A calibrated SED (from Calspec and others) of the observed star is converted from Flam units to photons number and convolved with a fiducial model of the atmosphere at the CTIO site. Then, the spectrum is converted into electrons using the QE of the camera and its resolution is downgraded to match the nominal (~.7pix) seeing of the observations.

The converging beam adds an extra dispersion as the defocus increases at larger wavelength \footnote

{Note on MonoCam tests, from S. Dagoret-Campagne \&al, LAL.}

.
$$\delta(\lambda) = \frac

{l} {f/D}

\left( \frac

{1}

{\sqrt{1-(\lambda/a)^2}} -1\right)$$
This corrective term is added to the forward modeling (result illustrated in file attached).