COS Data Processing Improvements Based on HST SMOV Results
Dr. Thomas Ake (STScI/CSC)
After the Cosmic Origins Spectrograph (COS) was installed onboard the Hubble Space Telescope (HST) in May 2009, it underwent an extensive calibration and characterization check-out during the Servicing Mission Observatory Verification (SMOV) period. The results from this program were used to update reference files and make changes to CALCOS, the COS data processing software. Improvements to the standard data products will be discussed. For the FUV channel, we have begun development of a flat field correction. As an intermediate step, grid wire shadows are now ignored when combining FP-POS exposures. Pulse-height filtering has been activated to reduce background features. For the NUV channel, vignetting corrections are incorporated in the flat field file. For both channels, improvements were made in the wavelength scales, flux calibration, and data quality flagging. Additional data are included in the FITS products to allow users to perform customized processing.
Dither Patterns for JWST's NIRCam
Dr. Jay Anderson (STScI)
This poster will present the dithering strategy for JWST's NIRCam. Except in rare circumstances, such as transit studies, all exposures with NIRCam will be dithered. We have designed an extensive set of dither patterns that should address the many varied scientific objectives users may want to pursue. The patterns are flexible enough to allow emphasis on small dithers to improve sampling, large dithers to cover field or mitigate L-flat and detector defects, or an optimal combination of the two. The canned dithers provided here should ensure that both PI-science and the archive are of the highest possible quality.
Detecting Cosmic Rays in Infrared Data
Miss Rachel Anderson (STScI)
Cosmic rays are a known problem in infrared astronomy, causing both loss of data and data accuracy. The problem becomes even more extreme when considering data from a high radiation environment such as in orbit around Earth, or outside the Earth's magnetic field altogether, unprotected, as the James Webb Space Telescope will be. To find the best method to correct for this disturbance we develop three cosmic ray detection methods, apply them to simulated multiaccum ramps with varying combinations of slope, number of frames, number of cosmic rays, and cosmic ray frame number and strength, and then we compare the results. The methods discussed include a 2-point difference method, a deviation from the fit method, and a y-intercept method.
WFC3 UVIS Detectors
Dr. Sylvia Baggett (STSCI)
Installed on the Hubble Space Telescope (HST) in May 2009, the Wide Field Camera 3 (WFC3) is a panchromatic camera possessing both a UVIS and IR channel. The UVIS channel contains a pair of 4096x2051 pixel e2v Technologies CCDs which cover a 160x160 arcsec field of view at 0.04 arcsec/pixel. A set of 62 filters (including the quads) plus an ultraviolet grism provide coverage of the full spectral range, from 200 to 1000nm. On-orbit performance has been excellent: read noise is ~3e-, dark current is now ~3 e-/pix/hr, and linearity is better than 1% up to 65K-70K e-, depending upon amp. The monthly anneal procedures have been effective, repairing a large fraction of the hot pixels that develop over time, the charge transfer efficiency is evolving as expected, and the 'bowtie' monitor program has successfully prevented any hysteresis issues. This poster will summarize the detector performance over the first year of on-orbit operations.
New Bad Pixel Mask Reference Files for the Post-NCS Era
Ms. Elizabeth Barker (STScI)
NICMOS calibration monitoring programs regularly obtained calibration images during the post-NCS era between 2002 and 2008. Both flat-fields and darks were taken during this long base-line, which allowed us to create high signal-to-noise bad pixel mask reference files, as well as investigate any temporal dependence of the mask files. We describe here the creation of new mask files based on this extended data set and compare the new masks with the previous versions. The new masks contain more bad pixels compared to the old versions, while there are fewer pixels thought to be affected by "grot".
Post-SM4 Flux Calibration of the STIS Echelle Modes
Ms. Azalee Bostroem (STScI)
Like all STIS spectroscopic modes, STIS echelle modes show a wavelength dependent decline in detector sensitivity with time. The echelle sensitivity is further affected by a time dependent shift in the blaze function. To better correct the effects of the echelle sensitivity loss and the blaze function changes, we derive new baselines for the echelle sensitivities from post HST Servicing Mission 4 observations of the standard star G191B2B. We present how these baseline sensitivities compare to pre-failure trends. Specifically, we highlight where the new results differ from expectations, discuss anomalous results found in E140H monitoring observations, and review plans for future monitoring and calibration of these modes.
JWST Fine Guidance Sensor Calibration
Dr. Pierre Chayer (STScI)
The primary function of the Fine Guidance Sensor (FGS) is to provide continuous pointing information to the Observatory. The FGS will image two separate regions of the JWST field of view onto two independent 2k x 2k infrared focal planes arrays, which will cover a wavelength range of 0.6 to 5 microns. Once in fine guiding, the FGS will provide continuous pointing information to a precision of < 5 milli-arc seconds at an update rate of 16 Hz for J ~ 18.5 magnitude stars. In order to fulfill its functions, i.e., Identification, Acquisition, Track, and Fine Guide, the FGS will require calibration. In this paper, we present a brief overview of the FGS, summarize its calibration plan, and highlight some results from the engineering test unit cryovac test that was performed during the spring of 2010.
