The James Webb Space Telescope (JWST) and its suite of instruments, modes and high contrast capabilities will enable imaging and characterization of faint and dusty astrophysical sources<sup>1-3</sup> (exoplanets, proto-planetary and debris disks, dust shells, etc.) in the vicinity of hosts (stars of all sorts, active galactic nuclei, etc.) with an unprecedented combination of sensitivity and angular resolution at wavelengths beyond 2 μm. Two of its four instruments, NIRCam<sup>4, 5</sup> and MIRI,<sup>6</sup> feature coronagraphs<sup>7, 8</sup> for wavelengths from 2 to 23 μm. JWST will stretch the current parameter space (contrast at a given separation) towards the infrared with respect to the Hubble Space Telescope (HST) and in sensitivity with respect to what is currently achievable from the ground with the best adaptive optics (AO) facilities. The Coronagraphs Working Group at the Space Telescope Science Institute (STScI) along with the Instruments Teams and internal/external partners coordinates efforts to provide the community with the best possible preparation tools, documentation, pipelines, etc. Here we give an update on user support and operational aspects related to coronagraphy. We aim at demonstrating an end to end observing strategy and data management chain for a few science use cases involving coronagraphs. This includes the choice of instrument modes as well as the observing and point-spread function (PSF) subtraction strategies (e.g. visibility, reference stars selection tools, small grid dithers), the design of the proposal with the Exposure Time Calculator (ETC), and the Astronomer's Proposal Tool (APT), the generation of realistic simulated data at small working angles and the generation of high level, science-grade data products enabling calibration and state of the art data-processing.
Proc. SPIE. 9904, Space Telescopes and Instrumentation 2016: Optical, Infrared, and Millimeter Wave
KEYWORDS: Data conversion, Infrared detectors, Infrared telescopes, James Webb Space Telescope, Sensors, Electrons, Calibration, Data conversion, Interference (communication), Signal detection, Near infrared
Conversion gain is a basic detector property which relates the raw counts in a pixel in data numbers (DN) to the number of electrons detected. The standard method for determining the gain is called the Photon Transfer Curve (PTC) method and involves the measurement the change in variance as a function of signal level. For non-linear IR detectors, this method depends strongly on the non-linearity correction and is therefore susceptible to systematic biases due to calibration issues. We have developed a new, robust, and fast method, the differential Photon Transfer Curve (dPTC) method, which is independent of non-linearity corrections, but still delivers gain values similar in precision but higher in accuracy.
We explore a new method to generate superbias files for the NIRCam detectors. Using data from Cryo-Vacuum 2 (CV2) testing, we subtract 1/f noise from NIRCam integrations before averaging the data to produce superbias maps. Our analysis shows that for a given dataset, using this method we are able to produce superbias images with significantly lower noise levels than those produced using the more traditional approach to superbias generation. We also find that we can produce a superbias which minimizes the noise in a superbias-subtracted file by using only the first 10 readouts from each of 15-20 dark current integrations. Our testing reveals that this method is successful for data from both the shortwave and longwave detectors on NIRCam.
Wide Field Camera 3 (WFC3) is the most used instrument on board the Hubble Space Telescope. Providing a broad range of high quality imaging capabilities from 200 to 1700mn using Silicon CCD and HgCdTe IR detectors, WFC3 is fulfilling both our expectations and its formal requirements. With the re-establishment of the observatory level "spatial scan" capability, we have extended the scientific potential ofWFC3 in multiple directions. These controlled scans, often in combination with low resolution slit-less spectroscopy, enable extremely high precision differential photometric measurements of transiting exo-planets and direct measurement of sources considerably brighter than originally anticipated. In addition, long scans permit the measurement of the separation of star images to accuracies approaching 25 micro-arc seconds (a factor of 10 better than prior FGS or imaging measurements) enables direct parallax observations out to 4 kilo-parsecs. In addition, we have employed this spatial scan capability to both assess and improve the mid spatial frequency flat field calibrations.
WFC3 uses a Teledyne HgCdTe 1014xl014 pixel Hawaii-lR infrared detector array developed for this mission. One aspect of this detector with implications for many types of science observations is the localized trapping of charge. This manifests itself as both image persistence lasting several hours and as an apparent response variation with photon arrival rate over a large dynamic range. Beyond a generally adopted observing strategy of obtaining multiple observations with small spatial offsets, we have developed a multi-parameter model that accounts for source flux, accumulated signal level, and decay time to predict image persistence at the pixel level. Using a running window through the entirety of the acquired data, we now provide observers with predictions for each individual exposure within several days of its acquisition.
