A photon counting camera based on a modified Teledyne-e2v CCD201-20 electron multiplying CCD (EMCCD) is being developed for the NASA WFIRST coronagraph, an exoplanet imaging technology development of the Jet Propulsion Laboratory (Pasadena, CA). The coronagraph is designed to demonstrate technologies required to directly image planets around nearby stars, and to characterize their spectra. The planets are exceedingly faint, providing signals similar to the detector dark current, and require the use of photon counting detectors. Red sensitivity (600-980nm) is preferred to capture spectral features of interest. EMCCDs are baselined both as science and wavefront sensors in the coronagraph in order to simplify the system architecture. We are engaged in a technology development program with Teledyne-e2v to ruggedize the sensors for use in space. The modified detectors minimize the required number of charge transfers and the exposure of signal charge to trap defects in order to extend their lifetime in space. In this paper we will summarize our progress, program status, and plans for flight development.
As it has for the past few years, numerical modeling is being used to predict the on-orbit, high-contrast imaging performance of the WFIRST coronagraph, which was recently defined to be a technology demonstrator with science capabilities. A consequence has been a realignment of modeling priorities and revised applications of modeling uncertainty factors and margins, which apply to multiple factors such as pointing and wavefront jitter, thermally-induced deformations, polarization, and aberration sensitivities. At the same time, the models have increased in fidelity as additional parameters have been added, such as time-dependent pupil shear and mid-spatial-frequency deformations of the primary and secondary mirrors, detector effects, and reaction-wheel-speed-dependent pointing and wavefront jitter.
A photon counting camera based on the Teledyne-e2v CCD201-20 electron multiplying CCD (EMCCD) is being developed for the NASA WFIRST coronagraph, an exoplanet imaging technology development of the Jet Propulsion Laboratory (Pasadena, CA) that is scheduled to launch in 2026. The coronagraph is designed to directly image planets around nearby stars, and to characterize their spectra. The planets are exceedingly faint, providing signals similar to the detector dark current, and require the use of photon counting detectors. Red sensitivity (600-980nm) is preferred to capture spectral features of interest. EMCCDs are baselined both as science and wavefront sensors in the coronagraph in order to simplify the system architecture. We are engaged in a test program to characterize the performance of the EMCCD in the required modes, as well as in a technology development program with Teledyne-e2v to ruggedize the sensors for use in space. In this paper we will summarize our progress, program status, and plans for flight development.
The WFIRST coronagraph is being designed to detect and characterize mature exoplanets through the starlight reflected from their surfaces and atmospheres. The light incident on the detector from these distant exoplanets is estimated to be on the order of a few photons per pixel per hour. To measure such small signals, the project has baselined the CCD201 detector made by e2v, a low-noise and high-efficiency electron-multiplying charge-coupled device (EMCCD), and has instituted a rigorous test and modeling program to characterize the device prior to flight. An important consideration is detector performance degradation over the proposed mission lifetime due to radiation exposure in space. To quantify this estimated loss in performance, the project has built a detector trap model that takes into account detailed trap interactions at the sub-pixel level, including stochastic trap capture and release, and the deferment of charge into subsequent pixels during parallel and serial clocking of the pseudo-two-phase CCD201 device. This paper describes recent detector trap model improvements and modeling results.
Full characterization of imaging detectors involves subjecting them to spatially and temporally varying illumination patterns over a large dynamic range. Here we present a scene generator that fulfills many of these functions. Based on a modern smartphone, it has a number of good features, including high spatial resolution (13 um), high dynamic range (∼104 ), near-Poisson limited illumination stability over time periods from 100 ms to many days, and no background noise. The system does not require any moving parts and may be constructed at modest cost. We present the optical, mechanical, and software design, test data validating the performance, and application examples.
We present the latest developments in our joint NASA/CNES suborbital project. This project is a balloon-borne UV multi-object spectrograph, which has been designed to detect faint emission from the circumgalactic medium (CGM) around low redshift galaxies. One major change from FIREBall-1 has been the use of a delta-doped Electron Multiplying CCD (EMCCD). EMCCDs can be used in photon-counting (PC) mode to achieve extremely low readout noise (¡ 1e-). Our testing initially focused on reducing clock-induced-charge (CIC) through wave shaping and well depth optimisation with the CCD Controller for Counting Photons (CCCP) from Nüvü. This optimisation also includes methods for reducing dark current, via cooling and substrate voltage adjustment. We present result of laboratory noise measurements including dark current. Furthermore, we will briefly present some initial results from our first set of on-sky observations using a delta-doped EMCCD on the 200 inch telescope at Palomar using the Palomar Cosmic Web Imager (PCWI).
