"Virtual MOONS", a focal plane simulator for the MOONS thousand-fiber NIR spectrograph

MOONS will be the next near infrared fiber fed multi-object spectrograph for the Very Large Telescope, that will offer a one thousand multiplexing capability and a simultaneous coverage of the wavelength range from 0.8 to 1.8 μm. With the aim of quantitatively i) assessing the instrument performances with respect to sensitivity and OH subtraction, ii) blind-testing the 1D spectra extraction and calibration, provided by the data reduction pipeline, and iii) testing the technical solutions adopted for reaching the outstanding instrument requirements, we have developed “Virtual MOONS”, an end-to-end software simulator, which quantitatively computes high fidelity focal plane raw images, emulating the output of the detector electronics. Starting from an ideal photon image derived from the geometrical optics propagation and Point Spread Function (PSF) variations computed by the ZEMAX optical design, the end-to-end optical budget is introduced along with the stray light contributions, resulting in the expected photon counts impinging the detector pixels. Then the photon image plus photon noise is converted to digital counts by means of a detailed detector simulation, including pixel-to-pixel response variation, dark, bias, read-out noise, cosmetics, charge diffusion, flatness and read-out schemes. Critical points like fiber differential response, PSF haloes and sky emission variations have been also taken into account. The current status of this work is presented with an example simulated image and numerical results.