While HST’s planning and scheduling processes are mature, JWST’s–with a planned 2018 launch–are still in
development. The STScI science, engineering, software, and operations teams are working together to get the JWST
planning and scheduling systems up and running in the next few years. Here, we review the improvements made to
HST’s planning and scheduling processes over the past three decades, as well as the current state of the observing
program. Also, differences between the two telescopes are discussed, as well as how they affect the creation of the
JWST planning and scheduling system.
Architecture choices impact planning and scheduling of activity sequences for two widely separated spacecraft
envisioned to be part of an astrophysics mission to observe extra-solar-planets. The two spacecraft consist of a large
space telescope and an external occulter, separated by tens of thousands of kilometres. The science need is to maintain
alignment at the tens of milliarcseconds level (~ metres) or less on given target stars after moving one of the spacecraft
tens of thousands of kilometres. Doing this efficiently presents operational and architectural design challenges that rely
on appropriate choice of navigation, propulsion, and alignment technologies, vehicle configuration, and activity
scheduling strategies—an extensive combination of which may potentially be chosen from for such a mission.
Challenges inherent in the general system architecture are described with emphasis on potential problems and the need
for sound and appropriate integration of architecture planning, subsystem choice, and activity scheduling.
The observer program implementation, planning, and scheduling subsystems are undergoing software development for
the James Webb Space Telescope front-end ground segment and are being tested in an integrated fashion. This part of
the ground system leverages what was developed and fine-tuned for the Hubble Space Telescope over previous decades.
This paper will describe the testing design, methods, results, plus the current capabilities and elements still to be
developed for these subsystems through the time of publication. We will point out elements from Hubble's systems,
from an operations perspective, which have been preserved for the new telescope, and those which require
A starshade with the James Webb Space Telescope (JWST) is the only possible path forward in the next
decade to obtain images and spectra of a planet similar to the Earth, to study its habitability, and search for
signs of alien life. While JWST was not specifically designed to observe using a starshade, its near-infrared
instrumentation is in principle capable of doing so and could achieve major results in the study of terrestrialmass
exoplanets. However, because of technical reasons associated with broadband starlight suppression and
filter red-leak, NIRSpec would need a slight modification to one of its target acquisition filters to enable feasible
observations of Earth-like planets. This upgrade would 1) retire the high risk associated with the effects of the
current filter red leak which are difficult to model given the current state of knowledge on instrument stray light
and line spread function at large separation angles, 2) enable access to the oxygen band at 0.76 μm in addition
to the 1.26 μm band, 3) enable a smaller starshade by relaxing requirements on bandwidth and suppression 4)
reduce detector saturation and associated long recovery times. The new filter would not affect neither NIRSpecs
scientific performance nor its operations, but it would dramatically reduce the risk of adding a starshade to JWST
in the future and enhance the performance of any starshade that is built. In combination with a starshade, JWST
could be the most capable and cost effective of all the exoplanet hunting missions proposed for the next decade,
including purpose built observatories for medium-size missions.
The James Webb Space Telescope will be an extraordinary observatory, providing a huge range of exciting new
astrophysical results. However, by itself it will not be capable of directly imaging planets in the habitable zone of
nearby stars, one of the most fascinating goals of astronomy for the coming decade. In this paper we discuss the
New Worlds Probe (NWP) concept whereby we use an external occulter (or starshade) to cast a shadow from
the star onto the telescope, therefore canceling the direct star light while the light from a planet is not affected.
This concept enables JWST to take images and spectra of extrasolar planets with sufficient contrast and inner
working angle to be able to discover planets down to the size of the Earth in the habitable zone around nearby
stars. JWST's instruments are appropriate to achieve low resolution spectroscopy (R ≅ 40) of these planets, and
address a series of fundamental questions: are there planets in the habitable zone around nearby stars? What
is the composition of their atmosphere? What are the brightness and structures of exozodiacal disks around
nearby stars? What is the mass and composition of currently known giant planets? In this paper we study the
starshade optimization for JWST given the instrumental constraints, and show that the modest optical quality
of the telescope at short wavelength does not impact the possibility of using a starshade. We propose a solution
to enable imaging and spectroscopy using target acquisition filters. We discuss possible time allocation among
science goals based on exposure time estimates and total available observing time. The starshade can be launched
up to 3 years after JWST and rendezvous with the telescope in orbit around L2.
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.
Introduction of the Large Proposal category for HST observing in Cycle 11 resulted in a significant migration toward multiple observing programs requiring 100 or more orbits on single target areas. While relatively benign in the inaugural Cycle, this policy created a formidable planning problem in Cycle 12 due to acceptance of several large programs with identical or closely located targets. The nature of this observing pool revealed shortcomings in the established processes for building an integrated HST science plan. Historically it has not been difficult to normalize individual programs within the overall HST observing plan. However, conflicts arising from competing demands and overlapping time windows in Cycle 12 necessitated compromises between programs at a more significant scale than experienced ever before. The planning tools and techniques needed to change rapidly in response, and communication both within the STScI and between the STScI and the affected observers was more crucial than ever before. Large and small-scale changes to major observing programs were necessary to create a viable integrated observing plan. This paper describes the major features of the Cycle 12 observing pool, the impact it had on the STScI front-end operations processes and how an executable Cycle 12 HST observing program was achieved.
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.
In an era of increasing pressure to do more with less and make the most out of every budget dollar, HST science operations have steadily been able to give its customers more by increasing observatory efficiency. While original mission goals for observatory efficiency were targeted at less than 35%, HST now consistently achieves weekly schedules which are greater than 50% efficient. Furthermore, special concentration on continuous viewing opportunities and science campaigns (i.e. -- the Hubble Deep Field) has yielded efficiencies exceeding 60%. More than fourteen years of applied operational experience and system analysis by HST ground, flight, instrument, and user support systems personnel have resulted in the success. However, these efficiency levels could be even higher were it not for the variety of constraints and unplanned events which affect how and when the observatory can be used. Certain known spacecraft and instrument constraints impact efficiency with little effect on long range plan stability since they can be accounted for in advance. For this class of concerns, planning scenarios can be developed and analyzed to see what efficiencies might be achieved without these constraints. Unpredictable events such as spacecraft safings and anomalies, targets of opportunity and quick turnaround director's discretionary science reduce the overall stability of an observatory's planned use as well as its efficiency. In this paper we will describe various constraints and unplanned events, show their effects on HST observatory efficiency and stability, and discuss specific efforts of the HST Long Range Planning group to minimize their impact.
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.