The WFIRST Coronagraph will be the most sensitive instrument ever built for direct imaging and characterization of extra-solar planets. With a design contrast expected to be better than 1e-9 after post processing, this instrument will directly image gas giants as far in as Jupiter's orbit. Direct imaging places high demand on optical detectors, not only in noise performance, but also in the need to be resistant to traps. Since the typical scene flux is measured in millielectrons per second, the signal collected in each practicable frame will be at most a few electrons. At such extremely small signal levels, traps and their effects on the image become extremely important. To investigate their impact on the WFIRST coronagraph mission science yield, we have constructed a detailed model of the coronagraph sensor performance in the presence of traps. Built in Matlab, this model incorporates the expected and measured trap capture and emission times and cross-sections, as well as occurrence densities after exposure to irradiation in the WFIRST space environment. The model also includes the detector architecture and operation as applicable to trapping phenomena. We describe the model, the results, and implications on sensing performance.
The Faint Intergalactic-medium Redshifted Emission Balloon (FIREBall-2) is an experiment designed to observe low density emission from HI, CIV, and OVI in the circum-galactic medium around low-redshift galaxies. To detect this diffuse emission, we use a high-efficiency photon-counting EMCCD as part of FIREBall-2's detector. The flight camera system includes a custom printed circuit board, a mechanical cryo-cooler, zeolite and charcoal getters, and a Nüvü controller, for fast read-out speeds and waveform shaping. Here we report on overall detector system performance, including pressure and temperature stability. We describe dark current and CIC measurements at several temperatures and substrate voltages, with the flight set-up.
The Keck Cosmic Web Imager (KCWI) is a new facility instrument being developed for the W. M. Keck Observatory
and funded for construction by the Telescope System Instrumentation Program (TSIP) of the National Science
Foundation (NSF). KCWI is a bench-mounted spectrograph for the Keck II right Nasmyth focal station, providing
integral field spectroscopy over a seeing-limited field up to 20"x33" in extent. Selectable Volume Phase Holographic
(VPH) gratings provide high efficiency and spectral resolution in the range of 1000 to 20000. The dual-beam design of
KCWI passed a Preliminary Design Review in summer 2011. The detailed design of the KCWI blue channel (350 to
700 nm) is now nearly complete, with the red channel (530 to 1050 nm) planned for a phased implementation contingent
upon additional funding. KCWI builds on the experience of the Caltech team in implementing the Cosmic Web Imager
(CWI), in operation since 2009 at Palomar Observatory. KCWI adds considerable flexibility to the CWI design, and will
take full advantage of the excellent seeing and dark sky above Mauna Kea with a selectable nod-and-shuffle observing
mode. In this paper, models of the expected KCWI sensitivity and background subtraction capability are presented,
along with a detailed description of the instrument design. The KCWI team is lead by Caltech (project management,
design and implementation) in partnership with the University of California at Santa Cruz (camera optical and
mechanical design) and the W. M. Keck Observatory (program oversight and observatory interfaces).
We are designing the Keck Cosmic Web Imager (KCWI) as a new facility instrument for the Keck II telescope at the
W. M. Keck Observatory (WMKO). KCWI is based on the Cosmic Web Imager (CWI), an instrument that has recently
had first light at the Hale Telescope. KCWI is a wide-field integral-field spectrograph (IFS) optimized for precision sky
limited spectroscopy of low surface brightness phenomena. KCWI will feature high throughput, and flexibility in field of
view (FOV), spatial sampling, bandpass, and spectral resolution. KCWI will provide full wavelength coverage (0.35 to
1.05 μm) using optimized blue and red channels. KCWI will provide a unique and complementary capability at WMKO
(optical band integral field spectroscopy) that is directly connected to one of the Observatory's strategic goals (faint
object, high precision spectroscopy), at a modest cost and on a competitive time scale, made possible by its simple
concept and the prior demonstration of CWI.
We describe the Cosmic Web Imager (CWI), a UV-VIS integral eld spectrograph designed for the Hale 200"
telescope at the Palomar Observatory. CWI has been built specically for the observation of diuse radiation.
The instrument eld of view is 60"40" with spectral resolving power of R5000 and seeing limited spatial
resolution. It utilizes volume phase holographic gratings and is intended to cover the spectral range 3800A to
9500A with an instantaneous bandwidth of 450A. CWI saw rst light in July 2009, and conducted its rst
successful scientic observations in May 2010.
We present proof-of-concept results for a novel ultraviolet-sensitive, photon counting, solar blind detector that
has the potential for high QE in a compact low voltage, low power, unsealed design. We utilize a delta-doped
back-illuminated CCD to read out low energy electrons from a photocathode. In parallel, a new generation
of high-QE ultraviolet-sensitive GaN photocathodes is being developed with initial success using delta-doping
technology rather than cesiation. In this paper we present results with the new readout using a CsI test cathode,
which produces events at under 1000 V accelerating potential.
GALEX is a NASA Small Explorer mission that was launched in April 2003 and is now performing a survey
of the sky in the far and near ultraviolet (FUV and NUV, 155 nm and 220 nm, respectively). The instrument
comprises a 50 cm Ritchey-Chretien telescope with selectable imaging window or objective grism feeding a pair
of photon-counting, microchannel-plate, delay-line readout detectors through a multilayer dichroic beamsplitter.
