UVES radial velocity accuracy from asteroid observations

Abstract

Context. High resolution observations of the asteroids Iris and Juno have been performed by means of the UV-Visual Echelle Spectrograph (UVES) at the European Southern Observatory (ESO) Very Large Telescope (VLT) to obtain the effective accuracy of the spectrograph's radial velocity. The knowledge of this quantity has important bearings on studies searching for a variability in the fine structure constant carried out with this instrument. Aims. Asteroids provide a precise radial velocity reference at the level of 1 m s −1 , which allows for instrumental calibration and the recognition of small instrumental drifts and calibration systematics. In particular, radial velocity drifts due to nonuniform slit illumination and slit optical misalignment in the two UVES spectrograph arms can be investigated. Methods. We compare the position of the solar spectrum reflected by the asteroids with the solar wavelength positions and with that of asteroid and twilight observations at other epochs to asses the UVES instrumental accuracy. Results. We observe radial velocities offsets in the range of ≈10−50 m s −1 , likely due to a nonuniform slit illumination. However, no radial velocity patterns with wavelengths are detected and the two UVES arms provide consistent radial velocities. These results suggest that the detected ∆α/α variability by Levshakov et al. (2007) deduced from a drift of −180 ± 85 m s −1 at z abs = 1.84, between two sets of Fe ii lines falling in the two UVES arms, may be real or induced by other kinds of systematics than those investigated here. The proposed technique allows us to make a real time quality check of the spectrograph and should be followed for very accurate measurements.

The UVES Large Program for testing fundamental physics I. Bounds on a change inαtowards quasar HE 2217−2818

“…This is not obvious a priori because the two arms of the spectrograph have two different entrance slits, and any optical misalignment will produce radial velocity differences between the two arms' spectra. Molaro et al (2008a) compared the radial velocity measured in the two arms using the sunlight reflected by an asteroid, and came to a similar conclusion. We note that with UVES, no telescope or instrument control is provided to control the relative slit alignment with the desirable accuracy, and the optical alignment can, in principle, change from one observing run to another.…”

The UVES Large Program for testing fundamental physics – III. Constraints on the fine-structure constant from three telescopes

“…That means that if some transitions are observed with the blue arm, and other transitions are observed with the red arm, any velocity offsets between the two arms would be interpreted as a velocity distortion across the whole spectrum, leading to an incorrect measurement of ∆α/α. Molaro et al (2008) were the first to use asteroids to check these velocity shifts between exposures taken on the two different arms of UVES. However, we do not need to account for this problem in our UVES spectra because all of the transitions that we use to measure ∆α/α are solely observed with the red arm of the spectrograph and therefore will not have any of these inter-arm distortions.…”

Fundamental constants and high‐resolution spectroscopy

“…These observations allow to generate a transfer function for correcting the comparison lamp wavelength scale. This technique was recently pioneered by us (Molaro et al 2008a) -we observed bright stars through an iodine gas absorption cell, as done for extrasolar planet searches, providing an even more precise transfer function for part of the wavelength range, important for varying constants; -we took a series of lamp exposures bracketing the quasar exposures to ensure the best possible starting point for this transfer function.…”