Sensing starlight rejected from a coronagraph is essential in stabilizing the telescope pointing and wavefront drift, but performance is degraded for dim stars. Laser Metrology (MET) provides a different, complementary sensing method, one that can be used to measure changes in the alignment of the optics at high bandwidth, independent of the magnitude of the host star. Laser metrology measures changes in the separation of optical fiducial pairs, which can be separated by many meters. The principle of operations is similar to the laser metrology system used in LISA-Pathfinder to measure the in-orbit displacement between two test masses to a precision of ~10 picometers. In closed loop with actuators, MET actively maintains rigid body alignment of the front-end optics, thereby eliminating the dominant source of wavefront drift. Because MET is not photon starved, it can operate at high bandwidth and feed-forward secondary-mirror jitter to a fast-steering mirror, correcting line-of-sight errors. In the case of a segmented, active primary mirror, MET provides six degrees of freedom sensing, replacing edge sensors. MET maintains wavefront control even during attitude maneuvers, such as slews between target stars, thereby avoiding the need to repeat time-consuming speckle suppression. These features can significantly improve the performance and observational efficiency of future large-aperture space telescopes equipped with internal coronagraphs. We evaluate MET trusses for various proposed monolithic and segmented spacebased coronagraphs and present the performance requirements necessary to maintain contrast drift below 10-11.
This paper presents results of the feedback control design for JPL's Fast Steering Mirror (FSM) for the WFIRST- AFTA coronagraph instrument. The objective of this controller is to cancel jitter disturbances in the beam from the spacecraft to a pointing stability of 0.4 masec over the duration of the observation using a momentum- compensated FSM. The plant model for the FSM was characterized experimentally, and the sensor model is based on simulated modeling. The control approach is divided between feedback compensation of low frequency attitude control system (ACS) drift, and feedforward cancellation of high frequency tonal disturbances originating from reaction wheel excitation of the telescope structure. This paper will present various aspects of the controller design, plant characterization, sensor modeling, disturbance estimation, performance simulation, and preliminary experimental testing results.
The SIM-Planetquest (Space Interferometry Mission), currently under development at the Jet Propulsion Laboratory,
consists of two 6-meter baseline interferometers on a flexible truss. SIM's science goals require 1μas accuracy in its
astrometric measurements. To achieve this level of accuracy for detecting planets SIM built the Spectrum Calibration
Development Unit (SCDU) testbed. The testbed requires a white light point source with broadband spectrum. Before
each long test the spectrum on the camera must be calibrated. To achieve this task a laser light visible to camera was
coupled to the white light source. The light system needed pointing stability of better than 4 micro-radians and a
minimum optical power level at the fringe tracking camera. Due to stability requirement of the experiment, the setup,
including the point source is in a vacuum chamber. To get a broadband spectrum point source inside the vacuum
chamber white light from a multimode fiber was combined with laser light in free space to a photonics crystal fiber
(PCF). The output is a single mode, broadband, and Gaussian beam. This paper explains the details of such a design and
shows some of the results.
SCDU (Spectral Calibration Development Unit) is a vacuum test bed that was built and operated for the SIM-Planetquest
Mission and has successfully demonstrated the calibration of spectral instrument error to an accuracy of
better than 20 picometers. This performance is consistent with the 1 micro-arc second goal of SIM. The calibration
procedure demonstrated in the test bed is traceable to the SIM flight instrument. This article is a review of all aspects of
the design and operation of the hardware as well as the methodology for spectral calibration. Spectral calibration to
better than 20 picometers and implications for flight are discussed.
The Space Interferometry Mission (SIM) requires the control of the optical path of each interferometer with picometer
accuracy. Laser metrology gauges are used to measure the path lengths to the fiducial corner cubes at the siderostats.
Due to the geometry of SIM a single corner cube does not have sufficient acceptance angle to work with all the gauges.
Therefore SIM employs a double corner cube. Current fabrication methods are in fact not capable of producing such a
double corner cube with vertices having sufficient commonality. The plan for SIM is to measure the non-commonalty of
the vertices and correct for the error in orbit. SIM requires that the non-common vertex error (NCVE) of the double
corner cube to be less than 6 μm. The required accuracy for the knowledge of the NCVE is less than 1 μm. This paper
explains a method of measuring non-common vertices of a brassboard double corner cube with sub-micron accuracy.
The results of such a measurement will be presented.
The Space Interferometry Mission (SIM) System Testbed-3 has been integrated in JPL's new Optical & Interferometry Development Laboratory. The testbed consists of a three baseline stellar interferometer whose optical layout is functionally equal to SIM's current flight layout. The main testbed objective is to demonstrate nanometer class stability of fringes in the dim star, or science, interferometer while using path length & angle feed-forward control, and while the instrument is integrated atop a flight-like flexible structure. This work marks the first time an astrometric 3-baseline interferometer is tested in air and on a flight-like structure rather than on rigid optical tables. This paper discusses the system architecture, dim star fringe tracking, and the testbed's latest experimental results.
