Abstract:
The Echelle spectrograph operating at Vainu Bappu Telescope (VBT)
is a general-purpose instrument designed for high-resolution spectroscopy.
It was considered for precision Radial Velocity (RV) measurements without
altering the existing design and primary usage. The design level limitations
and environmental perturbations at the spectrograph constitute a significant
source error for precise RV studies. Initially, the inherent stability of the
spectrograph was estimated. From the results of the stability analysis, a few
upgrade activities at the spectrograph were taken up. As part of the upgrade,
an autoguider was installed at the telescope prime focus. Along with this,
the wavelength calibration unit was upgraded; the alignment errors of the
instrument were also corrected. With these enhancements, the stability of
the spectrograph was improved from ± 1 pixel (1000 ms−1
) to ± 0.4 pixel
(400 ms−1
).
The next major limitation that was addressed is the lack of repeatability
of moving components in the spectrograph. For this purpose, the Zemax
model of the spectrograph was developed as a reference. ThAr spectral lines
obtained from observations were evaluated with respect to the Zemax reference positions, to estimate the drifts in the components. Apart from this, to
eliminate the instrumental drifts during observations, an iodine absorption
cell was integrated with the spectrograph. A generic algorithm was developed that uses iodine exposures to correct for the instrumental drifts from
RV estimates. Finally, with all the upgrades mentioned above, the RV precision limits of the spectrograph were pushed to a few 10-100 ms−1
regime.
The technique was validated on a well-studied exoplanet-hosting star. Later,
efforts were made towards the implementation of the iodine cell forward modelling technique to push the RV precision limits to a few ms−1
level. The
procedure was validated using synthetic stellar spectra. The algorithm was
developed on an open-source platform in Python to increase the accessibility
of the approach.
