COS FUV Flat Fields and Signal-to-Noise Characteristics
Dr. Thomas Ake (STScI/CSC)
The COS FUV channel employs a detector comprised of two microchannel plate (MCP) segments with cross delay line anodes. The detector shows several types of non-uniformities due to the hexagonal and moire patterns in the MCPs, dead spots, gain variations, and shadows from the wire grid installed in front of the MCPs to increase quantum efficiency. These features induce fixed-pattern noise in FUV spectra. The effects of these artifacts can be reduced by dividing the data by a flat field and combining exposures taken at different grating settings. A spectral iteration technique, similar to that used for GHRS and FOS, shows that S/N > 100 can be achieved in extracted spectra. Although flat field observations were obtained during SMOV using white dwarfs, a two dimensional flat field of sufficient quality for standard CALCOS processing was not achieved. Other methodologies are being explored for flat field correction and are expected to be installed in CALCOS to improve the S/N of data incrementally. As an initial step, CALCOS currently ignores grid wire regions when creating a summed spectrum from exposures taken at different FP-POS positions. Average one-dimensional flats, first removing the grid wires, then taking out the other fixed-pattern noise, are being investigated to correct individual exposures. These may require separate flat fields for different cross-dispersion locations. Two-dimensional flats will be examined to determine if additional observations are warranted.
The On-Orbit Performance of the Cosmic Origins Spectrograph
Dr. Alessandra Aloisi (Space Telescope Science Institute)
The Cosmic Origins Spectrograph (COS) was installed on board the Hubble Space Telescope (HST) in May 2009 as part of the most recent servicing mission 4, and is a third-generation instrument that has significantly extended HST UV spectroscopic capabilities. This contribution highlights the current instrument performance, with particular emphasis on the initial on-orbit characterization during the Servicing Mission Observatory Verification and the subsequent calibration during Cycle 17. Plans for future improvements in the COS on-orbit calibration are also included.
A Pixel-Based Correction for Imperfect CTE in ACS's WFC
Dr. Jay Anderson (STScI)
Based on a study of the trails seen behind warm pixels in long dark exposures, we have constructed an empirical model for the capture and release of charge that happens during readout of the WFC detector. The model assumes an even distribution of traps across the detector, and parametrizes the traps in terms of which electrons they impact and the profile of their release-time distribution. We adjust the parameters of our model to reduce the trailed warm pixels in the darks to delta functions, then use the same model to correct actual ACS images. We find that the model does a promising job restoring the CTE-trailed flux in real images to its original pixel, restoring photometry, astrometry, and even shape characteristics. Since any CTE-restoration process necessarily constitutes a deconvolution, it will be important to understand how it impacts noise and other issues.
Ironing out the Wrinkles in STIS
Dr. Thomas Ayres (University of Colorado (CASA))
On the J.J. Drake "Ladder of Cosmic Sexiness," the calibration rung falls even below the lowly ones of "Stars" and "The Sun." Nevertheless, we cognoscenti of such things know full well that without good calibrations, there will be little signal and much noise. Our knowledge of remote objects, environments, and events thus is only as good as our understanding of our instruments. A case in point is the solar system's premier high resolution ultraviolet spectrograph in space: STIS. The second generation HST instrument has unmatched spectral resolution, up to 114,000 in its echelle H modes, and full coverage of the key FUV and NUV wavelength regions with relatively few settings. For the initial seven years of its operations, 1997-2004, STIS was a workhorse in studies of stars, planets, interstellar matter, and even extragalactic sightlines. Now, a repaired STIS is teamed up with super high sensitivity, but lower resolution, partner COS to open new windows into the broad range of cosmic environments and processes that can be probed by ultraviolet spectroscopy. (Putting the "physics" into "astrophysics.") The present report describes an effort to push STIS beyond its already remarkable precision by carefully analyzing the way in which the echellegrams are processed, particularly with respect to wavelength assignments. The method is to run the CALSTIS pipeline on deep exposures of the Pt/Cr-Ne calibration lamps, treating them as if they were science images, and compare the resulting sharp-line spectra with laboratory measurements to map out any persistent deviations. Large scale coherent systematics, if present, can be modeled and compensated, either at the level of the pipeline dispersion constants, or as a post facto distortion correction. (The latter approach was followed in the so-called StarCAT project, which cataloged STIS echelle spectra of 545 objects classified as "stars.") Unfortunately, while there are excellent laboratory measurements of the STIS lamps below 1800 A, the situation at longer wavelengths is not as good, thanks mainly to the presence of chromium in the STIS sources, which has a rich, though poorly cataloged, spectrum at the longer wavelengths. This speed bump was circumvented by utilizing STIS as its own laboratory spectrometer, bootstrapping from the existing ground measurements of Pt and Ne lines. The large number of newly characterized reference wavelengths, especially in the NUV, coupled with the higher line densities achieved in coadded deep wavecal spectra, allowed some remarkable new insight into the dispersion properties of the instrument, even to the level of obtaining a simultaneous dispersion solution over all of the up to 26 independent settings of each of the four echelle modes. It is astonishing that this works as well as it does. Part of the reason is that the nonlinear terms in the dispersion model apparently are dominated by a quadrupolar geometrical distortion in the MAMA cameras, which is separable from the dominantly linear dispersion properties. The current set of dispersion constants in the calibration library could be significantly improved by exploiting the deep wavecal material and the new "laboratory" wavelengths to recalibrate; and the processing could be made more robust by performing a geometrical distortion correction prior to wavelength assignment. This work has resulted, as well, in lots of pretty pictures (depending on how one views the artistic merits of wavecal spectra).