The Space Interferometry mission's nano-meter class System Testbed has implemented an external metrology system to monitor changes in the length & orientation of the science interferometer baseline vector, which cannot be monitored directly. The output of the system is used in real time fringe tracking of dim stars. This paper describes the external metrology system, its mathematical representation, limitations, and method for estimating the length & orientation of the science baseline vector. Simulations and current system performance are presented and discussed.
Kite is a system level testbed for the External Metrology System of the Space Interferometry Mission (SIM). The External Metrology System is used to track the fiducials that are located at the centers of the interferometer's siderostats. The relative changes in their positions needs to be tracked to an accuracy of tens of picometers in order to correct for thermal deformations and attitude changes of the spacecraft. Because of the need for such high precision measurements, the Kite testbed was build to test both the metrology gauges and our ability to optically model the system at these levels. The Kite testbed is a redundant metrology truss, in which 6 lengths are measured, but only 5 are needed to define the system. The RMS error between the redundant measurements needs to be less than 140pm for the SIM Wide-Angle observing scenario and less than 8 pm for the Narrow-Angle observing scenario. With our current testbed layout, we have achieved an RMS of 85 pm in the Wide-Angle case, meeting the goal. For the Narrow-Angle case, we have reached 5.8 pm, but only for on-axis observations. We describe the testbed improvements that have been made since our initial results, and outline the future Kite changes that will add further effects that SIM faces in order to make the testbed more representative of SIM.
Researches have suggested several techniques (ie.: pupil masking, coronography, nulling interferometry) for high contrast imaging that permit the direct detection and characterization of extrasolar planets. Our team at JPL, in previous papers, has described an instrument that will combine the best of several of these techniques: a single aperture visible nulling corograph. The elegant simplicity of this design enables a powerful planet-imaging instrument at modest cost. The heart of this instrument is the visible light nulling interferometer for producing deep, achromatic nulls over a wide optical band pass, and a coherent array of single mode optical fibers 2 that is key to suppressing the level of scattered light. Both of these key components are currently being developed and have
produced intial results. This paper will review, in detail, the design of the nulling interferometer experiment and review the latest experimental results. These results illustrate that we are well on our way to developing the fundamental components necessary for planned mission. Likewise, our results demonstrate that the current nulling levels are already consistent with final requirements.
The Space Interferometry Mission's (SIM) shared-baseline astrometric interferometer System Test Bed 3 (STB3) has been constructed at JPL. STB3's objective is to use two of its interferometers (guides) for low frequency (0 to 1 Hz) fringe stabilization in the third one (science). This approach - being proposed for the first time in the context of space based observatories - is needed given the dim nature of science stars to be observed by SIM. Fringe stability is mostly affected by the low frequency attitude motion of the test bed's instrument table, with the inevitable exception of instrument vibration, thermal drift, and atmospheric fluctuations. Relative changes in table attitude cause optical path changes in the guide interferometers, which are tracked, linearly combined and fed forward to the science interferometer's active delay line to stabilize its optical path. This technique for tracking fringes in the science interferometer is possible because the position of the guide stars relative to the science star is well known. This open loop fringe tracking technique is dubbed Path-length Feed Forward, or PFF. In STB3, current fringe stability in the science interferometer using the PFF technique is at 50 to 60 nanometers RMS (from 0 to 500 Hz). Compare this to 15 to 20 nm RMS fringe stability in the guide interferometers, which operate in closed loop mode. Vibration, thermal drift and atmospherics in the science and guide interferometers are largely eliminated with the use of an internal metrology system. By design, mechanical vibrations are above the bandwidth of the interferometer system, and are passively rejected. Nevertheless, the internal metrology system can easily reject current low-level vibrations in STB3 down to the 6-nanometer RMS level.
Fringe tracking error in the science interferometer due to atmospherics is currently about 40 nanometers RMS at frequencies below 1.0 Hz. In SIM, the error in this low frequency band must be no more than 6 nm RMS. This error arises because the optical path stabilized by the internal metrology system is not equal to taht of the starlight, so not all atmospheric fluctuations in the starlight path can be stabilized. Therefore, there is a need to reduce the strength of atmospheric fluctuations or to filter them from the PFF command. In STB3 the strength of atmospheric fluctuations is already reduced with the use of optical path enclosures, which brought these fluctuations down from ~170nm RMS to their current levels of ~66nm RMS with a spread of 20nm. Simulations show that signal to noise ratios are generally not sufficient to filter atmospheric errors on-line.
The SIM system testbed III (STB3) is a 3-baseline interferometer mounted on a full-scale flexible structure developed at the Jet Propulsion Laboratory. The goal of the testbed is to demonstrate angle and fringe tracking of a dim star by feeding-forward the information from two interferometers looking at bright guide stars. This paper presents the optical architecture of STB3 and the first results obtained with the first interferometers running.