Measuring Aperture Locations in the HST Focal Plane
Dr. Colin Cox (Space Telescope Science Institute)
A knowledge of the positions of the HST cameras to within a few arc-seconds is clearly needed for targeting, but a more accurate knowledge is valuable for several reasons. First, the COS aperture is only 2.5 arcseconds in diameter and so requires accurate pointing to be sure not to miss a target. Additionally, multi-mission archives, multi-wavelength campaigns, coordinated observations and image mosaic generation all benefit by having the most accurate possible positions. Normal HST operations rely on the guide star catalog, (GSC2) in which stellar positions are known relatively to about 0.18 arc-seconds across an HST field of view. To obtain more accurate positions, we use as a target the open cluster M35 and an associated HST PASSOPS catalog. This catalog contains positions and proper motion measurements leading to a positional accuracy of 30 mas. By using stars from this catalog for both guiding and target we can expect to define each position to within about 50 mas. For the larger apertures we measure several stars simultaneously, statistically obtaining a further improvement in accuracy and coarsely tracking orientation. Since SM4 we have measured aperture locations for ACS, STIS, WFC3 and COS and show that positions are stable to within a few tenths of an arc-second and that, in general, overall pointing and astrometry for HST is dominated by GSC2 catalog errors. COS exhibits a trend in position not unusual for a newly flown instrument. Nevertheless we have established the position of the COS aperture well enough so that, for many observations, a blind pointing may be used in place of the full target acquisition procedure.
The NIRSpec calibration concept
Dr. Guido De Marchi (European Space Agency)
NIRSpec is the main near-infrared spectrograph on board the James Webb Space Telescope, a segmented off-axis telescope in deep space. Besides the traditional 'fixed slits' for long-slit spectroscopy, NIRSpec offers both integral field unit and multi-object spectroscopy capabilities. The highest level of multiplexing is provided by the Micro-Shutter Array, a fixed grid of micro-shutters allowing up to ~ 100 objects to be observed simultaneously over a large field of view (~ 10' square) and over nearly a factor of 10 in wavelength, since NIRSpec's detector response ranges from 0.6 to 5 micron. The combination of these factors results in a number of unique challenges for an efficient calibration of the instrument. In this paper we present a high-level description of the calibration and outline some of the challenges that it entails.
NIRSpec - the JWST Multi-Object Spectrograph
Dr. Guido De Marchi (European Space Agency)
The Near-Infrared Spectrograph (NIRSpec), to be launched in 2014 on the James Webb Space Telescope, will be the first slit-based multi-object spectrograph used in space, and is designed to simultaneously provide spectra of faint objects in the range 0.6 - 5.0 micron at resolutions of R=100, R=1000 and R=2700. The all-reflective wide-field optics, a novel MEMS-based programmable micro-shutter array slit selection device, and large format low-noise HgCdTe detectors combine to allow simultaneous observations of >100 objects within a 3.4x3.4 arcmin^2 field. A 3x3 arcsec^2 integral field unit and five fixed slits are also available. The NIRSpec sensitivity is expected to allow detection of a continuum flux of 20 nJy (AB>28) in R~100 mode, and a line flux of 6x10-19 erg s-1cm-2 in R~1000 mode, both at S/N > 3 in 104 s. NIRSpec is being built for the European Space Agency by EADS Astrium as part of the European contribution to the JWST mission, with the micro-shutter and detector arrays being provided by NASA/GSFC.
HST Cycle 19 Exposure Time Calculators
Dr. Rosa Diaz (STScI)
The Exposure Time Calculator (ETC) is a web-based application that assists users in calculating the exposure time needed for their HST observations, or the Signal-to-Noise Ratio (SNR) they can attain with a given HST observing time. These quantities are key for the preparation of proposals and observations during Phase I and Phase II of the proposing cycle and therefore have to be sufficiently accurate for each of the supported observing modes of all the HST instruments. Developing a general tool that shares communality among the different instruments is complicated, not only form the point of view of attaining accuracy of the calculations but also regarding reliability, portability, and maintainability. We are currently in the final stages of the development of the new ETC for Cycle 19 in Python. This new version improves these qualities and meets the needed level of reliability for the ETC. It also provides a basis for JWST Exposure Time Calculators; in preliminary development, with a limited-functionality prototype to be available in Spring 2011. This poster and demo shows the new tool, its improvements over the previous ETC and the current status of the new version.