JWST Science Operations: the Phase I & II Process
Dr. Tracy Beck (STScI)
The Space Telescope Science Institute will serve as the main Science and Operations Center for the James Webb Space Telescope, which is a segmented 6.5m telescope that will operate in deep space at L2. There are some telescope characteristics that will make observing with JWST different from using the Hubble Space Telescope. In this talk, I will highlight some of the operational observing constraints on JWST. I will also summarize the plans for the JWST proposal Phase I and Phase II process, including the use of templates to request science observations using the Astronomer's Proposal Tool (APT).
WFC3 Image Calibration and Reduction Software
Dr. Howard Bushouse (STScI)
Standard WFC3 image processing consists of the calwf3 task, which removes instrumental signatures from the images, and multidrizzle, which corrects images for geometric distortion and combines dithered sets of exposures. The status and future plans for calwf3 and multidrizzle will be discussed, including the status of the various calibration reference data used by them. Tips and tricks of using the software will also be discussed, including typical types of problems to look for in your processed images and how to fix them.
Cross Calibration of Space Infrared Telescopes
Dr. Sean Carey (Spitzer Science Center / Caltech)
I will discuss the cross calibration efforts between the various instruments aboard the Spitzer Space Telescope and to previous missions such as MSX and ISO, and the current mid-infrared observatories, WISE and AKARI. Current cross-calibration accuracy between the Spitzer instruments is of order 5%. While cross-calibration is conceptually simple, the details can be interesting. Complications such as the impact of errors in linearity solutions in measurements of common calibrators and differences in photometric truth used to calibrate individual instruments will be explored. Efforts to prepare cross-calibration to JWST will be described. The challenges of bootstrapping to extremely sensitive instruments (MIRI, NIRCAM and NIRSpec) and longer wavelengths (Herschel PACS and SPIRE) will be explored.
Cross-matching the Hubble Legacy Archive
Dr. Nathan Cole (Johns Hopkins University)
The Hubble Legacy Archive (HLA) is composed of hundreds of thousands of images taken by a number of different instruments all across the sky. A large number of these images are of overlapping fields and therefore contain many of the same objects. Through the use of a probabilistic cross-matching technique, it is possible to use these overlapping fields to determine a linkage between similar objects. This linkage can then be used to improve astrometry, as well as provide a means for connecting images taken via different instruments. We present a description of this cross-matching technique and discuss the specific choices made with regards to the first data used. We also present current results of implementing this technique to improve astrometry showing an improvement in astrometry to 0.004”. Future concerns and developments will also be discussed.
NICMOS Reference Files in the NCS Era, 2002-2008
Dr. Tomas Dahlen (STScI)
During the era after the installation of the NICMOS Cooling System (NCS) 2002-2008, the NICMOS calibration reference files have been regularly monitored to examine their stability over time. This includes repeated observations of dark frames, flat-fields images, and photometric standard stars. Over the seven-year span, a slight change in the A-to-D conversion, or gain, has been detected. This change can been expressed as a change in an effective temperature (or equivalently A-to-D conversion), referred to as biastemp, which is calculated directly from the bias level of the science exposures. The most probable cause for this behavior is a change in the detector bias voltage with time since the installation of the NCS. It is possible to estimate the A-to-D conversion at the time of observation for each science image by calculating the biastemp. Thereafter, reference files that are valid at biastemp matching that of the observations can be used. In this talk I will quantify how the change in A-to-D conversion has affected the reference files and describe how corrections have been implemented in the NICMOS calibration software calnica to account for this effect.
Red Leak Effects in Observations of Solar System Objects with ACS/SBC
Dr. Paul Feldman (Johns Hopkins University)
Following the failure of STIS in August 2004, attempts to obtain ultraviolet spectroscopy and photometry of solar system objects shifted to the Solar Blind Channel (SBC) of the ACS. These included spatially resolved spectroscopy using the PR130L prism of comet 9P/Tempel 1 (at the time of Deep Impact) and Europa, and imaging and photometry of the asteroid 21 Lutetia, one of the targets of ESA's Rosetta mission. Initial estimates of long wavelength ("red") contamination of the data due to impurities in the FUV MAMA detector suggested that these observations were feasible. Subsequent analyses produced better sampled, more reliable response curves (Boffi et al., TIR ACS 2008-002) that showed the long wavelength response to be much worse than expected. The analysis of differential photometry of Lutetia (Weaver et al., submitted to A&A) shows an effect that is 2.5 times larger than the published data. We also discuss the use of a PR130L spectrum of a solar analogue star 16CygB for the interpretation of the Europa emission spectrum.
A Redesign of MultDrizzle
Andrew Fruchter (STScI)
We describe new developments in MultiDrizzle intended to allow users to easily and accurately align and combine HST images taken at multiple epochs, and even with different instruments. In the first part of this program, the correction of the time-dependent distortion of ACS was introduced into MultiDrizzle to provide distortion corrections good to a few hundredths of a pixel level. Now, we are undertaking a more profound change in the underlying method that MultiDrizzle (and HST) use to represent astrometry and geometric distortion. We have developed new simple extensions of the fits fomat that allow us to fully represent the ACS and WFC3 distortions in the header of the image, meaning that a calibrated image needs no other files to describe its astrometry. Finally we describe new tools we are developing to allow users to more easily align images and images with catalogues, and to use those WCS based alignments for Drizzling. Time will be left to discuss these changes with the audience and to hear to what extent these improvements will meet their needs.
