Engineering topics

Lindler, Don

Publications and source records attributed to Lindler, Don.

18 records

NASA NTRS2017

Improved Reference Sampling and Subtraction: A Technique for Reducing the Read Noise of Near-Infrared Detector Systems

Near-infrared array detectors, like the James Webb Space Telescope (JWST) NIRSpec's Teledyne's H2RGs, often provide reference pixels and a reference output. These are used to remove correlated noise. Improved reference sampling and subtraction (IRS(exp 2)) is a statistical technique for using this reference information optimally in a leastsquares sense. Compared with the traditional H2RG readout, IRS(exp 2) uses a different clocking pattern to interleave many more reference pixels into the data than is otherwise possible. Compared with standard reference correction techniques, IRS(exp 2) subtracts the reference pixels and reference output using a statistically optimized set of frequency dependent weights. The benefits include somewhat lower noise variance and much less obvious correlated noise. NIRSpec's IRS(exp 2) images are cosmetically clean, with less 1/f banding than in traditional data from the same system. This article describes the IRS(exp 2) clocking pattern and presents the equations needed to use IRS(exp 2) in systems other than NIRSpec. For NIRSpec, applying these equations is already an option in the calibration pipeline. As an aid to instrument builders, we provide our prototype IRS(exp 2) calibration software and sample JWST NIRSpec data. The same techniques are applicable to other detector systems, including those based on Teledyne's H4RG arrays. The H4RG's interleaved reference pixel readout mode is effectively one IRS(exp 2) pattern.

Methods-statistical↗

NASA NTRS2016

Relative Throughput of the Near-IR Science Instruments for the James Webb Space Telescope as Measured During Ground Testing the Integrated Science Instrument Module

Data were obtained for the purpose of measuring the relative throughput of the Near-IR Science Instruments (SIs) of the James Webb Space Telescope (JWST) as part of the second and third cryogenic-vacuum tests (CV2CV3) of the Integrated Science Instrument Module (ISIM) conducted at the Goddard Space Flight Center (GSFC) in 2014 and 20152016, at the beginning and end of the environmental test program, respectively. This Poster focuses on data obtained as part of the Initial Optical Baseline and as part of the Final Performance test -- two epochs that roughly bracket the CV3 test. The purpose of the test is to trend relative throughput to monitor for any potential changes from gross problems such as contamination or degradation of an optical element. Point source data were taken at a variety of wavelengths for NIRCam Module A and Module B, NIRSpec, NIRISS, Guider 1 and Guider 2 using the Laser Diode (LD) 1.06 micron, LD 1.55 micron, 2.1 micron LED and 3.5 micron LED, as well as for NIRCam Mod A and B and NIRISS using a tungsten source and the F277W, and F480M filters. Spectra were taken using the G140M, G235M, and G395M gratings for NIRSpec, the GRISMR grism for NIRCam Mod A and B and the GR150C grism for NIRISS. The results of these measurements are compared to what would be expected given the efficiency of each of the optical elements in each SI. Although these data were taken as a check against gross problems, they can also be used to provide the first relative throughput estimate for each SI through the various filters source wavelengths measured in their flight-like configurations.

technology development↗

NASA NTRS2013

Principal Components Analysis of a JWST NIRSpec Detector Subsystem

We present principal component analysis (PCA) of a flight-representative James Webb Space Telescope NearInfrared Spectrograph (NIRSpec) Detector Subsystem. Although our results are specific to NIRSpec and its T - 40 K SIDECAR ASICs and 5 m cutoff H2RG detector arrays, the underlying technical approach is more general. We describe how we measured the systems response to small environmental perturbations by modulating a set of bias voltages and temperature. We used this information to compute the systems principal noise components. Together with information from the astronomical scene, we show how the zeroth principal component can be used to calibrate out the effects of small thermal and electrical instabilities to produce cosmetically cleaner images with significantly less correlated noise. Alternatively, if one were designing a new instrument, one could use a similar PCA approach to inform a set of environmental requirements (temperature stability, electrical stability, etc.) that enabled the planned instrument to meet performance requirements