The baseline mission is approximately 50% complete, with the instrument meeting its performance requirements
for astrometry, photometry and resolution. Operating GALEX with a very small team has been a challenge, yet
we have managed to resolve numerous satellite anomalies without loss of performance (only efficiency). Many of
the most significant operations issues of our successful ongoing mission will be reported here along with lessons
for future projects.
We describe the Galaxy Evolution Explorer (GALEX) satellite that was launched in April 2003 specifically to accomplish far ultraviolet (FUV) and near ultraviolet (NUV) imaging and spectroscopic sky-surveys. GALEX is currently providing new and significant information on how galaxies form and evolve over a period that encompasses 80% of the history of the Universe. This is being accomplished by the precise measurement of the UV brightness of galaxies which is a direct measurement of their rate of star formation. We briefly describe the design of the GALEX mission followed by an overview of the instrumentation that comprises the science payload. We then focus on a description of the development of the UV sealed tube micro-channel plate detectors and provide data that describe their on-orbit performance. Finally, we provide a short overview of some of the science highlights obtained with GALEX.
We describe the performance results for the Galaxy Evolution Explorer (GALEX) far ultraviolet (FUV) and near ultraviolet (NUV) detectors. The detectors were delivered to JPL/Caltech starting in the fall of 2000 and have undergone approximately 1000 hours of pre-flight system-level testing to date. The GALEX detectors are sealed tube micro-channel plate (MCP) delay line readout detectors. They have a 65 mm diameter active area, which will be the largest format on orbit. The FUV detector has a spectral bandpass from 115 - 180 nm and the NUV detector has a bandpass from 165 - 300 nm. We report here on the performance of the detectors before and after integration into the instrument. Characteristics measured include the background count rate and distribution, gain vs. applied high voltage, spatial resolution and linearity, flat fields, and quantum efficiency.
The Galaxy Evolution Explorer (GALEX), a NASA Small Explorer Mission planned for launch in Fall 2002, will perform the first Space Ultraviolet sky survey. Five imaging surveys in each of two bands (1350-1750Å and 1750-2800Å) will range from an all-sky survey (limit mAB~20-21) to an ultra-deep survey of 4 square degrees (limit mAB~26). Three spectroscopic grism surveys (R=100-300) will be performed with various depths (mAB~20-25) and sky coverage (100 to 2 square degrees) over the 1350-2800Å band. The instrument includes a 50 cm modified Ritchey-Chrétien telescope, a dichroic beam splitter and astigmatism corrector, two large sealed tube microchannel plate detectors to simultaneously cover the two bands and the 1.2 degree field of view. A rotating wheel provides either imaging or grism spectroscopy with transmitting optics. We will use the measured UV properties of local galaxies, along with corollary observations, to calibrate the UV-global star formation rate relationship in galaxies. We will apply this calibration to distant galaxies discovered in the deep imaging and spectroscopic surveys to map the history of star formation in the universe over the red shift range zero to two. The GALEX mission will include an Associate Investigator program for additional observations and supporting data analysis. This will support a wide variety of investigations made possible by the first UV sky survey.
At NASA GSFC we are developing a high resolution solar-blind photon counting detector system for UV space based astronomy. The detector comprises a high gain MCP intensifier fiber- optically coupled to a charge injection device (CID). The detector system utilizes an FPGA based centroiding system to locate the center of photon events from the intensifier to high accuracy. The photon event addresses are passed via a PCI interface with a GPS derived time stamp inserted per frame to an integrating memory. Here we present imaging performance data which show resolution of MCP tube pore structure at an MCP pore diameter of 8 micrometer. This data validates the ICID concept for intensified photon counting readout. We also discuss correction techniques used in the removal of fixed pattern noise effects inherent in the centroiding algorithms used and present data which shows the local dynamic range of the device. Progress towards development of a true random access CID (RACID 810) is also discussed and astronomical data taken with the ICID detector system demonstrating the photon event time-tagging mode of the system is also presented.
We are developing a novel solar blind, high resolution, photon counting detector for applications in space UV astronomy. Our concept is to utilize a charge injection device (CID) as the readout stage behind a microchannel plate (MCP) intensifier. This detector will take advantage of the flexible readout options afforded by the addressable CID architecture to provide high local frame rates around bright features in an image. In this concept, the detector bandwidth can be used most efficiently, reading pixels around a bright star more frequently than those in a nearby dim cloud of gas, for example. The demonstration apparatus described in this paper incorporates a 25 mm diameter intensifier tube fiber optically coupled to a commercially available 30 frame-s-1, 512 X 512 pixel, progressive-scan CID2250 camera. The 10 MHz analog video from this camera is digitized and processed by a centroider module that calculates the position of each event in real time with subpixel precision, thus providing high spatial resolution limited by the MCPs. The pore-resolved images presented in this paper validate the intensified CID concept. We plan to incorporates custom driving electronics and an experimental CID with on-chip address decoders for high speed random access of detector subarrays of arbitrary size and location. Our goal is to demonstrate a solar-blind UV photon counter with 200-300 counts-s-1 point source and 3 X 105 counts-s-1 global rate capability with up to 4000 by 4000 elements.