JWST Absolute Flux Calibration Planning
Dr. Karl Gordon (STScI)
The planning for the absolute flux calibration of all four JWST science instruments will be described. The main goal is to provide a uniform calibration (photometric and spectrophotometric) across all instruments using a common set of calibration stars. These stars will include white dwarfs, A0V, and solar type stars. The combination of these different types of stars will serve to check for systematic biases in the calibration as well as tie the Hubble (based on white dwarfs) and Spitzer (based on A0V and solar type stars) calibrations to the JWST calibration. An initial set of calibrators has been investigated and how they map to the sensitivity ranges of the JWST instruments will be detailed. Further work will concentrate on expanding the sample to more fully map the JWST instrument sensitivities and vetting individual calibration stars.
Science Capabilities of the Cosmic Origins Spectrograph
Prof. James Green (University of Colorado)
I will highlight the observational capabilities of COS, and the increase in sensitivity it provides in various observing situations. The scientific return of the instrument in its first year of operation will be quantified and presented, along with selected observations from the first year of COS data.
The JWST Integrated Science Instrument Module: Status and Test Plans
Dr. Matthew Greenhouse (NASA GSFC)
The Integrated Science Instrument Module (ISIM) of the James Webb Space Telescope (JWST) is discussed from a systems perspective with emphasis on development status and element-level I&T plans. The ISIM is one of three elements that comprise the JWST space vehicle and is the science instrument payload of the JWST. The science instruments, their supporting systems, and plans for element-level testing ahead of ISIM delivery to the Observatory will be discussed.
WFC3 IR Detector Behavior
Mr. Bryan Hilbert (STScI)
Using data taken during Servicing Mission Observatory Verification (SMOV4) and Cycle 17, we have characterized many aspects of the on-orbit behavior of the IR Channel. We find the dark current in the IR channel to be 0.043 - 0.050 e-/s/pixel. The correlated double sampling (CDS) read noise in RAPID sequences is 20-22 electrons, similar to that measured in ground testing. The effective noise measured in an image created from 16 reads of a SPARS200 ramp is 11.6 - 12.7 electrons. We have also recently finished the creation of a bad pixel mask for the IR detector, which contains a list of pixels with non-nominal behavior that should be ignored in WFC3/IR data analyses. Current analysis projects also include an investigation into the pixels' non-linearity behavior, as well as monitoring the rate and characteristics of "snowballs" which appear in IR channel data.
The Legacy of the Hubble Space Telescope Spectrographs
Prof. J. Christopher Howk (Univ. of Notre Dame)
The spectrographs of the Hubble Space Telescope have each offered order of magnitude improvements over their predecessors, including those space-based instruments before HST. They have provided a wealth of information on the local and distant universe, opening up new fields of study that were not possible prior to the launch of HST. They have, of course, come with their own set of design and use challenges. I will discuss the legacy of HST spectroscopy and look forward to the types of spectroscopy we might wish to see in future space missions.
JWST Pipeline: Discussion of Data Reduction
Robert Jedrzejewski (STScI)
The JWST Mission will, like HST, have a dedicated Calibration Pipeline that will be used to calibrate all science data from the observatory. Rather than follow the HST model of creating separate calibration pipeline programs for each Science Instrument, we will instead make use of the considerable simplification of separating the detector calibration from the instrument-specific calibration. Since JWST will only have two types of detectors, much of the detector calibration code will be common from instrument to instrument. Similarly, the instrument-specific behaviors can be classified as imaging, spectroscopic and coronagraphic, and we anticipate that, even though the architecture of the instruments may show significant variation, many of the primitive operations will be applicable to more than one instrument or mode and will allow re-using code. These simplifications will enable us to create a flexible, powerful pipeline that can be cleanly separated into independent calibration modules that can be evaluated, tested and improved straightforwardly over the life of the project.
ACCESS: Mission Overview, Design and Preliminary Performance
Dr. Mary Elizabeth Kaiser (Johns Hopkins University)
Access, Absolute Color Calibration Experiment for Standard Stars, is a series of rocket-borne sub-orbital missions and ground-based experiments designed to enable improvements in the precision of the astrophysical flux scale through the transfer of absolute laboratory detector standards from NIST to a network of stellar standards with a calibration accuracy of 1% and a spectral resolving power of 500 across the 0.35 to 1.7 micron bandpass. The ACCESS instrument is a Dall-Kirkham Cassegrain telescope with a 15.5-inch primary mirror, a feedback controlled optical-NIR performance monitoring system, and a compact spectrograph design based on a Rowland circle mount concave grating operating as a low order (m=1-4) echelle with a Fery prism for cross dispersion. The detector is an unfiltered substrate-removed HST/WFC3 heritage HgCdTe array. To achieve the goal of a 1% absolute flux calibration requires an extensive ground calibration of the ACCESS instrument. At the core of the ground calibration is the 'artificial star' optical delivery system. The ArtStar system feeds an absolute flux calibrated collimated beam to the ACCESS instrument for the determination of the end-to-end sensitivity. The flight detector has been selected, detector electronics are being tuned and measurements undertaken to confirm and further characterize detector performance. Optical system components have begun to arrive. We will present the ACCESS design, calibration strategy, preliminary sub-system ground performance data, and instrument status. NASA sounding rocket grant NNX08AI65G and DOE DE-FG02-07ER41506 support this work.