Analysis↗

NASA NTRS2013

The Complex Spin State of 103P-Hartley 2: Kinematics and Orientation in Space

We derive the spin state of the nucleus of Comet 103P/Hartley 2, its orientation in space, and its short-term temporal evolution from a mixture of observations taken from the DIXI (Deep Impact Extended Investigation) spacecraft and radar observations. The nucleus is found to spin in an excited long-axis mode (LAM) with its rotational angular momentum per unit mass, M, and rotational energy per unit mass, E, slowly decreasing while the degree of excitation in the spin increases through perihelion passage. M is directed toward (RA, Dec; J2000) = 8+/-+/- 4 deg., 54 +/- 1 deg. (obliquity = 48 +/- 1 deg.). This direction is likely changing, but the change is probably <6 deg. on the sky over the approx. 81.6 days of the DIXI encounter. The magnitudes of M and E at closest approach (JD 2455505.0831866 2011-11-04 13:59:47.310) are 30.0 +/- 0.2 sq. m/s and (1.56 +/- 0.02) X 10(exp -3) sq. m /sq. s respectively. The period of rotation about the instantaneous spin vector, which points in the direction (RA, Dec; J2000) = 300 +/- 3.2deg., 67 +/- 1.3 deg. at the time of closest approach, was 14.1 +/- 0.3 h. The instantaneous spin vector circulates around M, inclined at an average angle of 33.2 +/- 1.3 deg. with an average period of 18.40 +/- 0.13 h at the time of closest approach. The period of roll around the principal axis of minimum inertia (''long'' axis) at that time is 26.72 +/- 0.06 h. The long axis is inclined to M by approx. 81.2 +/- 0.6 deg. on average, slowly decreasing through encounter. We infer that there is a periodic nodding motion of the long axis with half the roll period, i.e., 13.36+/- 0.03 h, with amplitude of 1 again decreasing through encounter. The periodic variability in the circulation and roll rates during a cycle was at the 2% and 10-14% level respectively. During the encounter there was a secular lengthening of the circulation period of the long axis by 1.3 +/- 0.2 min/d, in agreement with ground-based estimates, while the period of roll around the long axis changed by approx. -4.4 min/d at perihelion. M decreased at a rate of 0.038 (sq m/s) per day in a roughly linear fashion. Assuming a bulk density between 230-300 kg/m3 and a total volume for the nucleus of 8.09 X 10(exp 8) cubic m, the net torque acting on the nucleus was in the range 0.8-1.1 X 10(exp 5) kg m(exp 2) /s(exp 2). In order to bring the spacecraft photometric and imaging data into alignment on the direction of M, the directions of the intermediate and short principal axes of inertia had to be adjusted by 33 deg (on the sky) from the values indicated by the shape model with an assumed homogeneous interior. The adjusted direction of the intermediate axis is RA, Dec = 302 deg., -16.5 deg.. The morning and evening terminators in the images are identified, and the variation of the insolation at three regions on the nucleus associated with active areas calculated. The plume of water vapor observed in the inner coma is found to be directed close to the direction of local gravity over the sub-solar region for a range of reasonable bulk densities. The plume does not follow the projected normal to the surface at the sub-solar point.

Comets↗

NASA NTRS2012

Reducing the Read Noise of HAWAII-2RG Detector Systems with Improved Reference Sampling and Subtraction (IRS2)

IRS2 is a Wiener-optimal approach to using all of the reference information that Teledyne's HAWAII-2RG detector arrays provide. Using a new readout pattern, IRS2 regularly interleaves reference pixels with the normal pixels during readout. This differs from conventional clocking, in which the reference pixels are read out infrequently, and only in a few rows and columns around the outside edges of the detector array. During calibration, the data are processed in Fourier space, which is <;:lose to the noise's eigenspace. Using IRS2, we have reduced the read noise of the James Webb Space Telescope Near Infrared Spectrograph by 15% compared to conventional readout. We are attempting to achieve further gains by calibrating out recently recognized non-stationary noise that appears at the frame rate.

