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This paper describes a space mission for visible direct detection and spectroscopy of Earth like extrasolar planets using a nulling coronagraph instrument behind a moderately sized telescope in space.
TPF-I has set for itself a host of challenging technical milestones along its path to demonstrating the feasibility of infrared nulling for planet detection Progress in each of these areas of technical development will be reviewed as well as progress in meeting the overarching technical milestones.
We report progress on a nulling coronagraph intended for direct imaging of extrasolar planets. White light is suppressed in an interferometer, and phase errors are measured by a second interferometer. A 1020-pixel MEMS deformable mirror in the first interferometer adjusts the path length across the pupil. A feedback control system reduces deflections of the deformable mirror to order of 1 nm rms.
The key to broadband operation of the Visible Nulling Coronagraph (VNC) is achieving a condition of quasi- achromatic destructive interference between combined beams. Here we present efforts towards meeting this goal using Fresnel rhombs in each interferometric arm as orthogonally aligned half wave phase retarders. The milestone goal of the demonstration is to achieve 1 × 10−9 contrast at 2/D over a 40 nm bandpass centered at 633 nm. Rhombs have been designed and fabricated, and a multi-step approach to alignment using coarse positioners for each rhomb and pair has been developed to get within range of piezo stages used for fine positioning. The previously demonstrated narrowband VNC sensing and control approach that uses a segmented deformable mirror is being adapted to broadband to include fine positioning of the piezo-mounted rhombs, all demonstrated in a low-pressure environment.
Planet detection around a bright star depends the resolution of the imaging system and the degree of light suppression of the star relative to the planet. We present a concept for a visible light Terrestrial Planet Finding (VTPF) mission. Its major feature is an imaging system for planet detection using a nulling interferometer behind a single aperture telescope. This configuration is capable of detecting earth-like planets with a 5m aperture using both imaging and spectroscopic imaging modes. We will describe the principles of the system, and show results of studies demonstrating its feasibility.
Null-type vacuum microbalance for measuring minute mechanical displacements
Manual control mechanism for adjusting control rod to null position
Geomagnetic tail structure near null sheet, indicating decrease and/or dissipation of plasma sheet particle energy density at lunar distances
High temperature thermal null strain gage with sensing unit and electronic control unit to measure mechanical strain in terms of induced thermal strain
Techniques for producing very low and zero magnetic fields are considered, giving attention to the compensation of the geomagnetic field by a Helmholtz coil system, approaches utilizing the shielding power of highly permeable alloys, and the complete exclusion of the geomagnetic field with the aid of a superconductive shield. Animal experiments in low magnetic fields are discussed, together with the exposure of man to 'null' magnetic fields and the Josephson junction as a possible biosensor of magnetic fields. It is found that neither the functions nor the behavior of man changes significantly during a two-week exposure to magnetic fields below 50 gammas.
A line list is presented for magnetic null lines found in the Ultraviolet Multiplet Table and in selected spectral analyses for use in detailed studies of magnetic peculiar A star atmospheres.
A test of the Einstein equivalence principle (EEP) was performed by carrying out a 'null' gravitational red-shift experiment. The experiment compared the rates of a pair of hydrogen maser clocks with those of a set of three superconducting-cavity stabilized oscillator clocks as a function of the solar gravitational potential. If EEP were not valid, the relative rates could vary with potential. During the experiment, the solar potential in the laboratory varied approximately linearly at 3 parts in 10 to the 12th per day because of the earth's orbital motion, and diurnally with an amplitude of 3 parts in 10 to the 13th because of the earth's rotation. An upper limit on the relative frequency variation of 1.7 parts in 100 of the external potential was set. The accuracy was limited by the frequency stability of the clocks and by unmodeled environmental effects. The result is consistent with the EEP at the 2 percent level. The experiment can also be viewed as setting a limit on a possible spatial variation of the fine-structure constant.
Two-dimensional aerodynamic skin-friction force measured directly. Disk-shaped prototype design has overall dimensions of 1.25 (3.18 centimeters) diameter and 0.5 inch (1.27 centimeters) height. Unique mechanism consisting of two flexural pivoted arms connected in tandem but at right angle with each other designed to impart plane motion needed to sense two-axis flow over sensing element of balance. Sensing element, 0.370 inch (0.940 centimeter) in diameter, attached to end of second arm is servoed by two restoring-force motors orthogonally mounted in plane of airflow. Mechanism allows free plane motion of sensing element with no friction, and balance self-nulled to provide direct plane skin-friction force measurements continuously.