The Photometric Performance of the WFC3 UVIS and IR Cameras
Dr. Jason Kalirai (STScI)
We characterize the total system throughput of HST/WFC3 by imaging both hot and cool HST spectrophotometric standard stars with both the UVIS and IR cameras. For the UVIS channel, our measurements indicate that efficiencies are higher than ground tests by 15-20% at central wavelengths and 5-10% at the blue and red ends of the UVIS spectral range. For the IR channel, the instrument throughput is 10-15% higher in all filters. Over its one year lifetime, WFC3's throughput is stable in wide and medium band filters to <1%. New photometric zero points have been calculated, and updates to the exposure time calculator have been implemented.
The NICMOS Legacy Archival Calibration Project
Dr. Anton Koekemoer (STScI)
The legacy recalibration and reprocessing of the entire archive of NICMOS data, consisting of more than 100,000 observations obtained since its installation on HST in 1997, has led to the production of a final archive that includes all the calibration and processing improvements that have been obtained as a result of our improved knowledge of the instrument performance during this time. This archive is unique in being the only set of HST observations in the 1 - 2 micron regime for over a decade until the installation of WFC3 in 2009, and provides the foundation for a wide range of follow-up science. The infrared detectors are susceptible to a wide range of calibration issues that are specific to infrared detectors, including persistence, temperature and bias-dependent effects, and an extensive program of calibration and software development has been carried out to correct these. These have been applied in a full recalibration and reprocessing of the entire archive of NICMOS data, including also MultiDrizzle combination and release in the HLA, resulting in a dataset with dramatically improved scientific value that will serve as a unique and extensive archival legacy in its own right, in addition to benefiting future infrared instruments and science programs.
Multi- Wavelength Geometric Distortion Solution for WFC3/UVIS and IR.
Vera Kozhurina-Platais (STScI)
The standard astrometric catalog based on ACS/WFC observations of Omega Cen field has been used to examine the geometric distortion of WFC3/UVIS and IR as function of wavelength. The multiple observations of this field taken with the large dither patterns and large range of the HST roll-angles were exposed through 10 pass-band UVIS filters and 5 IR filters. A 4th order polynomial model was used to derive the geometric distortion coefficients relative to the distortion - free coordinates of our astrometric field in the UVIS and IR channels. As a result: 1) the geometric distortion can be successfully corrected with 2-5 mas precision level in UVIS and IR channels respectively; 2) the non-perpendicularity of coordinates axes (skew) can be used to assess the scale change from filter to filter, time and HST roll-angle; 3) the coefficients of the geometric distortion in the UVIS and IR channels are used in STScI Multidrizzle software in order to correct WFC3 images for distortion.
WFPC2 Filters after 16 Years on Orbit
Ms. Pey Lian Lim (Space Telescope Science Institute)
Wide Field Planetary Camera 2 (WFPC2) was installed on Hubble Space Telescope (HST) in December 1993 during Servicing Mission 1 by the crew of Shuttle Mission STS-61. WFPC2 replaced Wide Field Planetary Camera 1 (WFPC1), providing improved UV performance, more advanced detectors, better contamination control, and its own corrective optics. After 16 years of exceptional service, WFPC2 was retired in May 2009 during Servicing Mission 4, when it was removed from HST in order to allow for the installation of Wide Field Camera 3 (WFC3). WFPC2 was carried back to Earth in the shuttle bay by the crew of Shuttle Mission STS-125. In a joint investigation by Goddard Space Flight Center (GSFC) and Space Telescope Science Institute (STScI), the Selectable Optical Filter Assembly (SOFA) of WFPC2 was extracted and the filter wheels removed and examined for any on-orbit changes. The filters were inspected, photographed and scanned with a spectrophotometer at GSFC. The data have been analyzed at STScI with a view towards understanding how prolonged exposure to the HST space environment affected the filters and what the resultant impacts are to WFPC2 calibrations. In this presentation, we will summarize our results from these post-SM4 laboratory studies, including a comparison of pre- to post-mission filter throughput measurements, evaluations of the UV filter red leaks, and assessment of the condition of the filter coatings.
Persistence in the WFC3 IR Detector
Dr. Knox Long (STScI)
As is the case for most if not all modern IR arrays, bright sources observed with the IR detector in WFC3 leave faint residual images in subsequent exposures. Persistence has been observed not only in dithered exposures by one observer of a single target within an orbit but also in exposures of another completely target by another observer in a subsequent orbit. The amount of image persistence in the WFC3 IR detector is a function of the degree of saturation of a pixel and time since the pixel was (over) exposed. The persistence decays roughly as a power law with time, and is typically 0.2 e/s for a pixel that was highly saturated 1000 s earlier. Here, we show examples of persistence which have been observed and characterize the effect. We also describe ways for observers to find the pixels that are likely to have been affected by persistence, and to mitigate the effects of persistence when planning observations and reducing their data.
Application of the SIDECAR ASIC as the detector controller for ACS and the JWST near-IR instruments
Dr. Markus Loose (Markury Scientific)
The SIDECAR ASIC is a fully integrated controller for high-performance optical and infrared detectors. It combines all functions on a single microchip, including output signal amplification and A/D conversion, bias voltage generation, clock generation, and housekeeping telemetry. The SIDECAR ASIC has been implemented into the CCD electronics box for ACS (during the SM4 repair of the instrument), and it is built into all of the JWST near-infrared instruments. The presentation will give a brief overview of the core capabilities and features of the SIDECAR ASIC, and then focus on the performance and operational aspects relevant to HST/ACS and JWST. Challenges with respect to 1/f noise on the bias voltages and overall strategies for further noise reduction will be discussed. In particular with respect to JWST, flexible data acquisition modes combined with elaborate post-processing have shown to provide significant noise improvements over the baseline approach. In this context, the full in-system programmability (even on-orbit) provides a valuable tool to adapt the operation and configuration of the SIDECAR ASIC to changing conditions.