On-Orbit Measurement of the Point Spread Function on the WFC3 Detectors
Dr. Linda Dressel (STScI)
Knowledge of the point spread function (PSF) on a detector is important for scientific applications such as photometric measurements and analysis of blended sources. It is also important for ensuring that good optical alignment of the instrument is achieved and maintained. The quality of the PSF on HST instruments varies over the detector and varies with time. The focus changes throughout an orbit as the optical bench "breathes", and the average focus changes slowly over time. Locally, the detected PSF is affected by pixelation and charge diffusion or interpixel sensitivity variation. The PSF is also dependent on wavelength. All of these factors both complicate PSF analysis and make it more challenging to provide useful PSFs to observers. We will review the on-going measurement and analysis of the PSF on the WFC3 detectors.
Anomalies and Artifacts of the WFC3 UVIS and IR Detectors: An Overview
Mr. Michael Dulude (STScI)
The WFC3 Quicklook project was designed to provide basic visual inspection of every UVIS and IR image acquired by WFC3. Since first light, a number of anomalies or artifacts have been found. These include trails from passing satellites, light leaks, scattered light artifacts, and a handful of detector idiosyncrasies. In this poster, we present a "rogue’s gallery" detailing the various types of anomalies found so far, their likely causes and possible remedies.
COS External Spectroscopic Performance: FUV spectral and Spatial Resolution
Dr. Parviz Ghavamian (STScI)
During the SM4 Servicing Mission Observatory Verification (SMOV) we discovered that the on-orbit shape of the COS LSF with the HST optical telescope assembly (OTA) deviates from the profile measured in ground testing due to the appearance of broad wings. The wings are caused by mid-frequency wave-front errors (MFWFEs) that are produced by the zonal (polishing) errors on the HST primary and secondary mirrors; these errors could not be simulated during ground testing. The MFWFE effects are particularly noticeable in the FUV. While the shape of the pre-launch LSF is well described by a Gaussian in the FUV, the on-orbit LSF has up to 40% of its total power distributed in non-Gaussian wings. The power in these wings is largest at the shortest wavelengths covered by the COS medium-resolution gratings (~ 1150 A). The effect decreases with increasing wavelength but has a non-negligible effect on encircled energies even at the longest wavelengths. We have calculated optical models incorporating the MFWFE effects into the LSF for the whole spectral range covered by the FUV and NUV medium-resolution gratings. We show that for the FUV, the convolution of these model LSFs with high-resolution STIS echelle spectra yields an excellent match to the on-orbit COS spectra of the same targets. The model LSFs are available online and can be used by COS observers to assess the impact of the MFWFE broadening on their COS spectra. In ground tests it was shown that COS can spatially resolve two equally bright objects separated by 1 arcseconds in the cross-dispersion direction in the FUV. Using the FUV spectra of white dwarfs acquired during the Cycle 17 COS Spectroscopic Sensitivity Monitoring program, we show the on-orbit spatial resolution (as defined by the full-width half maximum of the spectrum along cross-dispersion) meets this specification, but in a wavelength-dependent manner. The wavelength dependence is primarily due the astigmatism introduced by the FUV gratings along cross-dispersion. The spatial resolution also depends on the central wavelength (CENWAVE) setting used, with longer CENWAVE settings yielding progressively better spatial resolution. The spatial resolution ranges from 0.5'' to 1.8'' for G130M, 0.3'' to 0.6'' for G160M and 0.4'' to 2.0'' for G140L.
WFC3 UVIS Linearity Near and Beyond Saturation
Dr. Ronald Gilliland (STScI)
The linear behavior of the WFC3 CCDs near and beyond saturation will be characterized through comparison of counts in back-to-back short and long exposures on rich star fields where pairs of simple aperture sum photometry can be readily compared. If near perfect linearity exists, then the ratio of counts after summing over all pixels that have been bled into past saturation for the long exposure will simply reflect the relative exposure time. ACS and STIS both showed excellent linearity when using a gain keeping saturation on-chip. The default gain of WFC3 keeps saturation safely on-chip, however minor deviations from linearity at the ~10% level exist for stars that are more than 2 magnitudes brighter than saturation. This behavior is different on the two CCDs and will be characterized. A rough calibration sufficient to maintain photometric accuracy to 1-2% even for heavily saturated stars will be given. A map of the count level at which saturation sets in for point sources (CCD full well depth) will also be shown.
