The Lunar Surface Electromagnetics Experiment at Night (LuSEE-Night) is a project designed to investigate the feasibility of observing the Cosmic Dark Ages using an instrument on the lunar far-side. LuSEE-Night will measure the redshifted 21 cm transition of neutral hydrogen over a frequency range of 0.1-50 MHz, covering the redshift range 27 < z < 1100. The LuSEE-Night instrument is a radio frequency spectrometer, consisting of four horizontal monopole antennas, arranged to give wide zenith-pointing beams with two orthogonal linear polarizations. This combination of polarization, spectral, and angular sensitivity will be necessary to separate the cosmological signal from significantly stronger foreground emissions. LuSEE-Night will observe in drift scan during lunar night while the moon shields it from radio frequency interference from both the Earth and sun, and will transmit science and telemetry data back to Earth via an orbital relay during the lunar day. LuSEENight will have to operate in a challenging environment: its electronics must operate under hard radiation, the instrument must be thermally isolated during the cold 100 K lunar night, and have a thermal rejection path to survive the 390 K daytime temperature, and its photovoltaic and battery systems must provide sufficient power to operate during two weeks of lunar night. Furthermore, the instrument spectrometer must be powered throughout the lunar night using only a 7 kWh battery, due to mass limitations. Here we describe the power generation, storage, and delivery subsystems of the LuSEE-Night instrument, and the simulations of expected power generation, draw, and reserves over time which were performed in order to design the power subsystems, and ensure instrument survival and operation throughout the long lunar night. We also describe the Concept of Operations (ConOps) developed for the LuSEE-Night mission, which derives from the power management simulations.
Electron Multiplying Charge Coupled Devices, EMCCD are used as x-ray detectors. The NSLS-II Soft Inelastic x- ray Scattering (SIX) beam line uses two EMCCDs for x-ray detection. Electrons drift and diffuse from generation point toward pixel gates and are collected there. The diffused electrons form a charge cloud distributed over several neighboring pixels. This charge sharing enables coordinate measurements with accuracy better than the pixel pitch. The charge distribution shape has to be taken into account to achieve ultimate accuracy in coordinate measurements. In this paper, we present a method of the charge distribution shape analysis and demonstrate its applications. The number of electrons collected under a pixel is proportional to the shape function integral. These electron packets get transferred to the sense node of the output amplifier. The transfer process could introduce distortions to the original charge distribution. For example, during transfers, electrons in the packet could be exposed to traps if they are present in the sensor. The trapping and later the release processes distort the apparent shape of the charge distribution. Therefore, deviations of the charge distribution shape from the originally symmetrical form can indicate the presence of trap centers in the sensor and can be used for sensor diagnostics.
The pair creation energy,ω and the Fano factor of silicon were measured using a CCD sensor and X-rays from
an 55Fe source. The measurements were performed at a sensor temperature of 185K. The pair creation energy
was measured for X-rays in the 1.7-6.5 keV range. The measured pair creation energy is ω = (3:650 ± 0:009) eV
at the Mn Kα line energy. The Fano factor at this energy is F = 0:128±0:001. The agreement with theory and
previous measurements is satisfactory. The system gain was obtained from at field exposures using the Poisson
distribution properties. These results and the details of our measurement procedure are presented below.
We employ electrostatic conversion drift calculations to match CCD pixel signal covariances observed in at field exposures acquired using candidate sensor devices for the LSST Camera.1, 2 We thus constrain pixel geometry distortions present at the end of integration, based on signal images recorded. We use available data from several operational voltage parameter settings to validate our understanding. Our primary goal is to optimize flux point spread function (FPSF) estimation quantitatively, and thereby minimize sensor-induced errors which may limit performance in precision astronomy applications. We consider alternative compensation scenarios that will take maximum advantage of our understanding of this underlying mechanism in data processing pipelines currently under development. To quantitatively capture the pixel response in high-contrast/high dynamic range operational extrema, we propose herein some straightforward laboratory tests that involve altering the time order of source illumination on sensors, within individual test exposures. Hence the word hysteretic in the title of this paper.
The Large Synoptic Survey Telescope (LSST) camera will be made as a mosaic assembled of 189 large format Charge Coupled Devices (CCD). They are n-channel, 100 micron thick devices operated in the over depleted regime. There are 16 segments, 1 million pixels each, that are read out through separate amplifiers. The image quality and readout speed expected from LSST camera translates into strict acceptance requirements for individual sensors.
Prototype sensors and preproduction CCDs were delivered by vendors and they have been used for developing test procedures and protocols. Building upon this experience, two test stands were designed and commissioned at Brookhaven National Laboratory for production electro-optical testing.
