The impact of geometric distortions in multiconjugate adaptive optics astrometric observations with future extremely large telescopes

Abstract

Astrometry is one of the main science case which drives the requirements of the next multiconjugate adaptive optics (MCAO) systems for future extremely large telescopes. The small diffraction limited point-spread function (PSF) and the high Signal-to-Noise Ratio (SNR) of these instruments, promise astrometric precision at the level of microarcseconds. However, optical distortions have to be as low as possible to achieve the high demanding astrometry requirements. In addition to static distortions, the optomechanical instabilities cause astrometric errors that can be major contributors to the astrometry error budget. The present article describes the analysis, at design level, of the effects of opto-mechanical instabilities when coupled with optical surface irregularities due to the manufacturing process. We analyse the notable example of the Multi-conjugate Adaptive Optics RelaY (MAORY) for the extremely large telescope (ELT). Ray-tracing simulations combined with a Monte Carlo approach are used to estimate the geometrical structure and magnitude of field distortion resulting from the optical design. We consider the effects of distortion on the MCAO correction showing that it is possible achieve the micro-arcseconds astrometric precision once corresponding accuracy is obtained by both optical design and manufacturing. We predict that for single-epoch observations, an astrometric error below 50 µas can be achieved for exposure times up to 2 min, provided about 100 stars are available to remove fifth-order distortions. Such performance could be reproducible for multi-epoch observations despite the time-variable distortion induced by instrument instabilities.

Performance and limitations of using ELT and MCAO for 50 μas astrometry

“…Also the optomechanical misalingments happening at the two system interfaces, ELT-MAORY and MAORY-MICADO, can produce defocus, PS, and pointing error that are efficiently removed by the MCAO loop. A study of the optomechanical stability of the MCAO module has been assessed by Patti et al, 31 and in a subsequent work, 32 the same authors studied the impact of the optical manufacturing residuals aberrations on the astrometry. For both tolerance types, an astrometric error of ∼50 μas can be achieved for typical exposure times of ∼2 min using fifth-order polynomials.…”

“…The MICADO ∼50 μas astrometric requirement has been verified by simulation, and it also takes into account the variation of the distortion pattern at different field rotation positions. 32 In addition, the MAORY distortion on the NGS probes can also inject a spurious astrometric signal into the MICADO FoV. A rigid shift of the three NGS WFSs results in a drift of the field on MICADO, whereas a radial motion of the probes causes a PS change in the FoV.…”

“…A rigid shift of the three NGS WFSs results in a drift of the field on MICADO, whereas a radial motion of the probes causes a PS change in the FoV. 33 Patti et al 32 quantified at ∼0.5 mas (average value) the effect of tip-tilt and PS errors related to the NGS probe position variations. Higher order flexures have not been assessed yet and require finite-element analyses of the bench.…”