Rauscher, Bernard J.↗

NASA NTRS2012

From Hubble's NGSL to Absolute Fluxes

Hubble's Next Generation Spectral Library (NGSL) consists of R-l000 spectra of 374 stars of assorted temperature, gravity, and metallicity. Each spectrum covers the wavelength range, 0.18-1.00 microns. The library can be viewed and/or downloaded from the website, http://archive.stsci.edu/prepds/stisngsll. Stars in the NGSL are now being used as absolute flux standards at ground-based observatories. However, the uncertainty in the absolute flux is about 2%, which does not meet the requirements of dark-energy surveys. We are therefore developing an observing procedure that should yield fluxes with uncertainties less than 1 % and will take part in an HST proposal to observe up to 15 stars using this new procedure.

Heap, Sara R.↗

NASA NTRS2012

Reducing the Read Noise of the James Webb Space Telescope Near Infrared Spectrograph Detector Subsystem

We describe a Wiener optimal approach to using the reference output and reference pixels that are built into Teledyne's HAWAII-2RG detector arrays. In this way, we are reducing the total noise per approximately 1000 second 88 frame up-the-ramp dark integration from about 6.5 e- rms to roughly 5 e- rms. Using a principal components analysis formalism, we achieved these noise improvements without altering the hardware in any way. In addition to being lower, the noise is also cleaner with much less visible correlation. For example, the faint horizontal banding that is often seen in HAWAII-2RG images is almost completely removed. Preliminary testing suggests that the relative gains are even higher when using non flight grade components. We believe that these techniques are applicable to most HAWAII-2RG based instruments.

Rauscher, Bernard↗

NASA NTRS2012

JWST Near-Infrared Detector Degradation: Finding the Problem, Fixing the Problem, and Moving Forward

The James Webb Space Telescope (JWST) is the successor to the Hubble Space Telescope. JWST will be an infrared optimized telescope, with an approximately 6.5 m diameter primary mirror, that is located at the Sun-Earth L2 Lagrange point. Three of JWST's four science instruments use Teledyne HgCdTe HAWAII-2RG (H2RG) near infrared detector arrays. During 2010, the JWST Project noticed that a few of its 5 micron cutoff H2RG detectors were degrading during room temperature storage, and NASA chartered a "Detector Degradation Failure Review Board" (DD-FRB) to investigate. The DD-FRB determined that the root cause was a design flaw that allowed indium to interdiffuse with the gold contacts and migrate into the HgCdTe detector layer. Fortunately, Teledyne already had an improved design that eliminated this degradation mechanism. During early 2012, the improved H2RG design was qualified for flight and JWST began making additional H2RGs. In this article we present the two public DD-FRB "Executiye Summaries" that: (1) determined the root cause of the detector degradation and (2) defined tests to determine whether the existing detectors are qualified for flight. We supplement these with a brief introduction to H2RG detector arrays, and a discussion of how the JWST Project is using cryogenic storage to retard the degradation rate of the existing flight spare H2RGs.

Rauscher, Bernard J.↗

NASA NTRS2011

Reducing the Read Noise of H2RG Detector Arrays by more Efficient use of Reference Signals

We present a process for characterizing the correlation properties of the noise in large two-dimensional detector arrays, and describe an efficient process for its removal. In the case of the 2k x 2k HAWAII-2RG detectors (H2RG) detectors from Teledyne which are being used on the Near Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST), we find that we can reduce the read noise by thirty percent. Noise on large spatial scales is dramatically reduced. With this relatively simple process, we provide a performance improvement that is equivalent to a significant increase in telescope collecting area for high resolution spectroscopy with NIRSpec.

