The Dark Energy Survey Collaboration has completed construction of the Dark Energy Camera (DECam), a 3 square
degree, 570 Megapixel CCD camera which will be mounted on the Blanco 4-meter telescope at CTIO. DECam will be
used to perform the 5000 sq. deg. Dark Energy Survey with 30% of the telescope time over a 5 year period. During the
remainder of the time, and after the survey, DECam will be available as a community instrument. All components of
DECam have been shipped to Chile and post-shipping checkout finished in Jan. 2012. Installation is in progress. A
summary of lessons learned and an update of the performance of DECam and the status of the DECam installation and
commissioning will be presented.
The Dark Energy Survey is a Stage III Dark Energy Experiment that will obtain cosmological parameters by combining
four observational techniques; Galaxy Clusters, Weak Lensing, Type Ia Supernovae and Baryon Acoustic Oscillations.
The observations will be performed with a new wide field camera (DECam) that will be placed on the Blanco 4 m
telescope at CTIO. Here we describe the large format (600 mm clear aperture) Filter Changer Mechanism (FCM) for the
Dark Energy Survey Camera (DECam). The FCM, based on the Pan-STARRS design, is the largest ever constructed.
Fabrication of the filter changer has been completed and it has been tested under realistic conditions.
Flux dependent non-linearity (reciprocity failure) in HgCdTe NIR detectors with 1.7 μm cut-off was investigated.
A dedicated test station was designed and built to measure reciprocity failure over the full dynamic range of near
infrared detectors. For flux levels between 1 and 100,000 photons/sec a limiting sensitivity to reciprocity failure
of 0.3 %/decade was achieved. First measurements on several engineering grade 1.7 μm cut-off HgCdTe detectors
show a wide range of reciprocity failure, from less than 0.5 %/decade to about 10%/decade. For at least two
of the tested detectors, significant spatial variation in the effect was observed. No indication for wavelength
dependency was found. The origin of reciprocity failure is currently not well understood. In this paper we
present details of our experimental set-up and show the results of measurements for several detectors.
Many future space telescope missions are designed as wide-field surveys. The increased area of the survey is often
achieved by increasing the plate scale of the detectors. This can result in under-sampled instruments. Under these
conditions response variations within an individual pixel degrade photometric and shape information of observed
astronomical sources. These effects can be corrected for by mapping the sub-pixel response of all pixels on a detector.
Measuring sub-pixel sensitivity by projecting a single, micron-size spot is effective in understanding intrapixel response
variations, but the time required to create a detector-wide map is prohibitive. The existing Spot-O-Matic single spot
projector concept, has been extended to the design of a multi-spot projector, the Spots-O-Matic, enabling the mapping of
an entire detector. This new projector is under development to achieve the small spot size required for pixel
characterization over the field of view of an entire detector.
The Dark Energy Survey Collaboration is building the Dark Energy Camera (DECam), a 3 square degree, 520
Megapixel CCD camera which will be mounted on the Blanco 4-meter telescope at CTIO. DECam will be used to
perform the 5000 sq. deg. Dark Energy Survey with 30% of the telescope time over a 5 year period. During the
remainder of the time, and after the survey, DECam will be available as a community instrument. Construction of
DECam is well underway. Integration and testing of the major system components has already begun at Fermilab and
the collaborating institutions.
High detector quantum efficiency (QE) can greatly improve speed and performance of wide field instruments
that strive for fast precision photometry. SNAP, a proposed satellite mission dedicated to exploring the nature
of the dark energy will employ a very large focal plane instrumented with about equal number of CCD and
NIR sensors totaling more than 600 million pixels covering roughly 0.7 square degrees on the sky. To precisely
characterize the NIR detector QE, the SNAP project has put in place a test set-up capable of measuring absolute
QE at the 5% level with the goal of ultimately reaching a precision better than 2%. Illumination of the NIR
detectors is provided by either a quartz tungsten halogen lamp combined with a set of narrow band filters or
a manually tunable monochromator. The two light sources feed an integrating sphere at a distance of roughly
60 cm from the detector to be tested and a calibrated InGaAs photodiode, mounted adjacent to the NIR
detector provides absolute photon flux measurements. This paper describes instrumentation, performance and
measurement procedures and summarizes results of detailed characterization of the QE on several SNAP devices
as a function of wavelength.