Performance and Calibration of Wide Field Camera 3
Dr. John MacKenty (Space Telescope Science Institute)
The Wide Field Camera 3 was installed in HST in May 2009 during SM4. Designed to provide a factor of 10 or better increase in the near-ultraviolet and near-infrared imaging capabilities of the observatory, WFC3 has surpassed these expectations by large margins and is operating well. This talk will provide an overview of the instrument's design, its in-flight performance, and our strategy and priorities for its calibration.
Dr. John Mather (NASA GSFC)
The James Webb Space Telescope is now in the construction phase, having passed its Mission Critical Design Review in April 2010. I will review the key scientific objectives that were used to set the engineering parameters, and I will describe the observatory concept and the characteristics that are important to calibration. The JWST's optical system is adjustable after launch, so its optical performance is time-dependent. The optical train can be perturbed by temperature changes and gradients and by vibrations. In addition, the wide range of instrument capabilities from imaging to coronography, from tunable filter imaging to integral field and multislit spectroscopy, offer many chances for error and for cross-calibration to discover and compensate for such errors. Obtaining accuracy and precision beyond the basic requirements will be full of interesting challenges.
Calibrators for the in-orbit spectrophotometric calibration of the Medium Resolution Spectrograph of MIRI onboard the JWST.
Dr. Theodore Nakos (University of Ghent)
We present a list of 32 stellar sources selected for the in-orbit calibration/characterisation of the Relative Spectral Response Function (RSRF) of the Medium Resolution Spectrometer (MRS) of the Mid-InfraRed Instrument (MIRI) onboard the James Webb Space Telescope (JWST). As these sources will be used for spectrophotometric calibration, it is of key importance that they span a wide range of spectral types. The different selection criteria used for the list compilation and the analysis steps of the obtained data are described. The first results of the source modelling are also presented.
Calibration of HST's Fine Guidance Sensors
Edmund Nelan (STScI)
HST's three Fine Guidance Sensors are critical for the fine pointing and stabilization of the observatory. The ability of the FGS to measure the position of a guide star to ~1 milli-arcsecond at 40 Hz also empowers it to serve as an astrometric and high speed photometric science instrument. In addition, the interferometric design of the FGS enables it to resolve binary star systems with angular separations down to approximately 12 mas, well below the diffraction limit of HST. Over the years the FGSs have been calibrated as needed to support their role as guiders. FGS1r has been calibrated to support the FGS science program, including a re-calibration of its geometric distortion in late March 2010. In SM4 FGS2 was replaced by the refurbished FGS2r2. This new FGS was calibrated for guide duty after its on-orbit commissioning. This talk will outline the calibration history and status of HST's three FGSs.
NEW SCIENTIFIC CAPABILITIES OF THE HST WFC3
Prof. Robert O'Connell (University of Virginia)
I will describe some of the new scientific capabilities enabled by the extended wavelength coverage (0.2-1.7um), large suite of specialized filters, and improved "discovery efficiency" of HST's Wide Field Camera 3. Some examples from Cycle 17 programs include the star formation histories of nearby disk and early-type galaxies (from both resolved sources and integrated light), discovery of multi-age pre-main-sequence populations in massive Local Group star clusters, the evolutionary state of galaxies at intermediate redshifts (z ~ 1-4), and identification of candidates for the highest redshift galaxies at z ~ 8-9 in the Hubble Ultra Deep Field.
COS Sensitivity Trends in Cycle 17
Dr. Rachel Osten (STScI)
After the initial on-orbit determination of the absolute flux calibration of the Cosmic Origins Spectrograph was performed, we have been monitoring the instrument's spectroscopic sensitivity regularly. Observations of spectroscopic white dwarf standard stars have shown a decline in sensitivity in the bare Aluminum gratings G225M and G285M on the NUV channel, while the gratings with Al+MgF2 coatings (G230L and G185M) appear stable. The change in sensitivities of the G225M and G285M gratings are weighted means of -3.1 and -11.0 percent per year, respectively,while for the G230L and G185M they are -0.4 and -0.6 percent per year, respectively. The bare Al gratings exhibited sensitivity degradation on the ground; the rates of degradation were about 1.6% per year for the G225M and 4.5% per year for the G285M grating, measured during semi-annual grating efficiency tests. The trends seen in external targets appear to be wavelength-independent but grating dependent.All gratings on the FUV channel are experiencing wavelength-dependent sensitivity degradation, which is worse at longer wavelengths. Similar results are returned for the same wavelength ranges falling on different detector segments, and also made with different gratings and external targets. Below 1250 Angstroms, the sensitivity is declining at 5.6 percent per year, and from 1700--1800 Angstroms the decline is a weighted mean of 10.8 percent per year. The sensitivity changes on the NUV and FUV channels appear to be uncoupled. The sensitivity decline of the NUV bare Al gratings appears to be caused by continued growth of an oxide layer, either from additional deposition of atomic oxygen in space or migration of existing atomic oxygen in the system to the outside. Detector QE loss due to localized exposure of the FUV cross-delay line detector to UV irradiation can be ruled out by several tests which examine the sensitivity decline versus total counts in specific regions of the detector. The wavelength dependence of the FUV sensitivity degradation is the opposite sense for contamination to be an issue. The source of the FUV sensitivity decline does not appear to be due to shifts in the pulse-height distribution from gain sag, as such shifts are not observed (and would not explain the magnitude of the decrease). The loss of quantum efficiency shows the classic signature of photocathode degradation. I will present the latest results from sensitivity monitoring and discuss investigations of the observed trends.
