  Accurate wavelength calibration is an important factor for any measurement
with high resolution spectrographs. Stellar spectrum comprises of discrete
absorption or emission lines whose position is precisely determined by
calibrating the spectrograph using known reference lines generated from
laboratory sources. For the spectrograph to measure small variations in Doppler
shift, the wavelength calibration must be sufficiently stable during
observation time. Instrument instability, mainly due to environmental factors
like temperature and pressure variations, limitations of traditional
calibration methods, for example Th-Ar lamps, are the main challenges which
high precision spectroscopy. Through proper environmental control, by
maintaining pressure at few mbar and temperature fluctuations within
$\pm$0.05$^{\circ}$C, Fabry P\'erot etalon (FP) can yield a velocity precision
of 1-10 m/s, when used for wavelength calibration. We have developed a
passively stabilized FP based wavelength calibrator for Hanle Echelle
Spectrograph (HESP) installed on Himalayan Chandra Telescope (HCT) at Indian
Astronomical Observatory (IAO), Hanle, India. The etalon has been characterized
using Fourier Transform Spectrograph (FTS) and initial test runs have been
performed with HESP. In this paper we present the design and construction of
the instrument along with preliminary test results obtained from HESP.
