7 $\textrm{ms}^{-1}$. The echelle spectrograph operating in VBT is a general purpose instrument and is not designed for precision radial velocity measurements. Thus, the stability limitation is around 50 $\textrm{ms}^{-1}$ with the spectrograph. The model deviation is within the stability limits of the spectrograph.

  **Perturbed parameter**   **Actual parameter values**   **Values after optimization**   **Difference**
  ------------------------- ----------------------------- ------------------------------- ----------------
  prism\_x                  8.585863342                   8.585863042                     -3E-07
  prism\_y                  0                             -5.39281E-06                    -5.39281E-06
  prism\_z                  0                             1.06121E-06                     1.06121E-06
  grating\_x                25.852965                     25.85295965                     -5.35E-06
  grating\_y                -68.555                       -68.55499933                    6.7E-07
  grating\_z                90                            89.99999423                     -5.77E-06
  detector                  0                             4.65131E-07                     4.65131E-07

  :  \[tab:degeneracy\] The perturbed parameter values compared with the reference positions. The “difference” column shows the difference between the reference and the perturbed positions after optimization of the merit function. In the view of estimating the degeneracy in the system, the decimal positions are increased.

Sensitivity Analysis in Zemax {#sect:sensitivity}
=============================

Procedure
---------

The prerequisite of modeling the observations in Zemax is to estimate the RV sensitivities of the optical components [@tolerancing]. For matching observations with Zemax model and construction of *merit function*, the sensitivity of the optical components in echelle dispersion direction is estimated [@tolerancing].

A Th-Ar spectrum is taken for sensitivity analysis and the wavelength position of the isolated peaks is determined. Figure 