Observing with HST below 1150 Å: Extending COS/G130M and G140L coverage to 905 Å
Steve Osterman (CASA)
The FUV channel of the Cosmic Origins Spectrograph is designed to operate between 1150Å and 1800Å, limited at shorter wavelengths by the reflectivity of the MgF2 protected aluminum used on the optical telescope assembly and on the COS FUV diffraction gratings. However, because the detector for the FUV channel was windowless, it was recognized early on that there was the possibility that COS would have some sensitivity at shorter wavelengths due to the first surface reflection from the MgF2 coated optics. Preflight testing of the G140L grating suggested on order 5% efficiency at 1066Å, and early on-orbit observations verified that COS was sensitive down to at least the Lyman limit with 10-20 cm2 effective area between 912Å and 1070Å, and rising rapidly to over 1000 cm2 beyond 1150Å. We have also explored narrower band observations using the G130M grating out of band for coverage from 1020Å to 1170Å and from 905Å to 1055Å. We present ray trace simulations and calibration results for these observing modes, and explore alternative configurations to increase resolution and signal to noise.
An insight on the Gaia BP/RP and G-band flux calibration
Elena Pancino (INAF-OABO)
I will quickly present the Gaia mission, focussing on those technical aspects that are necessary to understand the details of its external (absolute) flux calibration. On board of Gaia there will be two (spectro)photometers, the blue one (BP) and the red one (RP) covering the range 330-1050 nm, and the white light (G-band) imager. Given the fact that the focal plane of Gaia will be constituted by 106 CCDs and the sources will cross the the focal plane at constant speed, at different positions in each of the foreseen passages (on average 80, but up to 350) in the mission lifetime, the "simple" problem of calibrating the integrated BP/RP and G-band magnitudes and the low resolution BP/RP spectra flux turns into a very delicate and complicated issue, including CTI effects, LSF (the linear version of the PSF) variations across the focal plane and with time, CCD gating to avoid saturation and the like.
WFC3 IR "Blobs", IR Sky Flats and the measured IR background levels
Dr. Nor Pirzkal (STScI)
The near infrared background, as seen using the WFC3 IR channel, varies by several orders of magnitude. Ever since WFC3 was installed on HST, we have been monitoring and examining incoming observations of sparse fields. This monitoring has enabled us to identify the WFC3 IR "Blobs" (Pirzkal et al., ISR 2010-06). It has also allowed us,for some of the broad band filters, to assemble IR Sky Flats. We have also been able to measure the observed IR background levels in many of the WFC3 IR filters. Here, we show how we have used WFC3 observations to construct the Sky Flats as well as how we detected and created maps of IR "Blobs". We finally show how the measurements of the IR background vary and how they correlate with HST pointing.
The Space Telescope Imaging Spectrograph After SM4 Repair
Dr. Charles Proffitt (STScI/CSC)
The Space Telescope Imaging Spectrograph (STIS) was originally installed in the Hubble Space Telescope (HST) in February of 1997. In May of 2001, the primary Side-1 electronics failed, but STIS continued operations using the redundant Side-2 electronics until August of 2004, when an electrical malfunction in a power supply forced cessation of operations. On May 17, 2009, during the fourth EVA of SM4, astronauts Michael Good and Mike Massimino replaced the STIS LVPS-2 circuit board containing the failed component, successfully repairing the Side-2 electronics and returning STIS to operations. STIS after this 2009 repair operates in much the same way as it did during its previous 2001-2004 period of operations with the Side-2 electronics. Internal and external alignments of the instrument are very similar to what they had been in 2004, and most changes in performance are modest. The STIS CCD detector continued to experience radiation damage throughout the hiatus in operations, leading to decreased charge transfer efficiency and an increased number of hot pixels. The sensitivities for most modes are surprisingly close to that expected from simple extrapolation of the 2003-2004 trends, although the echelle modes show somewhat more complex behavior. The biggest surprise was that the dark count rate for the NUV MAMA detector after SM4 has been much larger than had been expected; it is currently about three times bigger than it had been in 2004 and only slowly decreasing. We discuss how these changes will affect science with STIS now and in the future.
An Overview of Detectors
Dr. Bernard Rauscher (NASA Goddard Space Flight Center)
Removing the instrument signature of the detectors is an important part of calibrating any imager or spectrograph. When doing this, it helps to understand how the detectors work, and the kinds of artifacts that can manifest themselves. This talk will provide an overview of the hybrid HgCdTe and Si:As detectors that will be used by the James Webb Space Telescope, with an emphasis on calibration.