In this article, the sensor acceptance criteria are outlined and discussed, the test stand design and used equipment are presented and the results from commissioning sensor runs are shown.
Dynamic charge collection effects in thick CCDs have received interest in recent years, due to the performance implications for both ground and space based precision optical astronomy. The phenomena manifest as the "brighter - fatter" effect in Point Spread Function (PSF) measurements, and nonlinearity and signal dependence in spatial autocorrelation and photon transfer measurements. In this paper we present validation results from simple, analytically based predictive models for this effect, using an e2v CCD250. The model is intended to provide estimations for predicting device performance based on design parameters.
The LSST Camera science sensor array will incorporate 189 large format Charge Coupled Device (CCD) image sensors.
Each CCD will include over 16 million pixels and will be divided into 16 equally sized segments and each segment will
be read through a separate output amplifier.
The science goals of the project require CCD sensors with state of the art performance in many aspects. The broad
survey wavelength coverage requires fully depleted, 100 micrometer thick, high resistivity, bulk silicon as the imager
substrate. Image quality requirements place strict limits on the image degradation that may be caused by sensor effects:
optical, electronic, and mechanical.
In this paper we discuss the design of the prototype sensors, the hardware and software that has been used to perform
electro-optic testing of the sensors, and a selection of the results of the testing to date. The architectural features that lead
to internal electrostatic fields, the various effects on charge collection and transport that are caused by them, including
charge diffusion and redistribution, effects on delivered PSF, and potential impacts on delivered science data quality are
addressed.
Near-future astronomical survey experiments, such as LSST, possess system requirements of unprecedented
fidelity that span photometry, astrometry and shape transfer. Some of these requirements flow directly to the
array of science imaging sensors at the focal plane. Availability of high quality characterization data acquired
in the course of our sensor development program has given us an opportunity to develop and test a framework
for simulation and modeling that is based on a limited set of physical and geometric effects. In this paper we
describe those models, provide quantitative comparisons between data and modeled response, and extrapolate
the response model to predict imaging array response to astronomical exposure. The emergent picture departs
from the notion of a fixed, rectilinear grid that maps photo-conversions to the potential well of the channel.
In place of that, we have a situation where structures from device fabrication, local silicon bulk resistivity
variations and photo-converted carrier patterns still accumulating at the channel, together influence and distort
positions within the photosensitive volume that map to pixel boundaries. Strategies for efficient extraction of
modeling parameters from routinely acquired characterization data are described. Methods for high fidelity
illumination/image distribution parameter retrieval, in the presence of such distortions, are also discussed.
The Large Synoptic Survey Telescope instrument include four guiding and wavefront sensing subsystems called corner
raft subsystems, in addition to the main science array of 189 4K x 4K CCDs. These four subsystems are placed at the
four corners of the instrumented field of view. Each wavefront/guiding subsystem comprises a pair of 4K x 4K guide
sensors, capable of producing 9 frames/second, and a pair of offset 2K x 4K wavefront curvature sensors from which the
images are read out at the cadence of the main camera system, providing 15 sec integrations. These four
guider/wavefront corner rafts are mechanically and electrically isolated from the science sensor rafts and can be installed
or removed independently from any other focal plane subsystem. We present the implementation of this LSST
subsystem detailing both hardware and software development and status.
X-rays frames offer a lot of information about CCD. 55Fe sources are traditionally being used for CCD gain and charge transfer efficiency (CTE) measurements. The pixel size of modern scientific CCDs is getting smaller. The charge diffusion causes the charge spread among neighboring pixels especially in thick fully depleted sensors. This enables measurement of the charge diffusion using 55Fe X-rays. On the other hand, the usual CTE char- acterization method based on single pixel X-ray events becomes statistically deficient. A new way of measuring CTE using shape and amplitude analysis of X-ray clusters is presented and discussed. This method requires high statistical samples. Advances in test automation and express analysis technique allows for acquiring such statistical samples in a short period of time. The details of our measurement procedure are presented. The lateral diffusion measured using e2v CCD250 is presented and implications for X-ray cluster size and expected cluster shape are discussed. The CTE analysis using total X-ray cluster amplitude is presented. This analysis can reveal CTE problems for certain conditions. The statistical analysis of average X-ray cluster shape is presented. Characteristics X-rays can be used for the whole system absolute calibration. We demonstrate how spectral features of 55Fe and 241Am rad. sources are used for system linearity measurements.