Rauscher, Bernard J.↗

NASA NTRS2010

Observing the First Stars in Luminous, Red Galaxies

Modern cosmological simulations predict that the first stars are to be found today in luminous, red galaxies. Although observing such stars individually against a background of younger, metal-rich stars is impossible, the first stars should make their presence known by their strong, line-free ultraviolet flux. We have found evidence for a UV-bright stellar population in Sloan spectra of LRG's at z=0.4-0.5. We present arguments for interpreting this UV-bright stellar population as the oldest stars, rather than other types of stellar populations (e.g. young stars or blue straggler stars in the dominant, metal-rich stellar population

Heap, Sally↗

NASA NTRS2009

Ultraviolet Spectral Diagnostics

At redshifts, z>l, the rest-frame mid-UV is brought into view of large, ground-based telescopes. Here, we report on a study of the potential of the rest-frame UV spectrum for deriving the age since the last major episode of star formation in a galaxy. We base this investigation on wide-band (0.2-1.0 microns), low-resolution (R-1000) spectra of single stars in Hubble's Next Generation Spectral Library (NGSL). We find that a combination of mid-UV spectral indices and colors can indeed yield the age of a stellar population, but only if light from the stellar population is unreddened.

Heap, Sally↗

NASA NTRS2009

Luminous Red Galaxies at Z=0.4-0.5

We report on a study of approx.20,000 luminous red galaxies (LRG's) at z=0.4-0.5 observed by the Sloan Digital Sky Survey. In order to differentiate among them, we measured restframe magnitudes, u (3000-3500 A), b (4200-4800 A), and y (5700-6300 A) from the spectra themselves. The galaxies show a significant range in restframe colors and absolute magnitudes. We binned the spectra according to the restframe u-b color and y-band absolute magnitude in order to increase the S/N. We used 3 approaches to estimate the ages and metal content of these binned spectra: via their spectral energy distributions, from spectral-line indices, and by full spectral fitting. The three methods usually produce discordant results

Heap, Sally↗

NASA NTRS2006

Performance of the FOS and GHRS Pt/(Cr)-Ne Hollow-cathode Lamps after their Return from Space and Comparison with Archival Data

The Space Telescope European Coordinating Facility (ST-ECF) and National Institute of Standards and Technology (NIST) are collaborating to study hollow cathode calibration lamps as used onboard the Hubble Space Telescope (HST). As part of the STIS Calibration Enhancement (STIS-CE) Project we are trying to improve our understanding of the performance of hollow cathode lamps and the physical processes involved in their long term operation. The original flight lamps from the Faint Object Spectrograph (FOS) and the Goddard High Resolution Spectrograph (GHRS) are the only lamps that have ever been returned to Earth after extended operation in space. We have taken spectra of all four lamps using NIST s 10.7-m normal-incidence spectrograph and Fourier transform spectrometer (FTS) optimized for use in the ultraviolet (UV). These spectra, together with spectra archived from six years of on-orbit operations and pre-launch spectra, provide a unique data set - covering a period of about 20 years - for studying aging effects in these lamps. Our findings represent important lessons for the choice and design of calibration sources and their operation in future UV and optical spectrographs in space.

Kerber, Florian↗

NASA NTRS2003

UV Spectral Templates for High-Redshift Galaxies

New instrumentation such as DEIMOS on Keck-II now enable deep spectral surveys, and thereby samples of galaxies at younger ages. At a redshift, z = 1, all galaxies are less than 6 Gyr old and hence, have not yet formed horizontal-branch stars. Also, at z = 1, the restframe-UV comes into view, and with it, a new set of spectral diagnostics. UV spectral features are especially important because most of the UV flux comes from stars at the main-sequence turnoff (MSTO). Hence, UV spectral diagnostics enable the ages of z = 1 galaxies to be estimated directly from MSTO stars. In preparation for these high-redshift spectral surveys, we are developing UV spectral templates for stellar populations younger than 6 Gyr using UV-optical spectra of stars observed by HST/STIS. We are also planning to supplement these observations with theoretical spectral grids of stars of various metallicities. In this paper, we present a progress report on the observation-based spectral templates and spectral diagnostics.

