Estimating White-Light-lnterferometer Phases in Dim Light

A method has been developed to increase the accuracy of estimates of phase differences attributable to the optical-path-length difference between the arms of a long-baseline, white-light interferometer. The method is intended for use in the Space Interferometer Mission (SIM), in which there are requirements to (1) keep the phase error averaged over a 30-second integration time low enough to correspond to a path-length error-30 pm and (2) estimate phase differences at a rate of 1 kHz for use in feedback control of the optical-path-length difference of the arms of the interferometer, even when the light is from a distant star or other source that is so dim that the amount of light received from the source amounts to as few as 240 photons per millisecond. The algorithms were developed for monochromatic light, since the combined light is sent through a prism so that light impinging on a single pixel of the detector is nearly monochromatic. Techniques are then used to combine several different monochromatic results into a single, more accurate phase estimate.