Ongoing characterization of the sources on infrared background and the causes of its temporal and spatial variation has led to the appreciation of the impact of He I 1.083 micron emission from the earth's atmosphere. This adds a significant and variable background to the two filters and two grisms which include this spectral feature when the HST spacecraft is outside of the earth's shadow.
After nearly five years in orbit, long term trending of the scientific and engineering behavior of WFC3 demonstrates excellent stability other than the expected decline in CCD charge transfer efficiency. Addition of post-flash signal to images is shown to markedly improve the transfer efficiency for low level signals. Combined with a pixel based correction algorithm developed at STScl, CCD performance is stabilized at levels only slightly degraded from its initial values.
We now know that the flux of a source measured with HgCdTe arrays is not a simple, linear function, but depends on the
count-rate as well as the total number of counts. In addition to the count-rate non-linearity (and probably related to the
same physical mechanism), HgCdTe detectors are also susceptible to image persistence. Most of the persistence image
fades in a few minutes, but there is a longer-term component that can result in faint afterimages in the next orbit,
approximately 45 minutes later. For sources saturated at ~100 times full-well, the afterimages can persist for hours
afterwards. This report describes results from ground and on-orbit tests to characterize the persistence and the count-rate
non-linearity in the WFC3 IR detector during its first year of operation.
Installed in the Hubble Space Telescope (HST) in May 2009, the Wide Field Camera 3 (WFC3) is performing extremely
well on-orbit. Designed to complement the other instruments on-board the Hubble Space Telescope (HST) and enhance
the overall science performance of the observatory, WFC3 is effectively two instruments in one. The UVIS channel,
with its pair of e2v 4Kx2K CCD chips provides coverage from 200 to 1000 nm while the IR channel, with a Teledyne
HgCdTe focal plane array (FPA) on a Hawaii-1R multiplexer, covers the 800-1700 nm range. This report summarizes
the performance of the WFC3 detectors, including primary characteristics such as quantum efficiency, read noise, dark
current levels, and cosmetics, as well as hysteresis prevention and the impact of radiation damage in the CCDs. In
addition, we discuss effects in the IR detector such as persistence, count rate non-linearity, 'snowballs', and 'negative'
The Wide-field Camera 3 (WFC3) is a fourth-generation instrument planned for installation in Hubble Space Telescope
(HST). Designed as a panchromatic camera, WFC3's UVIS and IR channels will complement the other instruments onboard
HST and enhance the observatory's scientific performance. UVIS images are obtained via two 4096×2051 pixel
e2v CCDs while the IR images are taken with a 1024×1024 pixel HgCdTe focal plane array from Teledyne Imaging
Sensors. Based upon characterization tests performed at NASA/GSFC, the final flight detectors have been chosen and
installed in the instrument. This paper summarizes the performance characteristics of the WFC3 flight detectors based
upon component and instrument-level testing in ambient and thermal vacuum environments.
Wide Field Camera 3 (WFC3), a panchromatic imager developed for the Hubble Space Telescope (HST), is fully
integrated with its flight detectors and has undergone several rounds of ground testing and calibration at Goddard Space
Flight Center (GSFC). The testing processes are highly automated, with WFC3 and the optical stimulus, which is used to
provide external targets and illumination, being commanded by coordinated computer scripts. All test data are captured
and stored in the long-term Hubble Data Archive. A full suite of instrument characterization and calibration tests has
been performed, including the measurement of key detector properties such as dark current, read noise, flat field
response, gain, linearity, and persistence, as well as instrument-level properties like total system throughput, imaging
quality and encircled energy, grism dispersions, IR thermal background, and image stability. Nearly all instrument
characteristics have been shown to meet or exceed expectations and requirements.
Wide-Field Camera 3 (WFC3) has been built for installation on the Hubble Space Telescope (HST) during the next servicing mission. The WFC3 instrument consists of both a UVIS and an IR channel, each with its own complement of filters. On the UVIS side, a selectable optical filter assembly (SOFA) contains a set of 12 wheels that house 48 elements (42 full-frame filters, 5 quadrant filters, and 1 UV grism). The IR channel has one filter wheel which houses 17 elements (15 filters and 2 grisms). While the majority of UVIS filters exhibited excellent performance during ground testing, a subset of filters showed filter ghosting; improved replacements for these filters have been procured and installed. No filter ghosting was found in any of the IR filters; however, the new IR detector for WFC3 will have significantly more response blueward of 800 nm than the original detector, requiring that two filters originally constructed on a fused silica substrate be remade to block any visible light transmission. This paper summarizes the characterization of the final complement of the WFC3 UVIS and IR filters, highlighting improvements in the replacement filters and the projected benefit to science observations.