Heap, Sara↗

NASA NTRS2003

Time-Resolved UV Spectra of Pulsars with HST/STIS

We have used HST/STIS to observe the young Crab pulsar and the middle-aged PSR B0656+14. The Crab pulsar was observed in both the near- and far-UV, and together with ground-based data we establish a spectrum that covers 1140-9250 A. A flat spectrum with a power-law index of 0.11 +/- 0.04 fits the entire spectral region. With the time-tag mode of the spectrograph we obtain the pulse profile, and show that the near- and far-UV profiles are similar, although the primary peak is marginally narrower. In the far-UV spectrum we also see evidence of blueshifted absorption in C IV 1550 A out to 2500km/s, which could be a sign of the long-debated fast shell around the pulsar. PSR B0656+14 was observed using the prism mode of STIS/MAMA in the near-UV, and with this observation we have made it the first middle-aged for which a UV pulse profile has been established. Preliminary analysis shows two distinct pulses per the pulsar period in near-UV, which is in contrast to a single pulse detected in radio and soft X-ray ranges.

Gull, Theodore R.↗

NASA NTRS2002

Optimal Extraction with Sub-sampled Line-Spread Functions

STIS long-slit medium resolution spectra reduced in CALSTIS extended-source mode with narrow extraction heights (GWIDTH=3 pixels) show photometric uncertainties of +/- 3% relative to point-source extractions. These uncertainties are introduced through interpolation in the spectral image rectification processing stage, and are correlated with the number of pixel crossings the spectral profile core encounters in the spatial direction. The line-spread-function may be determined as a function of pixel crossing- position from calibration data sub-sampled in the spatial direction. This line spread function will be applied to science data to perform optimal extractions and point- source de-blending. Wavelength and breathing effects will be studied. Viability of the method to de-convolve extended source 'blobs' will be investigated.

Collins, Nicholas R.↗

NASA NTRS2002

In Pursuit of a Spectroscopic PSF for STlS First Order Modes

We are attempting to characterize the spectroscopic PSF of the STlS first order grating modes, primarily those that utilize the CCD. The goal is to extract spectroscopic information of sources approaching the diffraction limit of HST with STIS. Diffracted and scattered light of HST, scattered light effects within the STlS and performance of the detector are being considered. Filter fringing, CCD fringing, window reflections, scattering by the detector structure and other effects are being characterized. The response of the system when a fiducial is placed in the focal surface of STIS will be described.

Gull, Theodore R.↗

NASA NTRS2001

UV Timing and Spectroscopy of the Crab Nebula Pulsar

We have used the Hubble Space Telescope and Space Telescope Imaging Spectrograph to obtain Near Ultraviolet (NUV) (1600-3200 Angstroms) and Far Ultraviolet (FUV) (1140-1720 Angstroms) spectra and pulse profiles of the Crab Nebula's pulsar. The pulse period agrees well with the radio predictions. The NUV and FUV pulse profiles are little changed from the visible wavelength profile. Spectra obtained with the Nordic Optical Telescope were combined with the UV spectra for full coverage from 1140-9250Angstoms. Dereddening the spectrum with a standard extinction curve achieves a flat spectrum for E(B-V)=0.52, R=3.1. Lyman alpha absorption indicates a column density of 3.0=/-0.5 x 10(exp 21) cm -2, consistent with the E(B-V) of 0.52. The dereddened spectrum can be fitted by a power law with spectral index alpha=0.11+/-0.04. A broad, blueshifted absorption is seen in CIV (1550Angstroms), reaching a velocity of about 2500 kilometer per second.

Gull, Theodore R.↗