First On-orbit Measurements of the WFC3-IR Count-rate Non-Linearity
Dr. Adam Riess (STScI/JHU)
Previous HgCdTe detectors on HST have suffered from a count-rate dependent non-linearity, motivating an investigation of a similar effect on the WFC3-IR detector. An initial measurement of this effect was made by comparing the photometry of star clusters observed over a wide dynamic range and at overlapping wavelengths in WFC3-IR and NICMOS and/or ACS-WFC. Utilizing a color term to account for differences in the observed bandpasses, we find a significant detection of a non-linearity in WFC3-IR photometry which is in the same direction but a few times smaller than that of NICMOS. From 235 stars in 47Tuc observed with WFC3-IR in F110W and F160W and in similar bandpasses in NICMOS Camera 2, we measure a non-linearity of WFC3-IR of 0.011+/- 0.0023 and 0.010+/-0.0025 mag per dex, respectively, over a range of 10 magnitudes (4 dex). An independent measurement utilizes 1390 stars in NGC 3603 observed with ACS-WFC F850LP and WFC3-IR F098M and yields a very similar result, 0.010 +/- 0.0033 mag/dex. The consistency of this measurement from two different comparison detectors of different technology indicates this result is robust. The impact of this non-linearity is that photometry of faint (i.e., sky dominated) sources calibrated with WFC3-IR zeropoints will appear 0.04 +/-0.01 mag too faint.
Calibration of the HST NICMOS F110W Using High Redshift Red-Sequence Galaxies
Dr. Pascal Ripoche (LBL)
We present a new method to measure photometry zero-points using high redshift red-sequence galaxies. This method allows us to measure the HST NICMOS F110W zero-point close to the sky level and, thus, avoid calibration errors due to the poorly constrained HgCdTe non-linearity at these faint flux levels. This is the level at which almost all of the highest redshift supernovae observations are obtained with NICMOS camera. We combined HST optical and VLT near-infrared observations of distant red-sequence galaxies to constraint their spectral energy distribution and thus derived NICMOS calibration from the observed fluxes with NIC2 F110W. Using 23 red-sequence galaxies in three distant galaxy clusters, we determine the absolute NICMOS F110W zero point to 2.5% accuracy. This result shows that the non-linearity is over-corrected using the standard STScI pipeline leading to a significant zero-point offset at the sky level. This work has been supported by the Office of Science, U.S. Department of Energy, through contract DE-AC02-05CH11231 and in part by NASA through grants associated with HST-GO-10496 and HST-GO-11799.
WFC3 UVIS and IR channel flat fields
Dr. Elena Sabbi (STScI)
Flat fielding is a standard calibration of step for astronomical data, which allows us to correct for variations in the local response of a detector and improve the accuracy of photometric analysis. As for ACS, ground based flat-fields (Sabbi et al., ISR 2008-46, Bushouse, ISR 2008-28), acquired during the last thermal vacuum campaign (TV3), are, and will be, the base to remove the pixel-to-pixel variations in the WFC3 data. Tests performed during the Servicing Mission Observatory Verification (SMOV4), that followed the installation of WFC3 on Hubble, indicate that ground based flat-fields do not completely remove local variation in the response of the detector, but that low-frequency structures are still present on both the UVIS (Sabbi., ISR 2009-20) and IR (Hilbert et al., IRS 2009-93) data. As part of the standard calibration of WFC3 we have used observation of the rich globular cluster Omega Centauri to compare the flux of the same stars on different position of the detector and derive a correction to remove the remaining low-frequency structure. Here we will present the characteristics of the ground-based (better knows and P-flat) and the low-frequency (or L-flat) flat fields, how these files are created and their impact on the final astronomical data. At the end we will also present a summary for the future observational plan to further improve the quality of the WFC3 flat-fields.
SNDICE, a calibrated multi-wavelength light source for optical telescope calibration with a stability and a precision of 10-4
Dr. Kyan Schahmaneche (LPNHE-IN2P3-Paris 7 University)
The need for a precise photometric calibration has grown over the last few years. Several projects based on high precision photometry, like the measurement of the dark energy equation of state using type Ia supernovae have now reached a point where the photometric calibration must be better than the one percent. To achieve such a precision, usual astrophysical calibration procedures relying on star observations are limited by the knowledge of the emission spectra of the reference standards (Vega or the HST white dwarf calibrators). More precisely, as the cosmological parameters are measured comparing fluxes of nearby supernovae to fluxes of distant highly redshifted supernovae, the key-point is the inter-calibration between different bandwidths. Instrumental devices have been developed to study, monitor and model the transmission curves of the astronomical detector (telescope + camera) with an precision of the order of one per-thousand. Such instrumental devices are a first step in the setup of a new calibration procedure for astrophysical photometry measurements. In this paper, we present such a device called SNDICE (SuperNovae Direct Illumination Calibration Experiment) based on the concept of the direct illumination of the instrument by LEDs whose emitted light can be controlled with an precision of 10-4. The opportunity offered by the progress of LED technologies for supporting CCD photometry was underlined in a first paper which described SNDICE-type systems. The light beams emitted by these LED are monitored by photodiodes located along the path of light and the all calibration device has been first calibrated on test bench with respect to a NIST photodiode. The SNDICE project has started in the Spring 2007 and was installed in January 2008 at the Canada France Hawaii Telescope (CFHT) in Hawaii in order to complement the study of the photometric response of the extra wide field camera MegaCam in the SNLS experiment. We present here first results obtained with this prototype.
