Figure 1 shows the WFIRST CGI’s optical function diagram including LOWFS/C.
WFIRST Coronagraph LOWFS/C sensor uses the rejected starlight reflected off the coronagraph’s focal plane occulting mask for wavefront sensing. We rely on the observatory integrate modeling (IM) for the expected LoS and low order WFE disturbances used in our testbed dynamic tests. During the WFIRST CGI operation the reaction wheel speeds are limit to operate between -5 to +5 rev/sec in order to limit the LoS jitter.
The WFIRST observatory ACS uses six reaction wheels, each has its own speed which depends on the condition of the momentum compensation. While most of the RWA induced LoS jitter energies contain at the wheels’ fundamental frequencies, the LoS jitter also have multiple harmonic and sub-harmonic frequencies. Because the slow variation of RWA speed, the LoS jitter is tonal with fundamental frequency corresponding to the RWA’s wheel speed. The fast LoS jitter is caused by vibrations from the ACS’s reaction wheels assemblies. The slow (< 1 Hz) LoS drift is from the telescope’s attitude control system (ACS) residual pointing error. Depending on the sources of disturbances, these wavefront errors contain both low and high temporal frequency components, with the low frequency (milli-Hz) WFE coming mostly from thermal load variation, and high frequency WFE from the observatory vibration disturbances such as the reaction wheel assemblies (RWA) used for WFIRST telescope pointing. The wavefront dynamics presented to the WFIRST coronagraph consists of wavefront errors in both the line-of-sight error (wavefront tilt) and low order wavefront aberrations such as focus, astigmatism, and coma. From the coronagraph performance requirements and error budget, the WFIRST LOWFS/C needs to have the LoS post correction residual less than 0.5 mas (milli-arcsecond) per axis and low order wavefront error less than 0.25 nm rms. In order to maintain the coronagraph contrast level and stability WFIRST CGI needs a low order wavefront sensing and control (LOWFS/C) subsystem to sense and correct wavefront instability. This stringent contrast stability requirement drives a very tight tolerance for the line-of-sight (LoS) and low order wavefront error (WFE) drift. Additionally, in order to differentiate planets from residual star light speckles in the dark hole and to measure the planets with an adequate signal-to-noise ratio, the coronagraph contrast needs to be stable at a level of ~10 -9 during the science observation. The WFIRST CGI requires the coronagraph to have raw contrast better than 10 -8. We will present the LOWFS/C LoS feedback and feed forward loops testbed performance under these realistic LoS disturbances in which we have demonstrated the LoS control that meets the CGI requirement.ġ.1 Introduction of Low order wavefront sensing and control for WFIRST CGIĪ high contrast coronagraph instrument is very sensitive to the wavefront error. We will describe the LoS disturbances profile and testbed implementation approach. In this paper we will describe the LOWFS/C LoS performance test in which the injected LoS disturbances are derived from all six reaction wheels on WFIRST, each with an independent varying wheel speed profile modeled for a typical coronagraph observational scenario. However, in our previous tests the LoS disturbances simulated on the testbed were from the modeling of a single reaction wheel with a quasi-static wheel speed. We have demonstrated that LOWFS/C can maintain coronagraph contrast to better than 10 -8 in presence of WFIRST-like LoS and low order WFE disturbances for both Hybrid Lyot Coronagraph (HLC) and Shaped Pupil Coronagraph (SPC) modes. The Occulting Mask Coronagraph (OMC) dynamic testbed is used to demonstrate LOWFS/C performance together with the coronagraphs. WFIRST Coronagraph Instrument (CGI) uses its Low Order Wavefront Sensing and Control (LOWFS/C) subsystem to maintain the coronagraph contrast in presence of wavefront disturbances from the WFIRST observatory.
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