Exo-S is a direct imaging space-based mission to discover and characterize exoplanets. With its modest size, Exo-S bridges the gap between census missions like Kepler and a future space-based flagship direct imaging exoplanet mission. With the ability to reach down to Earth-size planets in the habitable zones of nearly two dozen nearby stars, Exo-S is a powerful first step in the search for and identification of Earth-like planets. Compelling science can be returned at the same time as the technological and scientific framework is developed for a larger flagship mission. The Exo-S Science and Technology Definition Team studied two viable starshade-telescope missions for exoplanet direct imaging, targeted to the $1B cost guideline. The first Exo-S mission concept is a starshade and telescope system dedicated to each other for the sole purpose of direct imaging for exoplanets (The "Starshade Dedicated Mission"). The starshade and commercial, 1.1-m diameter telescope co-launch, sharing the same low-cost launch vehicle, conserving cost. The Dedicated mission orbits in a heliocentric, Earth leading, Earth-drift away orbit. The telescope has a conventional instrument package that includes the planet camera, a basic spectrometer, and a guide camera. The second Exo-S mission concept is a starshade that launches separately to rendezvous with an existing on-orbit space telescope (the "Starshade Rendezvous Mission"). The existing telescope adopted for the study is the WFIRST-AFTA (Wide-Field Infrared Survey Telescope Astrophysics Focused Telescope Asset). The WFIRST-AFTA 2.4-m telescope is assumed to have previously launched to a Halo orbit about the Earth-Sun L2 point, away from the gravity gradient of Earth orbit which is unsuitable for formation flying of the starshade and telescope. The impact on WFIRST-AFTA for starshade readiness is minimized; the existing coronagraph instrument performs as the starshade science instrument, while formation guidance is handled by the existing coronagraph focal planes with minimal modification and an added transceiver.
Motivated by astrobiological remote sensing needs, Sparks et al. (2012)1 present an approach to spectropolarimetry which offers the prospect of high sensitivity over a very wide wavelength range (UV, optical, IR). Using static, robust components the polarization information is encoded onto one dimension of a two-dimensional data array, while the other dimension records the spectrum. A spatially varying retardance along the spectrograph slit, followed by a polarization analyzer, encodes the Stokes parameters as coefficients of orthogonal trigonometric functions perpendicular to the spectrum. No moving parts are required and all polarimetric information is available on a single data frame, hence the technique is immune to time dependencies, free of fragile modulating components, has the potential for high sensitivity while offering a wide wavelength range with full Stokes spectropolarimetry. Within the Solar System, spectropolarimetry offers diagnostics for dust (cometary, zodiacal, rings), surfaces (rocky, regolith, icy), aerosols (clouds, dust storms) and high energy plasma emission processes. Beyond the Solar System, space-based telescopic spectropolarimetry has important contributions to make in the detection of extrasolar planets and their characterization. There are astrobiological applications for full Stokes polarimetry stemming from the interaction of light with chiral living organisms, which offers the potential for a remote sensing detection capability for microbial life.
The infrastructure available on the ISS provides a unique opportunity to develop the technologies necessary to assemble
large space telescopes. Assembling telescopes in space is a game-changing approach to space astronomy. Using the ISS
as a testbed enables a concentration of resources on reducing the technical risks associated with integrating the
technologies, such as laser metrology and wavefront sensing and control (WFSandC), with the robotic assembly of major
components including very light-weight primary and secondary mirrors and the alignment of the optical elements to a
diffraction-limited optical system in space. The capability to assemble the optical system and remove and replace
components via the existing ISS robotic systems such as the Special Purpose Dexterous Manipulator (SPDM), or by the
ISS Flight Crew, allows for future experimentation as well as repair if necessary. In 2015, first light will be obtained by
the Optical Testbed and Integration on ISS eXperiment (OpTIIX), a small 1.5-meter optical telescope assembled on the
ISS. The primary objectives of OpTIIX include demonstrating telescope assembly technologies and end-to-end optical
system technologies that will advance future large optical telescopes.
We have developed a polarimeter for accurately measuring both the circular and linear polarization components of a
light beam from 400 nm to 800 nm. This polarimeter is designed to work at low light levels that are typical in
astronomical applications. It is optimized to detect the circular polarization signal that is orders of magnitude weaker
than the linear polarization signal. Two photoelastic modulators (PEMs) are the key polarization components employed
in this polarimeter to afford the high sensitivity required for the application. Using this instrument, we have quantified
the circular polarization signal produced by astrobiologically relevant microorganisms and compared the results to
macroscopic vegetation (such as leaves) and abiotic minerals. Our aim is to understand whether circular polarization
offers a viable technique for remote detection of chiral signatures and hence will be useful as an element of telescopic
searches for life elsewhere in the Universe. We see unambiguous circular polarization from photosynthetic microbes.
