Miltiadis Papalexandris

Overview of SIM wide-angle astrometric calibration strategies

This paper summarizes two different strategies envisioned for calibrating the systematic field dependent biases present in the Space Interferometry Mission (SIM) instrument. The Internal Calibration strategy is based on pre-launch measurements combined with a set of on-orbit measurements generated by a source internal to the instrument. The External Calibration strategy uses stars as an external source for generating the calibration function. Both approaches demand a significant amount of innovation given that SIMs calibration strategy requires a post-calibration error of 100 picometers over a 15 degree field of regard while the uncalibrated instrument introduces tens to hundreds of nanometers of error. The calibration strategies are discussed in the context of the wide angle astrometric mode of the instrument, although variations on both strategies have been proposed for doing narrow angle astrometry.

Description of a proposed on-orbit calibration procedure for SIM based on spacecraft maneuver

The astrometric performance of the SIM relies on precise measurements of the optical pathlength difference of the starlight through the arms of the interferometers that comprise the SIM instrument, and on precise relative distance betweeen a set of fiducials that define the baselines of the interferometers. The accuracy of these measurements can be affected by various phenomena. Some of them are time-dependent, while others are relatively static and repeatable. In this work we are concerned with the instrument errors of the latter type and in their compensation. In particular, a procedure for on-orbit calibration of the instrument error function is defined, and a proof of concept of its viability is presented. On a given grid of stars, the proposed procedure generates approximations of the gradient of the instrument error function at a discrete set of field points corresponding to the star locations via a specialized set of maneuvers of the spacecraft. These gradient approximations are then used to estimate the error function via a least squares procedure in a manner that is very analogous to the wavefront reconstruction problem in adaptive optics systems. An error analysis of the procedure is presented providing further insights into the connections between instrument errors and the grid reduction solution. Finally, numerical results are presented on a randomly generated grid of stars that demonstrate the feasibility of the method.

Average phase calculations with near-field diffraction algorithms for the Space Interferometry Mission

This work reports on the computation of the average phase of a beam over an optical element via discrete Fourier transform techniques. The objective is to develop accurate diffraction models for the Space Interferometry Mission (SIM). Applications related to SIM include calibration of metrology measurements, evaluation of cornercube diffraction effects, and others. The algorithms that are used to compute the field are described and numerical tests that assess their accuracy are presented.

LISA telescope sensitivity analysis

The Laser Interferometer Space Antenna (LISA) for the detection of Gravitational Waves is a very long baseline interferometer which will measure the changes in the distance of a five million kilometer arm to picometer accuracies. As with any optical system, even one with such very large separations between the transmitting and receiving telescopes, a sensitivity analysis has to be performed to see how, in this case, the far field phase varies when the telescope parameters change as a result of structural stress relief or temperature changes. The results of the sensitivity analysis are presented.

Numerical phase front propagation for the laser interferometer space antenna

The present article reports on numerical studies of phase front propagation for the Laser Interferometer Space Antenna (LISA). The main objective is to determine the sensitivity of the average phase of the metrology beam with respect to fluctuations of the pointing of the beam. For this purpose, the metrology beam is propagated numerically along the interferometric arm of the instrument. The effects of the obscurations from the secondary mirror and its supporting struts are studied in detail. Further, the effects of random wavefront distortions that occur due to imperfections of the optical elements are estimated through a series of Monte Carlo simulations. The results of this study can be used to determine design requirements for the instrument.

Optimization methods for thermal modeling of optomechanical systems

Numerical techniques for a class of optimization problems associated with the thermal modeling of optomechanical systems are presented. Emphasis is placed on applications where radiation plays a dominant role. This work is motivated by the need for incorporating thermal analysis into integrated modeling of high-precision, space-borne optical systems. The specific problems of interest are thermal control to minimize the wavefront error by application of external heat loads, and the temperature estimation problem of predicting temperatures at arbitrary nodes of the model given noisy measurements on a subset of nodes. The proposed numerical techniques are briefly described and compared to existing algorithms. Their accuracy and robustness are demonstrated through numerical tests with models form ongoing NASA missions.