Estimated performance of PRIMA with the UTs

Simulations of PRIMA performance with the UTs have not been undertaken, however some estimates of the probable performance can be made using general properties of optical interferometers. Figure 25 shows the light collected through a spatial filter per coherence time of the interference fringes for a typical interferometer with different levels of AO correction (but all under the same seeing conditions). In this case the AO correction involved ideal compensation of a finite number of Zernike modes (with the modes matched to the aperture diameter used). The decrease in signal for large apertures and low-order AO correction is due to the reduced coherence time for the fringes when the variance in the wavefront phase across the aperture increases beyond $1$ radian. It is not clear which (if any) of these curves would correspond to the MACAO system. It is interesting to note that the S/N ratio is optimised when the Strehl ratio is $\sim 30\%$ for all the AO correction models shown here. More realistic simulations of the VLTI would probably be required in order to optimise the operation of PRIMA using the UTs.

図 25: The photon count through a spatial filter per coherence timescale normalised so that a diffraction-limited aperture of diameter $r_{0}$ would give unity. The labels on the lines indicate the number of Zernike modes corrected from tip-tilt upwards (describing the level of AO correction). The curve labelled ``P'' corresponds to the result expected for piston mode only simulations (i.e. with ideal AO correction - see e.g. [9]). All curves are for the same seeing conditions.