The pocket pumping technique is used to detect small electron trap sites. These traps, if present, degrade CCD charge transfer efficiency. To reveal traps in the active area, a CCD is illuminated with a flat field and, before image is read out, accumulated charges are moved back and forth number of times in parallel direction. As charges are moved over a trap, an electron is removed from the original pocket and re-emitted in the following pocket. As process repeats one pocket gets depleted and the neighboring pocket gets excess of charges. As a result a dipole" signal appears on the otherwise at background level. The amplitude of the dipole signal depends on the trap pumping efficiency. This paper is focused on trap identification technique and particularly on new methods developed for this purpose. The sensor with bad segments was deliberately chosen for algorithms development and to demonstrate sensitivity and power of new methods in uncovering sensor defects.
Tight requirements on the Large Synoptic Survey Telescope point spread function (PSF) demand sensor contribution
to PSF be both small and well characterized. The sensor PSF is determined by the lateral charge
diffusion on the drift path from the photon conversion point to the gates. The maximum drift path occurs
for photons converted at the window, for blue optical photons in particular. Charges generated at the window
surface undergo "worst case" charge spreading and the blue optical PSF is used to characterize the sensor's PSF.
Different techniques for charge diffusion characterization have been developed, each with its own systematics
and measurement difficulties. A new way to measure charge diffusion using an X-ray source is presented. We
demonstrate the effectiveness and limitations of our technique and discuss relation of charge diffusion value
obtained with X-ray measurements to sensor PSF.
As part of the LSST sensor development program we have developed an advanced CCD emulator for testing new
multichannel readout electronics. The emulator, based on an Altera Stratix II FPGA for timing and control, produces 4
channels of simulated video waveforms in response to an appropriate sequence of horizontal and vertical clocks. It
features 40MHz, 16-bit DACs for reset and video generation, 32MB of image memory for storage of arbitrary grayscale
bitmaps, and provision to simulate reset and clock feedthrough ("glitches") on the video channels. Clock inputs are
qualified for proper sequences and levels before video output is generated. Binning, region of interest, and reverse clock
sequences are correctly recognized and appropriate video output will be produced. Clock transitions are timestamped
and can be played back to a control PC.
A simplified user interface is provided via a daughter card having an ARM M3 Cortex microprocessor and miniature
color LCD display and joystick. The user can select video modes from stored bitmap images, or flat, gradient, bar, chirp,
or checkerboard test patterns; set clock thresholds and video output levels; and set row/column formats for image
outputs. Multiple emulators can be operated in parallel to simulate complex CCDs or CCD arrays.
The science focal plane of the Large Synoptic Survey Telescope is made up of 21 modules designated "raft towers".
Each raft tower module (RTM) is an autonomous, fully-testable and serviceable 144 Mpixel imager consisting of nine
highly-segmented CCDs with complete readout electronics chain. To minimize noise and obscuration the RTM is
housed in a compact enclosure fully contained within the camera cryostat. The RTM is required to meet strict
performance goals for image plane flatness, readout speed, noise, and power dissipation. Key components include the
4K × 4K fully-depleted CCDs with 16 outputs each, ceramic CCD support structure, and ASIC electronics for video
processing and clock/bias generation. In addition to CCD signal handling, the RTM electronics also includes monitoring
for temperature, voltage, and current, makeup heater control, ASIC configuration and readback, powerdown modes, and
specialized diagnostic outputs. Digitized data are transmitted out of the camera cryostat over a single 3Gb/s serial link.
RTS2, or Remote Telescope System 2nd Version, is a modular observatory control system. Development of RTS2 began in 2003 and since then it has been used at more than 20 observatories world-wide. Its main users are small, fully autonomous observatories, performing target of opportunity observations.
Since June 2007 RTS2 has been used at Brookhaven National Laboratory (BNL) to control the acquisition of images for the Large Synoptic Survey Telescope (LSST) CCD characterisation. The CCD test laboratory includes multiple devices which need to be controlled in order to perform the electro-optical testing of the CCD.
The configuration of the devices must be recorded in order for that information to be used later during data analysis.
The main factors leading to use of RTS2 were its availability, open - source code, and modular design which allows its fast customisation to fit changing needs of a R&D project.
This article focuses on the description of changes to the system which allow for the integration of LSST's
multiple output CCD imagers. The text provides details of the multiple channel implementation, which parts
of the system were affected, and how these changes influenced overall system design. It also describes how easy
and fast it was to run the multiple channel instrument on night and twilight sky during prototype CCD testing,
and demonstrates how the complex routines, such as twilight skyflats acquisitions, worked out of the box.