NICMOS Coronagraphy: Recalibration and the NICMOS Legacy Archive PSF Library
Dr. Glenn Schneider (Steward Observatory, University of Arizona)
NICMOS coronagraphy, with well-matched template Point Spread Function (PSF) subtraction, probes the closest environments of occulted targets with the highest imaging sensitivity in intrinsically high contrast fields at the smallest radial distances afforded, uniquely, by HST. NICMOS PSF-subtracted coronagraphy has been invoked in a wide variety of HST programs with science themes as divergent as detecting and characterizing disks of circumstellar material in neo-natal stellar environments, to studying faint nebulosity associated with luminous active galaxies, to searching for planetary-mass companions in extrasolar planetary systems recently born and in the "stellar graveyard." The investment in HST time in the execution of these and other NICMOS coronagraphic programs, has met with mixed returns. Stunning successes importantly advancing their fields highlight more frequent, and unfortunately somewhat common, failures arising from highly compromised technically-achievable performance due to the lack of suitable template PSFs required to produce high-fidelity, photometrically robust, high contrast coronagraphic images. To remedy this situation are undertaking a rigorous, homogeneous, and complete recalibration and analysis of the full archival set of raw NICMOS coronagraphic images (through HST Cycle 15) previously obtained and residing in the MAST to create a Legacy library of template PSFs enabling the recovery of the large body of science otherwise lost. This PSF library and enhanced data recalibration processes, along with generically applicable analysis software we have created, will: (1) critically augment the needs of observational programs reliant on high fidelity PSF subtractions, (2) increase survey yields and improve photometric efficacy, (3) reduce the observing time (HST orbit allocations) otherwise required for near-contemporaneous reference PSF observations, and (4) greatly enrich the yet-unrealized potential of the many NICMOS coronagraphic observations already acquired from the broad spectrum of science programs previously executed. Here we discuss the recalibration mythologies, data products, and analysis tools that we have developed under HST AR program 11279 and are delivering to STScI/MAST.
A New Technique for Measuring Absolute Proper Motions with HST: Using Background Galaxies as Positional References
Dr. S. Tony Sohn (STScI)
Over the past few years, it has been demonstrated that HST is well suited for high accuracy astrometric sciences. For example, HST's imagers have been used to measure relative proper motions of stars for deriving the internal dynamics of globular clusters. Absolute proper motions have also been measured in several studies but so far, they relied on a single reference object, a quasi-stellar object (QSO) within the field. In this talk, we present a new way of measuring proper motions of stars. Instead of relying on background QSOs, we developed a technique that uses numerous background galaxies as positional references. We discuss this technique in detail and present preliminary results for one test case, proper motion of M31. Our new method can in principle be applied to any existing multi-epoch HST data and opens up a possibility to measure accurate transverse motions for galaxies out to a few Mpc.
The Astrometric Context of HST in 2010
Prof. William van Altena (Yale University)
For the past 20 years, the HST has occupied a unique niche in astrometry, that of high-resolution imaging and interferometry-based astrometry. As a consequence of its small field of view, HST astrometry has been primarily oriented towards the observation of carefully selected targets rather than surveys searching for new classes of objects. Ground-based astrometric, photometric and radial velocity surveys have played crucial roles in characterizing classes of objects and identifying astrophysically-interesting targets worthy of observation by the HST. The symbiotic nature of HST and ground-based astrometry is illustrated by surveys of visual and spectroscopic binaries that lead to the identification of binaries in critical parts of their orbits where one or a few HST Fine-Guidance Sensor observations of their angular separation can lead to a definitive mass determination. Photometric surveys have identified numerous Cepheid variables in the Milky Way that might be used to calibrate the cosmic distance scale, however their distances generally remain estimated only from their spectro-photometric characteristics. HST-FGS observations of carefully selected and relatively nearby Cepheids have led to a solid independent calibration of the cosmic distance scale. Ground-based observations of galactic globular clusters have determined the physical makeup of those clusters and characterized the evolutionary tracks of stars with differing masses and metallicities, but only HST imaging astrometry in conjunction with radial velocity observations have made it possible to determine the dynamical state of the clusters. The above examples highlight only a few contributions of HST astrometry to the critical calibration of astronomical objects, while in this talk I will try to place HST in the context of how we are using astrometry to understand our local universe.
Slitless Spectroscopy with HST Instruments
Dr. Jeremy Walsh (Space Telescope European Co-ordinating Facility)
Slitless spectroscopy is generally regarded as a niche, perhaps 'difficult', astronomical observation technique. However the low background from space and the high spatial resolution offered by HST instruments has enabled it to become a powerful survey tool, with some applications to single object work. The ST-ECF has been involved with slitless spectroscopy from NICMOS, through ACS to WFC3. The techniques and software have also been developed for the bulk extraction of NICMOS and ACS slitless spectra ingested into the Hubble Legacy Archive. Slitless spectroscopy has been selected for the spectroscopic channel of the Euclid Dark Energy mission currently under study for an ESA Cosmic Vision M-Class mission. The critical choices made in the development of calibration techniques and extraction software for HST slitless spectra are reviewed, with particular emphasis on ACS and WFC3. Possible future developments are outlined.
Fringing in the WFC3/UVIS detector
Michael H. Wong (UC Berkeley)
In late 2010, a star cluster will be observed with narrowband red filters to determine the impact of fringing (position- and wavelength-dependent patterns of brightness variation) on flight data. Ground flat fields show peak-to-trough amplitude variations of 0.5% to 16% (among 12 affected filters). Different "fringe flat fields" will be applied to the flight data to determine the best model fringing correction. The difference between models comes from two detector thickness maps, derived from two separate sets of ground test data. The disagreement between the thickness maps is consistent with an error in the monochromatic illumination wavelengths in one of the data sets, but an unexplained corner-to-corner slope across the detector remains even after correcting for the wavelength error. Flight data are needed to determine which thickness map produces the best correction for fringing.