The standard approach to achieving TPF-level starlight suppression has been to couple a few techniques together. Deployment of a
low- or medium-performance external occulter as the first stage of starlight suppression reduces manufacturing challenges, mitigates
under-performance risks, lowers development costs, and hastens launch date for TPF. This paper describes the important aspects of a
conceptual 4-metre apodized square aperture telescope system utilizing a low-performance external occulter. Adding an external
occulter to such a standard TPF design provides a benefit that no other technique offers: scattered and diffracted on-axis starlight
is suppressed by orders of magnitude before reaching the telescope. This translates directly into relaxed requirements on the
remainder of the optical system.
APEX is a proposed mission for a Small Explorer (SMEX) satellite. APEX will investigate the density, temperature, composition, magnetic field, structure, and dynamics of hot astrophysical plasmas (log T = ~5-7), which emit the bulk of their radiation at EUV wavelengths and produce critical spectral diagnostics not found at other wavelengths. APEX addresses basic questions of stellar evolution and galactic structure through high-resolution spectroscopy of white dwarf stars, cataclysmic variables, the local interstellar medium, and stellar coronae. Thus APEX complements the Chandra, Newton-XMM, FUSE, and CHIPS missions. The instrument is a suite of 8 near-normal incidence spectrometers (~90-275 Angstroms, resolving power ~10,000, effective area 30-50 cm2) each of which employs a multilayer-coated ion-etched blazed diffraction grating and a microchannel plate detector of high quantum efficiency and high spatial resolution. The instrument is mounted on a 3-axis stabilized commercial spacecraft bus with a precision pointing system. The spacecraft is launched by a Taurus vehicle, and payload size and weight fit comfortably within limits for the 2210 fairing. Of order 100 targets will be observed over the baseline mission of 2 years. These are selected carefully to maximize scientific return, and all were detected in the EUVE and the ROSAT WFC surveys.
In this manuscript, we further develop our concepts for the free-flying occulter space-based mission, the Umbral Missions Blocking Radiating Astronomical Sources (UMBRAS). Our optical simulations clearly show that an UMBRAS-like mission designed around a 4-m telescope and 10-m occulter could directly image terrestrial planets. Such a mission utilizing existing technology could be built and flown by the end of the decade. Moreover, many of the other proposed concepts for Terrestrial Planet Finder (TPF) could significantly benefit by using an external occulter.
We present simultations for an optical design comprising a square aperture telescope plus square external occulter. We show that the entire diffraction pattern, which is propagated from occulter to telescope and through telescope to focal plane, may be characterized by two parameters, the Fresnel number and the ratio of the telescope diameter to the occulter width. Combining the effects of a square occulter with apodization provides a much more rapid roll-off in the PSF intensity between the diffraction spikes than may be achieved with an unapodized telecope aperture and occulter. We parameterize our results with respect to wavefront quality and compare them against other competing methods for exo-planet imaging. The combination of external occulter and apodization yields the required contrast in the region of the PSF essential for exo-planet detection.
An occulter external to the telescope (i.e., in a separate spacecraft, as opposed to a classical coronagraph with internal occulter) reduces light scatter within the telescope by approximately 2 orders of magnitude. This is due to less light actually entering the telescope. Reduced scattered light significantly relaxes the constraints on the mirror surface roughness, especially in the mid-spatial frequencies critical for planet detection. This study, plus our previous investigations of engineering as well as spacecraft
rendezvous and formation flying clearly indicates that the UMBRAS concept is very competitive with, or superior to, other proposed concepts for TPF missions.
We describe a 1-meter space telescope plus free-flying occulter craft mission that would provide direct imaging and spectroscopic observations of Jovian and Uranus-sized planets about nearby stars not detectable by Doppler techniques. The Doppler technique is most sensitive for the detection of massive, close-in extrasolar planets while the use of a free-flying occulter would make it possible to image and study stellar systems with planets comparable to our own Solar System. Such a mission with a larger telescope has the potential to detect earth-like planets. Previous studies of free-flying occulters reported advantages in having the occulting spot outside the telescope compared to a classical coronagraph onboard a space telescope. Using an external occulter means light scatter within the telescope is reduced due to fewer internal obstructions and less light entering the telescope and the polishing tolerances of the primary mirror and the supporting optics can be less stringent, thereby providing higher contrast and fainter detection limits.