Post-SM4 ACS/WFC Bias Striping: Characterization And Mitigation
Dr. Norman Grogin (STScI)
All images taken with the restored ACS Wide Field Camera (WFC) exhibit a modest additional noise component in the bias level that is constant along rows of the two WFC CCDs (spanning both amplifier quadrants on each CCD). We believe this to be caused by low-frequency (1 mHz to 1 Hz) 1/f-noise on the reference voltage from the new Application-Specific Integrated Circuit (ASIC) that governs the offset of the pixel signal subsequent to its correlated double-sampling. The striping amplitude is roughly Gaussian with sigma=0.9-, as compared with the underlying WFC read noise of 4.2e- (gain=2). The OPUS pipeline superbias and superdark images, averaged from dozens of individual biases and darks, show very little residual striping and thus do not exacerbate the striping in the pipeline-processed science images. The impact to most science programs will be insignificant; however we recognize that the highly correlated nature of this striping may complicate analyses at very low surface brightness, particularly for images where the background counts are also low. In addition to our detailed characterizations of the striping and its evolution during Cycle 17, we present results from our testing of stripe-removal codes to correct a broad variety of science images. We have determined that the WFC overscan is a poor estimator of the stripe amplitude, requiring the stripes to be fit across the imaging area where care is needed to avoid removing source flux as well. STScI is making the stripe-removal code available to the community as stand-alone software to correct flat-fielded ACS/WFC images (FLTs) prior to drizzling.
Measurement of the Count-Rate Non-linearity in WFC3-IR Detectors
Dr. Robert Hill (NASA/GSFC)
The Detector Characterization Laboratory at NASA/GSFC has investigated the count-rate non-linearity (reciprocity failure) characteristics of 1.7μm cut-off HgCdTe devices that are very similar to the WFC3-IR detector. The reciprocity failure follows a power law behavior over the range of fluxes tested (0.1-10^4 photons/second). The slope of the power law varies among detectors, ranging from ~0.3-1%/dex at 1.1μm, which is much smaller than the ~6%/dex effect observed with the HST NICMOS 2.5μm cut-off detectors. Reciprocity failure is nevertheless an important effect in the calibration of WFC3 data, as well as in other applications in which there is a large difference in flux between the photometric standards and the scientific sources of interest. Furthermore, the variation among detectors demonstrates that demonstrates that a measurement made with the WFC3-IR detector is necessary in order to provide a proper calibration of the WFC3-IR data.
The Detection and Removal of Large-scale Background Structures in NICMOS Observations
Dr. Eric Hsiao (LBNL)
After applying the standard corrections for well-studied NICMOS anomalies, significant large-scale spatial background variation remains. We report on the detections of fringing in the F110W filter, and a time-dependent residual amplifier glow in the calibrated NICMOS deep science images. We also describe methods developed to further correct these anomalies. Principal component analysis applied to our science images revealed that the intensity of the residual amplifier glow decays exponentially with time and resets every orbit. The images can be separated into two groups according to the glow level. A model of the background structure is derived from the algebraic manipulation of stacked science images and consists of the following components. A constant component is dominated by the residual amplifier glow as high as 20 DN at the corners and residual persistence structure at the center. A second component which scales with sky level displays clear fringe pattern with 15% variation for F110W images, but none is detected for F160W images. Using these model components to correct for the anomalies significantly improves the cosmetic appearance of NICMOS images and reduces the magnitude scatter in the photometry of faint galaxies by 20%. 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.
Removing the herring-bone pattern-noise from all HST/STIS Side-2 CCD data: a factor ~3 enhancement in sensitivity
Dr. Rolf Jansen (ASU, School of Earth & Space Exploration)
When STIS resumed operations in July 2001 using its redundant "Side-2" electronics, the read-noise of the CCD detector appeared to have increased by ~1 e- due to a superimposed and highly variable "herring-bone" pattern-noise. For the majority of programs aiming to detect signals near the STIS design limits, the impact of this noise is far more serious than implied by a mere 1 e- increase in amplitude of the read-noise, as it is of a systematic nature and can result in ~8 e- relative deviations (peak-to-valley). On this poster, I will summarize a method to cleanly remove this pattern-noise from raw STIS CCD frames (Jansen et al. 2002; Brown 2001), report on an Archival Calibration Legacy program to (semi-)automatically remove the herring-bone pattern-noise from *all* raw, un-binned, Side-2 STIS/CCD frames taken between Jul 2001 and Aug 2004, and present trends in characteristics of the noise pattern. This work was funded by grant HST-AR-11258 from STScI, which is operated by AURA under NASA contract NAS5-26555.
Cosmic Origins Spectrograph Commissioning: Overview of COS Servicing Mission Observatory Verification
Dr. Charles (Tony) Keyes (Space Telescope Science Institute)
The Cosmic Origins Spectrograph (COS) was installed on the Hubble Space Telescope (HST) during Servicing Mission 4 (SM4) in May 2009. COS is designed and optimized to obtain spectra of faint objects at moderate spectral resolution (R>16,000) with two channels: FUV, covering wavelengths from 1150 to 1450 Å; and NUV, covering 1700 – 3200 Å. Two low resolution gratings (R>1500) cover the <900 – 2050 Å (FUV) and 1650 – 3200 Å (NUV) wavelength regions. An imaging capability is also available on the NUV channel. As part of the HST Servicing Mission Observatory Verification (SMOV) program, an extensive period of checkout, fine-tuning, and preliminary characterization began after the installation of COS. The COS SMOV program was a cooperative effort between the Space Telescope Science Institute and the Instrument Definition Team based at the University of Colorado. Nearly 2800 COS exposures in 34 separate observing programs were obtained during the course of SMOV. Early activities included an initial instrument functional checkout, high-voltage turn-on and initial characterization of the detectors, NUV and FUV channel focus and alignment, wavelength calibration, and target acquisition verification and assessment. Once this initial period was completed, science-related calibrations and verifications were performed in order to prepare the instrument for normal operations. These activities included flux calibration, detector flat field characterization, spectroscopic performance verification, high S/N operation, and thermal and structural stability measurements. We present the design, execution and results of the SMOV program, including the interrelationships and dependencies between the various tasks, and how the pre-launch plan was adjusted in real-time due to changing conditions.
