Estimation and correction of the instrumental perturbations of Vainu Bappu Telescope Echelle spectrograph using a model-based approach. (arXiv:1906.05064v1 [astro-ph.IM])

Estimation and correction of the instrumental perturbations of Vainu Bappu Telescope Echelle spectrograph using a model-based approach. (arXiv:1906.05064v1 [astro-ph.IM])
<a href="http://arxiv.org/find/astro-ph/1/au:+Chamarthi_S/0/1/0/all/0/1">Sireesha Chamarthi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Banyal_R/0/1/0/all/0/1">Ravinder K. Banyal</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sripadmanabhan_S/0/1/0/all/0/1">Sriram Sripadmanabhan</a>

The Echelle spectrograph operating at Vainu Bappu Telescope (VBT), India, is
a general purpose instrument used for many high-resolution spectroscopic
observations. A concerted effort is being made to expand the scientific
capability of the instrument in emerging areas of observational astronomy. The
present study is aimed at evaluating the feasibility of the spectrograph to
carry out precision Radial Velocity (RV) measurements. In the current design,
major factors limiting the RV precision of the spectrograph arise from the
movable grating and slit; optical aberrations; positional uncertainty
associated with optomechanical mounts and environmental and thermal
instabilities in the spectrograph room. RV instabilities due to temperature and
pressure variations in the environment are estimated to vary around 120 $
textrm{ms}^{-1} $ and 400 $ textrm{ms}^{-1} $ respectively. The positional
uncertainty of the grating in the spectrograph could induce a spectral shift
$sim1.4,textrm{kms}^{-1} $ across the Echelle orders. A Zemax model is used
to overcome the uncertainty in the zero-positioning and lack of repeatability
of the moving components. We propose to obtain the ThAr lamp observations and
using the Zemax model as the reference, predict the drifts in the positions of
the optical components. The perturbations of the optical components from the
nominal position are corrected at the beginning of the observational run. After
a good match is obtained between the model and the observations, we propose to
use a Zemax model to improve the wavelength calibration solution. We could
match the observations and model within $pm$~1 pixels accuracy after the model
parameters were perturbed in a real-time setup of the spectrograph. In this
paper, we present the estimation of the perturbations of optical components and
the effect on the RV obtained.

The Echelle spectrograph operating at Vainu Bappu Telescope (VBT), India, is
a general purpose instrument used for many high-resolution spectroscopic
observations. A concerted effort is being made to expand the scientific
capability of the instrument in emerging areas of observational astronomy. The
present study is aimed at evaluating the feasibility of the spectrograph to
carry out precision Radial Velocity (RV) measurements. In the current design,
major factors limiting the RV precision of the spectrograph arise from the
movable grating and slit; optical aberrations; positional uncertainty
associated with optomechanical mounts and environmental and thermal
instabilities in the spectrograph room. RV instabilities due to temperature and
pressure variations in the environment are estimated to vary around 120 $
textrm{ms}^{-1} $ and 400 $ textrm{ms}^{-1} $ respectively. The positional
uncertainty of the grating in the spectrograph could induce a spectral shift
$sim1.4,textrm{kms}^{-1} $ across the Echelle orders. A Zemax model is used
to overcome the uncertainty in the zero-positioning and lack of repeatability
of the moving components. We propose to obtain the ThAr lamp observations and
using the Zemax model as the reference, predict the drifts in the positions of
the optical components. The perturbations of the optical components from the
nominal position are corrected at the beginning of the observational run. After
a good match is obtained between the model and the observations, we propose to
use a Zemax model to improve the wavelength calibration solution. We could
match the observations and model within $pm$~1 pixels accuracy after the model
parameters were perturbed in a real-time setup of the spectrograph. In this
paper, we present the estimation of the perturbations of optical components and
the effect on the RV obtained.

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