Over the past decade scientists have collected convincing evidence that the content of our universe is dominated
by a mysterious dark energy. Understanding the nature of dark energy is a very difficult task, and requires a
variety of independent experimental approaches. Most of these approaches rely on photometric calibrations over
a wide range of intensities using standardized stars and internal reference sources, and hence on a complete
understanding of the linearity of the detectors. The SNAP near infrared (NIR) instrument team has performed
a comprehensive study of precision photometry on 1.7 micron cut-off HgCdTe detectors. Among those studies
are the count rate dependent detector non-linearity that was recently discovered with the NICMOS array on
the Hubble Space Telescope, and possible pixel size variations seen in quantum efficiency (QE) data. The nonlinearity
on NICMOS exhibits an unexpected behavior, where pixels with high (low) count rates detect slightly
more (less) flux than expected for a linear system. To test this count rate dependent non-linearity a dedicated
setup was built that produces a known amount of light on a detector, and measures its response as a function of
light intensity and wavelength. If the pixel response variations seen in QE data are due to pixel area variations,
standard flat-fielding will degrade photometry precision for point sources in an undersampled telescope. Studies
have been performed to estimate the magnitude of pixel area variations.
Precision near infrared (NIR) measurements are essential for the next generation of ground and space based instruments. The SuperNova Acceleration Probe (SNAP) will measure thousands of type Ia supernovae up to a redshift of 1.7. The highest redshift supernovae provide the most leverage for determining cosmological parameters, in particular the dark energy equation of state and its possible time evolution. Accurate NIR observations are needed to utilize the full potential of the highest redshift supernovae. Technological improvements in NIR detector fabrication have lead to high quantum efficiency, low noise detectors using a HgCdTe diode with a band-gap that is tuned to cutoff at 1.7 μm. The effects of detector quantum efficiency, read noise, and dark current on lightcurve signal to noise, lightcurve parameter errors, and distance modulus fits are simulated in the SNAPsim framework. Results show that improving quantum efficiency leads to the largest gains in photometric accuracy for type Ia supernovae. High quantum efficiency in the NIR reduces statistical errors and helps control systematic uncertainties at the levels necessary to achieve the primary SNAP science goals.
Large format (1k × 1k and 2k × 2k) near infrared detectors manufactured by Rockwell Scientific Center and Raytheon Vision Systems are characterized as part of the near infrared R&D effort for SNAP (the Super-Nova/Acceleration Probe). These are hybridized HgCdTe focal plane arrays with a sharp high wavelength cut-off at 1.7 μm. This cut-off provides a sufficiently deep reach in redshift while it allows at the same time low dark current operation of the passively cooled detectors at 140 K. Here the baseline SNAP near infrared system is briefly described and the science driven requirements for the near infrared detectors are summarized. A few results obtained during the testing of engineering grade near infrared devices procured for the SNAP project are highlighted. In particular some recent measurements that target correlated noise between adjacent detector pixels due to capacitive coupling and the response uniformity within individual detector pixels are discussed.
Mission requirements, the baseline design, and optical systems budgets for the SuperNova/Acceleration Probe (SNAP) telescope are presented. SNAP is a proposed space-based experiment designed to study dark energy and alternate explanations of the acceleration of the universe’s expansion by performing a series of complementary systematics-controlled astrophysical measurements. The goals of the mission are a Type Ia supernova Hubble diagram and a wide-field weak gravitational lensing survey. A 2m widefield three-mirror telescope feeds a focal plane consisting of 36 CCDs and 36 HgCdTe detectors and a high-efficiency, low resolution integral field spectrograph. Details of the maturing optical system, with emphasis on structural stability during terrestrial testing as well as expected environments during operations at L2 are discussed. The overall stray light mitigation system, including illuminated surfaces and visible objects are also presented.
A well-adapted spectrograph concept has been developed for the SNAP (SuperNova/Acceleration Probe) experiment. The goal is to ensure proper identification of Type Iz supernovae and to standardize the magnitude of each candidate by determining explosion parameters. The spectrograph is also a key element for the calibration of the science mission. An instrument based on an integral field method with the powerful concept of imager slicing has been designed and is presented in this paper. The spectrograph concept is optimized to have high efficiency and low spectral resolution (R~100), constant through the wavelength range (0.35-1.7μm), adapted to the scientific goals of the mission.