The circular polarization of reflected light is related to the circular dichroism of photosynthetic molecules. Therefore,
circular polarization spectroscopy offers the prospect of remotely sensing life's unique chiral signature.
The Extrasolar Planetary Imaging Coronagraph (EPIC) is a proposed NASA Discovery mission to image
and characterize extrasolar giant planets in orbits with semi-major axes between 2 and 10 AU. EPIC will
provide insights into the physical nature of a variety of planets in other solar systems complimenting radial
velocity (RV) and astrometric planet searches. It will detect and characterize the atmospheres of planets
identified by radial velocity surveys, determine orbital inclinations and masses, characterize the
atmospheres around A and F type stars which cannot be found with RV techniques, and observe the inner
spatial structure and colors of debris disks. EPIC has a proposed launch date of 2012 to heliocentric Earth
trailing drift-away orbit, with a 3 year mission lifetime (5 year goal), and will revisit planets at least three
times at intervals of 9 months. The robust mission design is simple and flexible ensuring mission success
while minimizing cost and risk. The science payload consists of a heritage optical telescope assembly
(OTA), and visible nulling coronagraph (VNC) instrument. The instrument achieves a contrast ratio of 109
over a 4.84 arcsecond field-of-view with an unprecedented inner working angle of 0.14 arcseconds over the
spectral range of 440-880 nm, with spectral resolutions from 10 - 150. The telescope is a 1.5 meter offaxis
Cassegrain with an OTA wavefront error of λ/9, which when coupled to the VNC greatly reduces the
requirements on the large scale optics, compressing them to stability requirements within the relatively
compact VNC optical chain. The VNC features two integrated modular nullers, a spatial filter array (SFA),
and an E2V-L3 photon counting CCD. Direct null control is accomplished from the science focal
mitigating against complex wavefront and amplitude sensing and control strategies.
The CorECam Instrument Concept Study (ICS) addressed the requirements and science program for the
Terrestrial Planet Finder Coronagraph's (TPF-C) primary camera. CorECam provides a simple interface to
TPF-C's Starlight Suppression System (SSS) which would be provided by the TPF-C Program, and
comprises camera modules providing visible, and near-infrared (NIR) camera focal plane imaging. In its
primary operating mode, CorECam will conduct the core science program of TPF-C, detecting terrestrial
planets at visible wavelengths. CorECam additionally provides the imaging capabilities to characterize
terrestrial planets, and conduct an extended science program focused on investigating the nature of the
exosolar systems in which terrestrial planets are detected. In order to evaluate the performance of CorECam
we developed a comprehensive, end-to-end model using OSCAR which has provided a number of key
conclusions on the robustness of the TPF-C baseline design, and allows investigation of alternative
techniques for wavefront sensing and control. CorECam recommends photon counting detectors be
baselined for imaging with TPF-C since they provide mitigations against the background radiation
environment, improved sensitivity and facilitate alternative WFSC approaches.
The Extrasolar Planetary Imaging Coronagraph (EPIC) will provide the first direct measurements of a broad range of fundamental physical characteristics of giant planets in other solar systems. These characteristics include orbital inclination, mass, brightness, color, the presence (or absence) of CH4 and H2O, and orbital or rotational-driven variability. EPIC utilizes a 1.5 meter telescope coupled to a Visible Nulling Coronagraph to achieve these science goals. EPIC has been proposed as a Discovery Mission.
We present an overview of the ACS on-orbit performance based on the calibration observations taken during the first three months of ACS operations. The ACS meets or exceeds all of its important performance specifications. The WFC and HRC FWHM and 50% encircled energy diameters at 555 nm are 0.088" and 0.14", and 0.050" and 0.10". The average rms WFC and HRC read noises are 5.0 e- and 4.7 e-. The WFC and HRC average dark currents are ~ 7.5 and ~ 9.1 e-/pixel/hour at their operating temperatures of - 76°C and - 80°C. The SBC + HST throughput is 0.0476 and 0.0292 through the F125LP and F150LP filters. The lower than expected SBC operating temperature of 15 to 27°C gives a dark current of 0.038 e-/pix/hour. The SBC just misses its image specification with an observed 50% encircled energy diameter of 0.24" at 121.6 nm. The ACS HRC coronagraph provides a 6 to 16 direct reduction of a stellar PSF, and a ~1000 to ~9000 PSF-subtracted reduction, depending on the size of the coronagraphic spot and the wavelength. The ACS grism has a position dependent dispersion with an average value of 3.95 nm/pixel. The average resolution λ/Δλ for stellar sources is 65, 87, and 78 at wavelengths of 594 nm, 802 nm, and 978 nm.
