Study of CCD mosaic configurations for the ILMT: Astrometry and photometry of point sources in the absence of a TDI corrector

The image deformations induced by the time-delay integration (TDI) mode, used with a CCD camera at the prime focus of a Liquid Mirror Telescope (LMT), can be corrected for by a system of lenses, simultaneously with the optical aberrations. However, before adopting the design of a TDI corrector, we may wonder what is the importance of the TDI image deformations and how to possibly reduce these by choosing optimal mosaic configurations of CCDs, covering an equivalent field of view. In a first step, we have simulated the point spread function (PSF) deformation due to the TDI mode only, for different selected mosaic configurations. We have then estimated the importance of the astrometric and photometric errors induced by the TDI deformation. By simulating star fields with the same synthetic PSF and taking into account realistic sources of noise (cf. CCD readout noise, photon noise and sky background), we have derived the limiting magnitudes for photometry and astrometry for the case of a 4 m LMT.