Internal Monitoring of WFC3/UVIS Charge Transfer Efficiency
Vera Kozhurina-Platais (STScI)
We summarize the results of one year of Charge Transfer Efficiency (CTE) monitoring of the WFC3 UVIS in-flight detector, based on data acquired during the monthly internal Extended Pixel Edge Response (EPER) test. We present an algorithm for CTE assessment and fit a power-law functional dependency of CTE on signal level. We find that at each signal level, CTE declines linearly over time and CTE losses are worst at the lowest signal levels. We compare our in-flight results to similar pre-flight baseline data based on radiation tests performed on a flight-like device.
An example reduction of WFC3/IR slitless data
Dr. Martin Kuemmel (Space Telescope - European Coordinating Facility)
The G102 and G141 grisms in the WFC3 IR channel together cover the wavelength range 0.8-1.6 micron with resolutions of R~210 and R~130, respectively. In this contribution we present an example reduction of G102 and G141 slitless data taken during the WFC3 Early Release Science programme. All core tasks are done by the spectroscopic extraction software aXe, which was specifically designed to handle data from the HST slitless modes. The reduction scheme includes a sky background subtraction with a master sky image to achieve a homogeneous, flat background for the spectral extraction and the most recent in-orbit calibrations. Moreover we present a new method for co-adding data from individual slitless images that, similar to MultiDrizzle in direct imaging, allows the rejection of deviating pixels from e.g. cosmic ray hits. As results we show the spectra of some emission line galaxies down to m(F140)_AB=24.5, demonstrating the remarkable efficiency and capability of the WFC3 NIR grisms.
WFC3 Low-Frequency Flat Field Corrections
Ms. Jennifer Mack (STScI)
Multiple dithered observations of the globular cluster Omega Centauri (NGC 5139) have been used to measure inflight corrections to the WFC3 UVIS and IR ground flat fields for a subset of key filters. To obtain an adequate characterization of the flat field over the detector field of view (FOV), 9 pointings were obtained for each filter using a 3x3 box dither pattern with steps of approximately 25% of the FOV. By measuring relative changes in the brightness of a star over different portions of the detector, low-frequency spatial variations in the detector response (L-flats) have been used to correct the flat fields obtained during ground testing. The broad wavelength range covered by these observations allow an interpolation of the L-flat correction for the remaining wide, medium and narrow-band filters, assuming a simple linear dependence with pivot wavelength. Initial results indicate that the required L-flat corrections are ±1.5% (standard deviation) in the IR and ±1.0% in the UVIS, and that the photometric response for a given star after applying the L-flat correction is now stable to better than 1% for any position in the field of view. Followup observations of the same field at multiple orientations will be used to verify the accuracy of the L-flat solutions and to quantify any temporal changes in the detector response while in orbit.
The JWST Tunable Filter Imager and its Calibration
Dr. Andre Martel (STScI)
The JWST Tunable Filter Imager (TFI) is a sensitive camera that shares the optical bench of the Fine Guidance Sensor (FGS). It provides JWST with the capability of imaging a 2.2'x2.2' field-of-view through narrow-band filters over a wavelength range of 1.5 – 5.0 um, with a non-functional region between 2.5 and 3.2 um. The narrow-band filters are characterized by spectroscopic resolving powers of ~75 to ~120 for central wavelengths selected by the user with the aid of a tunable filter. The TFI is also capable of high-contrast imaging with a choice of four coronagraphs and a non-redundant mask. Here, we provide a brief summary of the technical specifications and scientific capabilities of the TFI and present an overview of the upcoming ground calibration and on-orbit commissioning activities.
Dither and drizzle strategies for Wide Field Camera 3
Mr. Max Mutchler (Space Telescope Science Institute)
Observing strategies and data reduction methods for the ultraviolet-visible (UVIS) channel of Hubble's Wide Field Camera 3 (WFC3) inherit much from the Advanced Camera for Surveys (ACS), since they are similar cameras. The infrared (IR) channel of WFC3 is an extraordinary new capability for Hubble, and there is less conventional wisdom established for it's usage. The author presents the relevant issues and strategies for obtaining optimal results from both the UVIS and IR channels of WFC3. This is based on experience with the Early Release Observations (ERO), the Early Release Science (ERS) program, Hubble's 20th Anniversary mosaic of HH901 in the Carina Nebula, and moving-target Solar System observations. The author will further illustrate these topics, and assist users with their own HST data sets, in the MultiDrizzle course being offered on the Tuesday afternoon before the HST Calibration Workshop.
