object for the range in between giant planets and brown dwarfs.
We report the discovery of three new substellar companions to solar-type stars, HD 191806, HD 214823, and HD 221585, based on radial velocity measurements obtained at the Haute-Provence Observatory. Data from the SOPHIE spectrograph are combined with observations acquired with its predecessor, ELODIE, to detect and characterise the orbital parameters of three new gaseous giant and brown dwarf candidates. Additionally, we combine SOPHIE data with velocities obtained at the Lick Observatory to improve the parameters of an already known giant planet companion, HD 16175 b. Thanks to the use of different instruments, the data sets of all four targets span more than ten years. Zero-point offsets between instruments are dealt with using Bayesian priors to incorporate the information we possess on the SOPHIE/ELODIE offset based on previous studies. The reported companions have orbital periods between three and five years and minimum masses between 1.6 MJup and 19 MJup. Additionally, we find that the star HD 191806 is experiencing a secular acceleration of over 11 m s-1 per year, potentially due to an additional stellar or substellar companion. A search for the astrometric signature of these companions was carried out using Hipparcos data. No orbit was detected, but a significant upper limit to the companion mass can be set for HD 221585, whose companion must be substellar. With the exception of HD 191806 b, the companions are located within the habitable zone of their host star. Therefore, satellites orbiting these objects could be a propitious place for life to develop.
The gravitational microlensing technique allows the discovery of exoplanets
around stars distributed in the disk of the galaxy towards the bulge. However,
the alignment of two stars that led to the discovery is unique over the
timescale of a human life and cannot be re-observed. Moreover, the target host
is often very faint and located in a crowded region. These difficulties hamper
and often make impossible the follow-up of the target and study of its possible
companions. Gould et al. (2013) predicted the radial-velocity curve of a binary
system, OGLE-2011-BLG-0417, discovered and characterised from a microlensing
event by Shin et al. (2012). We used the UVES spectrograph mounted at the VLT,
ESO to derive precise radial-velocity measurements of OGLE-2011-BLG-0417. To
gather high-precision on faint targets of microlensing events, we proposed to
use the source star as a reference to measure the lens radial velocities. We
obtained ten radial velocities on the putative V=18 lens with a dispersion of
~100 m/s, spread over one year. Our measurements do not confirm the
microlensing prediction for this binary system. The most likely scenario is
that the assumed V=18 mag lens is actually a blend and not the primary lens
that is 2 magnitude fainter. Further observations and analyses are needed to
understand the microlensing observation and infer on the nature and
characteristics of the lens itself.
The statistical validation of transiting exoplanets proved to be an efficient
technique to secure the nature of small exoplanet signals which cannot be
established by purely spectroscopic means. However, the spectroscopic diagnoses
are providing us with useful constraints on the presence of blended stellar
contaminants. In this paper, we present how a contaminating star affects the
measurements of the various spectroscopic diagnoses as function of the
parameters of the target and contaminating stars using the model implemented
into the PASTIS planet-validation software. We find particular cases for which
a blend might produce a large radial velocity signal but no bisector variation.
It might also produce a bisector variation anti-correlated with the radial
velocity one, as in the case of stellar spots. In those cases, the full width
half maximum variation provides complementary constraints. These results can be
used to constrain blend scenarios for transiting planet candidates or radial
velocity planets. We review all the spectroscopic diagnoses reported in the
literature so far, especially the ones to monitor the line asymmetry. We
estimate their uncertainty and compare their sensitivity to blends. Based on
that, we recommend the use of BiGauss which is the most sensitive diagnosis to
monitor line-profile asymmetry. In this paper, we also investigate the
sensitivity of the radial velocities to constrain blend scenarios and develop a
formalism to estimate the level of dilution of a blended signal. Finally, we
apply our blend model to re-analyse the spectroscopic diagnoses of HD16702, an
unresolved face-on binary which exhibits bisector variations.