In this concept, the occulting spot is positioned over the star by translating the occulter craft, at distances of 1,000 to 15,000 kms from the telescope, on the sky instead of by moving the telescope. Any source within the telescope field-of-view can be occulted without moving the telescope. In this paper, we present our current concept for a 1-m space telescope matched to a free-flying occulter, the Umbral Missions Blocking Radiating Astronomical Sources (UMBRAS) space mission. An UMBRAS space mission consists of a Solar Powered Ion Driven Eclipsing Rover (SPIDER) occulter craft and a matched (apodized) telescope. The occulter spacecraft would be semi-autonomous, with its own propulsion systems, internal power
(solar cells), communications, and navigation capability. Spacecraft rendezvous and formation flying would be achieved with the aid of telescope imaging, RF or laser ranging, celestial navigation inputs, and formation control algorithms.
We present a novel coronagraphic imaging technique and design for space-based telescopes. The Umbral Mission
Blocking Radiating Astronomical Sources (UMBRAS) is a space mission design consisting of a free flying occulter, the
Solar Powered Ion Driven Eclipsing Rover (SPIDER), and possibly one or two metrology platforms. The UMBRAS
spacecraft operate in conjunction with a space-based telescope. The size of the occulting SPIDER is dictated by
the size of the telescope with which it will work. The goal of UMBRAS is to provide "paleolithic" (i.e., non-focal
plane) coronagraphic capability to enable direct imaging of extrasolar Jovian planets and other bright substellar
companions such as brown dwarfs.
We discuss two aspects of the operation of a free flying occulter: acquisition of targets and station keeping. Target
acquisition is modeled after the onboard schemes used by Hubble Space Telescope (HST) science instruments. For
UMBRAS, the onboard commanding sequences would include imaging the field using instruments on the telescope,
locating the target and the occulter in the field, and accurately positioning the occulter over the target. Station
keeping consists of actively maintaining the occulter position in the telescope line of sight to the target.
Velocity matching of the c)cculter with the space-based telescope is essential to mission performance. An appropriate
combination of solar electric and cold gas thrusters provide the ability to match velocities using position
information derived from communication and from ranging data between telescope, occulter and any metrology
The accuracy requirements for target acquisition and station keeping depend upon the science requirements,
the occultation geometry, and the sensitivity of the science to changes in occultation geometry during an exposure
sequence. Observing modes other than the ideal centered occultation of a target will be discussed.
In this paper we discuss operational considerations for the free-flying occulter. Operations consist of maneuvering the Solar-Powered Ion-Driven Eclipsing Rover (SPIDER) between targets, alignment with the space-based telescope line of sight to the target, and stationkeeping target-to-target maneuvers need to be optimized to conserve propellant. A reasonable balance needs to be determined between target observation rate and the number of targets that are observable during mission lifetime. Velocity matching of the SPIDER with the telescope is essential to mission performance. An appropriate combination of solar electric and cold-gas thrusters provides the ability to match velocities using positional information derived from comminution and ranging between telescope, occulter and any metrology stations. Desirable features of using an external coronagraphic vehicle include the ability to obtain coronagraphic data with any instrument on the telescope-- imaging, spectroscopic, or interferometric.
Direct imaging of terrestrial and Jupiter-size planets about other stars is a major goal of NASA's Origins Program and should be as well for the next generation of spaceborne telescopes. In this paper, we discuss a free-flying occulter to augment the design and imaging capability of space-based telescopes. The Umbral Mission Blocking Radiating Astronomical Sources (UMBRAS) space mission would consist of a Solar- Powered Ion-Driven Eclipsing Rover (SPIDER) and possibly one or two metrology platforms. The UMBRAS spacecraft would be semi-autonomous, with their own propulsion systems, internal power (solar cells), communications, and navigation capability. The spacecraft (the telescope, SPIDER, and any metrology platform) would define a reference frame for aligning the telescope and the SPIDER with the observed target. When stationed at distances of 1,000 to 15,000 km from a telescope, the occulter will enable an 8 m telescope to image very faint sources as close as 0.15' from the target stars. Three of the Doppler-detected planets about nearby stars are at this separation and could be directly imaged with this observing technique. It would be possible to image giant planets as close as 5 Au from parent stars at distances from the Sun as great as 30 pc. With this technique, terrestrial- size planets could be detected around nearby stars within the next decade. We briefly discuss the diffraction effects caused by the occulter and a preliminary proof-of-concept design for the UMBRAS spacecraft. Finally, we suggest types of observations other than planet finding that could be performed with UMBRAS.