The Space Telescope Imaging Spectrograph Flat Fielding
Mr. Sami-Matias Niemi (STScI)
The Space Telescope Imaging Spectrograph (STIS) has two types of reference files that are related to flat-fielding: L- and P-flats. A low-order flat field (L-flat) consists of a map of the large scale (> 64 pixels) variations in the sensitivity across a detector and contains the wavelength-dependent, low spatial frequency information about the uniformity of the detector. Instead, a pixel-to-pixel flat field reference file contains the high-frequency pixel-to-pixel variations and the small scale blemishes like dead and bright pixels, thinning differences, and dust motes. The main features of the P-flats are the high frequency odd-even pattern seen in the high-resolution MAMA exposures and the small scale blemishes from dead and bright pixels, dust motes, etc. seen in the charge-coupled device (CCD) images. We describe the generation of post Servicing Mission 4 STIS P-flats. Application and the effectiveness of the new P-flats is also discussed.
Wavelength Calibration of the Cosmic Origins Spectrograph
Cristina Oliveira (STScI)
The Cosmic Origins Spectrograph (COS) was installed during the most recent servicing mission of the Hubble Space Telescope. The FUV channel, providing more than 10x the FUV throughput of STIS, covers the wavelength range from 1150 to 1800 A with the medium resolution gratings (G130M/G160M) and from ~900 to 2400 A with the low resolution grating (G140L). The medium resolution gratings in the NUV channel (G185M/G225M/G285M) cover ~1700 to 3200 A, while the low resolution grating (G230L) covers 1700 to 3200 A. We describe the wavelength calibration of the FUV and NUV channels, which uses a combination of ground and on-orbit data obtained during SMOV. We consider the effect of drift of the Optics Select Mechanisms (OSM) on the wavelength scales and describe how the onboard wavecal lamps allow us to correct for the OSM drift. We report the accuracy achieved with the current wavelength calibration strategy and summarize our cycle 17 wavelength calibration program.
Monitoring of the Wavelength Calibration Lamps for the Hubble Space Telescope
Dr. Ilaria Pascucci (Space Telescope Science Institute)
The Space Telescope Imaging Spectrograph (STIS) and the Cosmic Origins Spectrograph (COS) are the two optical-UV spectrographs on board the Hubble Space Telescope. To determine the wavelength scale for individual science observations, internal arc lamp spectra accompany most observations of external targets. Here we present a detailed analysis of the changes in the COS and STIS internal lamp fluxes and spectra over time, and also compare our results to pre-launch ground testing, and to laboratory accelerated aging testing of similar lamps. Most of the analysis presented here focuses on the behaviour of the lamps in the far-UV (FUV). We find that the STIS LINE lamp has faded by a factor of about 15 in the very short FUV wavelengths (1150-1200 Angstrom) over the 13-year period on which STIS was in space, a much steeper fading than predicted from accelerated aging tests in the laboratory. We also find that all STIS lamps have faded during the period in which the spectrograph was not operational (2004-2009) thus pointing to on-orbit conditions as an additional and important cause of lamp fading. We report that the COS P1 lamp output appears to decline with usage with a similar slope as the LINE and HITM1 lamps on STIS. Finally, we recommend switching from the LINE to the HITM2 lamp for a more efficient wavelength calibration of the STIS settings covering the very short FUV wavelengths.
On-Orbit Performance of COS Target Acquisitions
Dr. Steven Penton (University of Colorado)
We present an analysis of the on-orbit performance of the COS target acquisition (TA) procedures, both imaging and spectroscopic. In particular, we will discuss the impact of the Cycle 17 FGS-to-COS science instrument aperture file (SIAF) adjustments and their impact on Cycle 18 COS TAs. Many observers will be able to skip the time consuming ACQ/SEARCH procedure and proceed directly to the imaging (ACQ/IMAGE) or spectroscopic (ACQ/PEAKXD+ACQ/PEAD) peakup stages. Observers requiring the best photometric accuracies will be advised as to the best TA strategies for NUV and FUV observations.
UBVRI-ZY and ugriz calibrated zero-points from 20 calspec standards covering a wide range of color.