Measurement of planar skin friction forces in aerodynamic testing currently requires installation of two perpendicularly mounted, single-axis balances; consequently, force components must be sensed at two distinct locations. A two-axis instrument developed at the Langley Research Center to overcome this disadvantage allows measurement of a two-dimensional force at one location. This paper describes a feedback-controlled nulling circuit developed for the NASA two-axis balance which, without external compensation, is inherently unstable because of its low friction mechanical design. Linear multivariable control theory is applied to an experimentally validated mathematical model of the balance to synthesize a state-variable feedback control law. Pole placement techniques and computer simulation studies are employed to select eigenvalues which provide ideal transient response with decoupled sensing dynamics.
In three dimensions, magnetic reconnection may take place in a sheared magnetic field at any singular field line, where the nearby field has X-type topology in planes perpendicular to the field line and where an electric field is present parallel to the field line. In the ideal region around the singular line there will, in general, be singularities in the plasma flow and electric field, both at the singular line and at 'magnetic flipping layers', which are remnants of local magnetic separatrices. In the absence of a three-dimensional magnetic neutral point or null point, reconnection of field lines can still occur by a process of magnetic flipping, in which the plasma crosses the flipping layers but the field lines rapidly flip along them by magnetic diffusion. Depending on the boundary conditions, there may be two or four flipping layers which converge on the singular line. A boundary layer analysis of a flipping layer is given, in which the magnetic field parallel to the layer decreases as one crosses it while the plasma pressure (or magnetic pressure associated with the field along the singular line) increases. The width of the flipping layer decreases with distance from the singular line.
Circuit measures electrical current via combination of Hall-effect-sensing and magnetic-field-nulling techniques. Known current generated by feedback circuit adjusted until it causes cancellation or near cancellation of magnetic field produced in toroidal ferrite core by current measured. Remaining magnetic field measured by Hall-effect sensor. Circuit puts out analog signal and digital signal proportional to current measured. Accuracy of measurement does not depend on linearity of sensing components.
The optical design of the second-generation wide-field/planetary-camera instrument for the Hubble Space Telescope has been modified to compensate for the spherical aberration of the optical telescope assembly (OTA) by introduction of undercorrected spherical aberration into the wave front. This instrument can be tested in a simple manner to ensure that its aberration contribution has the proper sign and magnitude. We present designs for a near-zero power doublet lens that can be used to generate a spherically aberrated wave front that is similar to the OTA wave front. When this lens is used in combination with the instrument, a near-perfect or nulled wave front should be produced, resulting in a high-quality point image on axis. We also present lens designs for a similar test that can be performed on the OTA simulators now being built to verify the other second-generation instruments.
It should be possible to discover and to analyze the atmospheres of Earth-like planets of nearby stars, provided their thermal emission is not overwhelmed by a bright zodiacal cloud in the same system. Nulling interferometers in space, such as the Terrestrial Planet Finder, can efficiently suppress a stellar point source, but there is no way to suppress diffuse emission underlying the planet under study. A cloud much brighter than solar level would cause serious reduction in sensitivity, by its photon shot noise. Since some clouds have been detected that are orders of magnitude brighter, zodiacal measurements of TPF candidates to solar level are critically needed. These could be obtained relatively soon with the largest aperture ground-based interferometers observing in the 10 micron atmospheric window. A cloud would be detectable as an infrared excess, provided the stellar emission (some 10(exp 4) times brighter) is adequately suppressed by destructive interference. Ideally, the ground interferometer should have angular response similar to that of TPF, so as to measure directly the troublesome zodiacal component. The individual elements must be large, for even with approx. 8 m apertures, star cancellation and low thermal background, integration times of at least several hours will be needed to sense the clouds against shot noise from telescope thermal emission. In this paper we compare the sensitivity of TPF and ground based interferometers to a twin of the solar system at 10 pc, using Good's (1994) model of the zodiacal cloud.