The Advanced Camera for Surveys (ACS) is a third generation instrument for the Hubble Space Telescope (HST). It is currently planned for installation in HST during the fourth servicing mission in Summer 2001. The ACS will have three cameras.
The Advanced Camera for the Hubble Space Telescope has three cameras. The first, the Wide Field Camera, will be a high- throughput, wide field, 4096 X 4096 pixel CCD optical and I-band camera that is half-critically sampled at 500 nm. The second, the High Resolution Camera (HRC), is a 1024 X 1024 pixel CCD camera that is critically sampled at 500 nm. The HRC has a 26 inch X 29 inch field of view and 29 percent throughput at 250 nm. The HRC optical path includes a coronagraph that will improve the HST contrast near bright objects by a factor of approximately 10 at 900 nm. The third camera, the solar-blind camera, is a far-UV, pulse-counting array that has a relatively high throughput over a 26 inch X 29 inch field of view. The advanced camera for surveys will increase HST's capability for surveys and discovery by a factor of approximately 10 at 800 nm.
A review of the capabilities of the near-IR camera and multi-object spectrometer (NICMOS) on the Hubble Space Telescope and of the environment in which the instrument operates is presented, together with the observing strategies which can be adopted to maximize the scientific return from the instrument. The wavelength dependence and intensity of the background detected by NICMOS, cosmic-ray impacts, image quality, and photometric calibrations are discussed. Examples of actual observations performed with NICMOS in wavebands which are difficult to access from the ground are shown.
We describe the on-orbit characterization of the HgCdTe detectors aboard NICMOS. The flat-field response is strongly wavelength dependent, and we show the effect of this on the photometric uncertainties in data, as well as the complications it introduces into calibration of slitless grism observations. We present the first rigorous treatment of the dark current as a function of exposure time for HgCdTe array detectors, and show that they consist of three independent components which we have fully characterized - a constant component which is the true dark current, an 'amplifier glow' component which results from operation of the four readout amplifiers situated near the detector corners and injects a spatially dependent signal each time the detector is non-destructively read out, and finally the 'shading', a component well known in HgCdTe detectors which we show is simply a pixel dependent bias change whose amplitude is a function of the time since the detector was last non-destructively read out. We show that with these three components fully characterized, we are able to generate 'synthetic' dark current images for calibration purposes which accurately predict the actual performance of the three flight detectors. In addition, we present linearity curves produced in ground testing before launch. Finally, we report a number of detector related anomalies which we have observed with NICMOS some of which have limited the observed sensitivity of the instrument, and which at the time of writing are still not fully understood.
The Advanced Camera for the Hubble Space Telescope will have three cameras. The first, the Wide Field Camera, will be a high throughput (45% at 700 nm, including the HST optical telescope assembly), wide field (200' X 204'), optical and I-band camera that is half critically sampled at 500 nm. The second, the High Resolution Camera (HRC), is critically sampled at 500 nm, and has a 26' X 29' field of view and 25% throughput at 600 nm. The HRC optical path will include a coronagraph which will improve the HST contrast near bright objects by a factor of approximately 10. The third camera is a far ultraviolet, Solar-Blind Camera that has a relatively high throughput (6% at 121.6 nm) over a 26' X 29' field of view. The Advanced Camera for Surveys will increase HST's capability for surveys and discovery by at least a factor of ten.
The Wide Field Planetary Camera (WF/PC) onboard the Hubble Space Telescope contains contaminants which condense on the windows in front of each CCD detector. These contaminants are UV opaque and increase with time to the extent that after several months they block 50% of the flux at 300 nm. Also, when the contaminants are warmed above -40 degree(s)C and then returned to the normal CCD operating temperature of -87 degree(s)C, particles form and severely degrade the image quality. The windows may be temporarily cleaned by raising their temperature to 0 degree(s)C. However, this results in a change in the structure of the flat field due to the partial removal of the UV flood which was applied after launch to suppress Quantum Efficiency Hysteresis in the CCDs. Repeated decontaminations will reintroduce the QEH and necessitate another time consuming UV flood and recalibration of the instrument. After 22 months of on-orbit operation, the contaminants could no longer be fully removed by the decontamination procedure. This paper describes the current state of the contaminants, what has been deduced concerning their properties and sources, the results of our efforts to remove them, and some lessons for future space-based instruments using cryogenic UV sensitive detectors.
An overview of the Faint Object Camera and its performance to date is presented. In particular, the detector's efficiency, the spatial uniformity of response, distortion characteristics, detector and sky background, detector linearity, spectrography, and operation are discussed. The effect of the severe spherical aberration of the telescope's primary mirror on the camera's point spread function is reviewed, as well as the impact it has on the camera's general performance. The scientific implications of the performance and the spherical aberration are outlined, with emphasis on possible remedies for spherical aberration, hardware remedies, and stellar population studies.