Dr. Andrew Pickles (Las Cumbres Observatory Global Telescope. LCOGT)
We present UBVRI-ZY and ugriz magnitudes for a library of digital spectra, calibrated on 20 calspec standards with standard photometry and covering a wide range of color, from DA white dwarfs through G/K stars to VB8 (M7V). The zero point dispersion is 0.027, 0.020, 0.008, 0.014 and 0.016 mag respectively in UBVRI, where the UmBmVcRcIc filter profiles minimize these dispersions. We find zero points for u=u'=0.03 +/- 0.03 mag, and zero points for griz and g'r'i'z' of 0.0 +/-0.02 mag. With these zero-points we find Vc(Vega) = 0.020 +/- 0.008 mag from an average of 17 calspec standards, and colors which are close to zero. We can derive VL(Vega) = 0.013 +/- 0.02 mag using the Landolt/Cohen VL filter profile and 17 calspec standards, or VL(Vega) = 0.023 +/- 0.02 mag using only DA WD standards, but these latter values should be corrected for color.
WFC3 UVIS Linearity Low to Near Saturation
Mr. Abhijith Rajan (Space Telescope Science Institute)
We present measurement results of the linearity of the WFC3 UVIS detectors by sampling over the response curve from very low counts up to near saturation. We use data collected from the last thermal vacuum test (TV3) before SM4 and from the Cycle 17 calibration proposal 11925. TV3 data tested the response at high count levels; the in-orbit program obtained exposures of NGC 1850 to verify the linearity response and to characterize the linearity at very low counts between a few hundred and a few thousand electrons. Our results indicate that the CCDs are linear to < 1% between a few thousand and ~65,000 electrons.
WFC3/IR CHANNEL FILTER WEDGE
Dr. Elena Sabbi (STScI)
In order to verify and quantify if any of the WFC3/IR imaging filter presents variations in its thickness (filter wedge) we observed the young and bright star cluster NGC 1850 consecutively through all the WFC3/IR filters without moving the telescope. By comparing the stellar coordinates in an exposure with the respect of another we found a weak relation between shift and wavelength that may be ascribed to the fact that the WFC3/IR channel is not exactly in the focal plane of the telescope.
On-Orbit Performance of the COS Detectors
Dr. David Sahnow (Johns Hopkins University)
The Cosmic Origins Spectrograph includes two microchannel plate detectors. The NUV channel uses a MAMA, similar to those used by other HST instruments, while the FUV channel uses a two-segment cross delay line (XDL) device. Since SM4, both detectors have been characterized in detail, using data collected as part of specific calibration programs and in the course of normal science observations. We will describe the on-orbit performance of both detectors since the installation of COS, and describe ongoing detector characterization work. This will include a discussion of the sensitivity, stability, dead time, dark rates (FUV), and flat fields (NUV).
Calibration of B,V,R,I Photometry for the Wide Field Channel of the HST Advanced Camera for Surveys
Dr. Richard Shaw (NOAO)
We present new observations of two Galactic globular clusters, PAL4 and PAL14, using the Wide-Field Channel of the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope, and reanalyze archival data from a third, NGC2419. We matched our photometry of hundreds of stars in these fields from the ACS images to existing, ground-based photometry of faint sequences which were calibrated on the standard B,V,R,I system of Landolt. These stars are significantly fainter than those generally used for HST calibration purposes, and therefore are much better matched to supporting precision photometry of ACS science targets. We were able to derive substantially more accurate photometric transformation coefficients for several commonly used ACS filters compared to those published by Sirianni, et al. (2005), owing to the use of a factor of several more calibration stars which span a greater range of color. We find that the inferred transformations from each cluster individually do not vary significantly from the average, except for a small offset of the photometric zeropoint in the F850LP filter. Our results show that the published prescriptions for the time-dependent correction of CCD charge-transfer efficiency appear to work very well over the ~3.5 yr interval that spans our observations of PAL4 and PAL14 and the archived images of NGC2419.
Dr. Anatoly Suchkov (JHU)
The ACS/WFC detector consists of two CCDs, each of which is read out through two amplifiers. While reading each quadrant of the detector, the electronic crosstalk between the amplifiers induces faint, typically negative, mirror-symmetric ghost images on the other three quadrants. The effect is strongest for high-signal offending (source) pixels. Analysis of crosstalk before Servicing Mission 4 (SM4) showed that its impact on ACS/WFC science is not significant and can be ignored in most science applications. After SM4, crosstalk due to low-signal offenders is much weaker than before SM4 and does not produce ghosts similar to those seen in pre-SM4 images. For high-signal offending pixels, there are substantial differences between the gain=1 eˉ/DN and gain=2 eˉ/DN cases. For the default gain setting of 2, the crosstalk is similar to what it was before the SM4, up to 5–8 eˉ per pixel on the same CCD. For gain=1, the crosstalk is ~100 eˉ per pixel for saturated offending pixels on the same CCD, which is more than an order of magnitude above the pre-SM4 level. The crosstalk from saturated pixels is ~20–30 eˉ per pixel on the other CCD, which is also much higher than it was before SM4.