We present the latest modifications of the open source observatory control software package RTS2. New features
were developed specifically for the automated testing of CCD chips for the mosaic camera of the Large Synoptic
Survey Telescope. Currently, the system is in operation at Brookhaven National Laboratory in Upton, USA and
at Laboratoire de Physique Nucl´eaire et des Hautes ´Energies in Paris, France. RTS2 software is currently used to
characterize the sensors from various vendors and will be used first for selection and then for testing of production
CCD sensors. With our system we are able to automatically obtain a series of images for analysis. Data is used
to study many aspects of sensor characteristics, including wavelength dependence of quantum efficiency, the dark
current, and the linearity of the CCD response as a function of back-bias voltage and temperature. We also can
measure a point spread function over the whole surface of the CCD sensors.
KEYWORDS: Observatories, Telescopes, Charge-coupled devices, Sensors, Space telescopes, Control systems design, C++, CCD image sensors, Image processing, Control systems
For almost a decade we have been developing an open source control system for autonomous observatories
called Remote Telescope System, 2nd version - RTS2. The system is currently used to operate about dozen
observatories. It was designed from the beginning as the ultimate tool for autonomously performing any possible
observing plan on any hardware. Its modular design allows exactly this and enables even more. Currently it
is used to control not only observatories but also CCD testing laboratories. We present the internal design of
this open source observatory and laboratory control package, and discuss its overall structure. We emphasise
new developments and our experiences building a community of users and developers of the package. Design
of the system modularity is explained in detail, and various approaches to software reuse are discussed, with a
demonstration of how the best solution emerged. We describe problems that were encountered as mirror sizes
and associated operational complexity grew. We also describe how the system is being used at a CCD testing
laboratory, and detail the quick transition from previously unsupported hardware to fully automated operation.
We discuss how the system's evolution has affected code design, and present unexpected benefits it is brought.
Our experience with use of open source code and libraries are discussed.
Future wide field astronomical surveys, like Large Synoptic Survey Telescope (LSST), require photometric precision
on the percent level. The accuracy of sensor calibration procedures should match these requirements. Pixel
size variations found in CCDs from different manufacturers are the source of systematic errors in the flat field
calibration procedure. To achieve the calibration accuracy required to meet the most demanding science goals
this effect should be taken into account.
The study of pixel area variations was performed for fully depleted, thick CCDs produced in a technology
study for LSST. These are n-channel, 100μm thick devices.
We find pixel size variations in both row and column directions. The size variation magnitude is smaller in
the row direction. In addition, diffusion is found to smooth out electron density variations. It is shown that the
characteristic diffusion width can be extracted from the flat field data.
Results on pixel area variations and diffusion, data features, analysis technique and modeling technique are
presented and discussed.
Knowledge of the point spread function (PSF) of the sensors to be used in the Large Synoptic Survey Telescope (LSST)
camera is essential for optimal extraction of subtle galaxy shape distortions caused by gravitational weak lensing. We
have developed a number of techniques for measuring the PSF of candidate CCD sensors to be used in the LSST camera,
each with its own strengths and weaknesses. The two main optical PSF measurement techniques that we use are the
direct Virtual Knife Edge (VKE) scan as developed by Karcher, et al.1 and the indirect interference fringe method after
Andersen and Sorensen2 that measures the modulation transfer function (MTF) directly. The PSF is derived from the
MTF by Fourier transform. Other non-optical PSF measurement techniques that we employ include 55Fe x-ray cluster
image size measurements and statistical distribution analysis, and cosmic ray muon track size measurements, but are not
addressed here.
The VKE technique utilizes a diffraction-limited spot produced by a Point-Projection Microscope (PPM) that is scanned
across the sensor with sub-pixel resolution. This technique closely simulates the actual operating condition of the sensor
in the telescope with the source spot size having an f/# close to the actual telescope design value. The interference fringe
method uses a simple equal-optical-path Michelson-type interferometer with a single-mode fiber source that produces
interference fringes with 100% contrast over a wide spatial frequency range sufficient to measure the MTF of the sensor
directly. The merits of each measurement technique and results from the various measurement techniques on prototype
LSST sensors are presented and compared.
We present characterization methods and results on a number of new devices produced specifically to address LSST's
performance goals, including flatness, QE, PSF, dark current, read noise, CTE, cosmetics, and crosstalk. The results
indicate that commercially produced, thick n-channel over-depleted CCDs with excellent red response can achieve tight
PSF at moderate applied substrate bias with no evidence of persistent image artifacts. We will also report ongoing
studies of mosaic assembly techniques to achieve chip-to-chip
co-planarity, high fill factor, and thermal stability.
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