Wide Field Camera 3 (WFC3), a panchromatic imager being developed for the Hubble Space Telescope (HST), is now
fully integrated and has undergone extensive ground testing at Goddard Space Flight Center, in both ambient and
thermal-vacuum test environments. The thermal-vacuum testing marks the first time that both of the WFC3 UV/Visible
and IR channels have been operated and characterized in flight-like conditions. The testing processes are completely
automated, with WFC3 and the optical stimulus that is used to provide external targets and sources being commanded
by coordinated computer scripts. All test data are captured and stored in the long-term Hubble Data Archive. A full suite
of instrument calibration tests have been performed, including measurements of detector properties such as dark current,
read noise, flat field response, gain, linearity, and persistence, as well as total system throughput, encircled energy,
grism dispersions, IR thermal background, and image stability tests. Nearly all instrument characteristics have been
shown to meet or exceed expectations and requirements. Solutions to all issues discovered during testing are in the
process of being implemented and will be verified during future ground tests.
We present the performance of the IR detectors developed for the WFC3 project. These are HgCdTe 1Kx1K devices with cutoff wavelength at 1.7 μm and 150K operating temperature. The two selected flight parts, FPA#64 (prime) and FPA#59 (spare) show quantum efficiency higher than 80% at λ=1.6 μm and greater than 40% at λ>1.1μm, readout noise of ~25 e<sup>-</sup> rms with double correlated sampling, and mean dark current of ~0.04 e/s/pix at 150K. We also report the results obtained at NASA GSFC/DCL on these and other similar devices in what concerns the QE long-term stability, intra-pixel response, and dark current variation following illumination or reset.
Wide Field Camera 3 is a fourth generation instrument for the
Hubble Space Telescope (HST), to be installed during the next HST Servicing Mission 4. For its infrared channel Rockwell Scientific Company has developed a new type of HgCdTe 1Kx1K detector, called WFC3-1R, with cutoff wavelength at 1.7μm and 150K operating temperature. The WFC3-IR detectors are based on HgCdTe MBE grown on a CdZnTe substrate and use a new type of multiplexer, the Hawaii-1R
MUX. Two flight detectors, a prime and a spare, have been recently selected on the basis of the measures performed at NASA Goddard Research Center - Detector Characterization Laboratory. These parts show quantum efficiency higher than 80% at λ=1.6μm and greater than 40% at λ>1.1μm, readout noise of ~25 e- rms with double correlated sampling, and mean dark current of ~0.04 e/s/pix at 150K. We show that the IR channel of WFC3, equipped with one of these flight detectors, beats the instrument requirements in all configurations and promises to have a discovery efficiency
significantly higher than NICMOS. In particular, a two-band
wide-area, deep survey made with WFC3 exceeds the discovery
efficiency of NICMOS before and after the installation of NCS
by a factor of 15 and 10, respectively.
Rockwell Scientific Company is developing a new type of HgCdTe 1K 1K detector, called WFC3-1R, with cutoff
wavelength at 1.7 m and 150K operating temperature. The detector will be installed on the Wide Field Camera 3, the
fourth generation panchromatic instrument for the Hubble Space Telescope (HST) to be installed during HST Servicing
Mission 4, currently scheduled for 2004. The detector uses HgCdTe MBE grown on a CdZnTe substrate and a new type
of multiplexer, the Hawaii-1R MUX. Six lots of detectors have been produced so far, and have demonstrated the
capability to meet or exceed the project requirements. In particular, detectors show quantum efficiency as high as ~90%
at =1.4-1.6 m and greater than 50% at >1.0 m, readout noise of 30 e- rms with double correlated sampling, and dark
current <0.2 e/s/pix at 150K. We illustrate the behavior of the reference pixels, showing that they allow the
compensation of drifts in the dc output level. A number of detectors show a peculiar instability related to the variations
of diode polarization, still under investigation. We also report on the environmental testing needed to qualify the WFC3-
1R detectors as suitable for flight on the HST. We finally provide an update of the project status.