New NICMOS Flat-fields
Deepashri Thatte (Space Telescope Science Institute)
The A-to-D conversion, or gain, of the NICMOS detectors depends on a combination of detector temperature and bias voltage. The response of the three NICMOS detectors changed slightly due to changes in the A-to-D conversion over both the era before the installation of the NICMOS Cooling System (NCS) 1997-1998 and after the installation 2002-2008. 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. Here we discuss the creation of new flat-field reference files used to correct for changes in the structure of flat- fields caused by the change in A-to-D conversion. These reference files (*_tdf.fits) populate the keyword TDFFILE in the headers of pre-NCS data retrieved from the OTFR after January 23, 2009 and post-NCS data retrieved after November 19, 2008 and are used by the FLATCORR step in calnica version 4.4 and later. Each *_tdf.fits file consists of five different extensions each valid at a different biastemp range. During calibration, the pipeline calculates the biastemp of the science image and then uses the flat-field extension with the best matching biastemp. For consistency check we also created epoch dependent flat-fields for post-NCS data and compared them with the biastemp dependent flat-fields.
Recovery of very low signal-to-noise transiting lightcurves using self-coherence
Mr. Ingo Waldmann (University College London)
The detection and characterisation of telluric planets from space and ground facilities is one of the main goals of future exoplanet research, thus the importance of being able to fully recover and understand data in low signal-to-noise (SNR) conditions is crucial. Methods such as ’self-coherence’ allow for the retrieval of signals buried far below the noise level. In particular, at SNRs < 1 the noise may suppress high-order coefficients in the eclipse signal’s Fourier series. As a consequence the shape of the retrieved transit-curve becomes inherently linked with noise, as well as the overall morphology of the timeseries. Understanding the details of this process is key to fully recovering the shape and depth of the original signal. In this paper we have used Monte-Carlo simulations of very low SNR data, to investigate morphological changes of the original eclipse at low-SNRs. Furthermore, we demonstrate the full recovery of the eclipse’ transit-depth down to SNRs of 0.15 per spectral channel using calibration techniques described here. A clear understanding of methods such as self-coherence is essential for probing ever weaker atmospheric signatures such as the ones observed for extrasolar planets.
Updated Status and Performance of the Space Telescope Imaging Spectrograph's Charged-Coupled Device
Mr. Michael Wolfe (STScI)
A description is provided of the overall performance of the STIS CCD after HST Servicing Mission 4 (SM4) and during Cycle 17 calibrations. Most aspects of CCD performance are found to be consistent with extrapolations of the trends seen prior to the failure of STIS in August 2004. The gain values for 1 and 4 have not changed since the Servicing Mission Orbital Verification after SM4 and the Read Noise through Amp D, as determined from unbinned bias images taken during Cycle 17 calibrations, has for the most part remained constant. As is expected due to the on-orbit radiation environment the dark current is perpetually increasing. Additionally, the slope is still present in the dark current along CCD columns, resulting in a dark current near the detector readout amplifier that is about 32% lower than the mean over the whole detector. The spurious charge has increased as well and has a slope that results in a smaller value at the top of the chip (near the readout amplifier) when compared to the center of the chip. The spurious charge is approximately 38% lower at the top of the chip (row 900) than it is in the center of the chip (row 512) for gain = 1. The same behavior for the gain = 4 spurious charge is also present but the percentage difference between the top of the chip and the center is 6.5%. The charge transfer inefficiency (CTI) has also continued to increase as well, although the increased dark current will significantly moderate the actual CTI losses for typical observations. The currently measured CTI values are in excellent agreement with a simple extrapolation of trends seen during Cycle 17 calibrations. Additionally, the number of hot pixels has continued to increase despite the monthly annealing of hot pixels. Although the gain and read noise have stayed essentially the same, the number of hot pixels, dark current, spurious charge, and CTI have continued to increase. The current sensitivity trends of all CCD modes will also be presented and it will be shown that these trends have essentially remained the same.
Trend of Dark Rates of the COS and STIS MAMA Detectors
Dr. Wei Zheng (Johns Hopkins University)
The dark rate of the STIS NUV MAMA detector was about an order of magnitude higher after the SM4 repair than before, with an initial rate of ~0.01 count/pixel/sec. Measurements over the past year show a dual-component exponential decline with e-folding timescale of ~40 and 400 days. The most recent measurements show a rate of 3.6E-3 count/pixel/sec, which is ~3 times the historical average value. The COS NUV detector dark rate started at a very low value of 6E-5 count/pixel/sec, and has displayed a steady increase, approaching the ground-tested level of 3.7E-4 count/pixel/sec. Still, the rate of COS NUV detector is considerably lower than that of its STIS counterpart. The rates for both detectors are sensitive to detector temperature, and the rate fluctuations can be fit with an empirical model. We will discuss the measurements and trends of the COS and STIS MAMA detector dark rates and their implications for data analysis.