We describe demonstrations of remarkable robustness to instrumental noises by using a multiple delay externally dispersed interferometer (EDI) on stellar observations at the Hale telescope. Previous observatory EDI demonstrations used a single delay. The EDI (also called “TEDI”) boosted the 2,700 resolution of the native TripleSpec NIR spectrograph (950-2450 nm) by as much as 10x to 27,000, using 7 overlapping delays up to 3 cm. We observed superb rejection of fixed pattern noises due to bad pixels, since the fringing signal responds only to changes in multiple exposures synchronous to the applied delay dithering. Remarkably, we observed a ~20x reduction of reaction in the output spectrum to PSF shifts of the native spectrograph along the dispersion direction, using our standard processing. This allowed high resolution observations under conditions of severe and irregular PSF drift otherwise not possible without the interferometer. Furthermore, we recently discovered an improved method of weighting and mixing data between pairs of delays that can theoretically further reduce the net reaction to PSF drift to zero. We demonstrate a 350x reduction in reaction to a native PSF shift using a simple simulation. This technique could similarly reduce radial velocity noise for future EDI’s that use two delays overlapped in delay space (or a single delay overlapping the native peak). Finally, we show an extremely high dynamic range EDI measurement of our ThAr lamp compared to a literature ThAr spectrum, observing weak features (~0.001x height of nearest strong line) that occur between the major lines. Because of individuality of each reference lamp, accurate knowledge of its spectrum between the (unfortunately) sparse major lines is important for precision radial velocimetry.
The Dark Energy Survey (DES) is an operating optical survey aimed at understanding the accelerating expansion of the universe using four complementary methods: weak gravitational lensing, galaxy cluster counts, baryon acoustic oscillations, and Type Ia supernovae. To perform the 5000 sq-degree wide field and 30 sq-degree supernova surveys, the DES Collaboration built the Dark Energy Camera (DECam), a 3 square-degree, 570-Megapixel CCD camera that was installed at the prime focus of the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory (CTIO). DES has completed its third observing season out of a nominal five. This paper describes DES “Year 1” (Y1) to “Year 3” (Y3), the strategy, an outline of the survey operations procedures, the efficiency of operations and the causes of lost observing time. It provides details about the quality of the first three season's data, and describes how we are adjusting the survey strategy in the face of the El Niño Southern Oscillation.
The Dark Energy Survey (DES) is a next generation optical survey aimed at understanding the accelerating expansion of the universe using four complementary methods: weak gravitational lensing, galaxy cluster counts, baryon acoustic oscillations, and Type Ia supernovae. To perform the 5000 sq-degree wide field and 30 sq-degree supernova surveys, the DES Collaboration built the Dark Energy Camera (DECam), a 3 square-degree, 570-Megapixel CCD camera that was installed at the prime focus of the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory (CTIO). DES started its first observing season on August 31, 2013 and observed for 105 nights through mid-February 2014. This paper describes DES “Year 1” (Y1), the strategy and goals for the first year's data, provides an outline of the operations procedures, lists the efficiency of survey operations and the causes of lost observing time, provides details about the quality of the first year's data, and hints at the “Year 2” plan and outlook.
Scientists are interested in using digital micromirror devices (DMD) as slit-masks in multiobject spectrometers on future space missions. A favored orbit is at the second Lagrangian point (L2). A requirement for mission planning is to determine how long such microelectrical mechanical systems devices would remain operational given the L2 radiation environment, which is primarily composed of solar protons and cosmic rays. To this end, we initiated DMD proton testing. Three DMDs were irradiated with high-energy protons (35 to 50 MeV) at the Lawrence Berkeley National Laboratory 88 in. Cyclotron. Assuming a typical spacecraft shielding of 100 mils of aluminum, our tests imply that DMDs remain fully operable in a five-year mission at L2 with a margin of safety of 4.5.
Scientists conceiving future space missions are interested in using DMDs as a multi-object spectrometer (MOS) slit mask. The main uncertainties in utilizing DMDs in a space-based instrument are associated with their operational longevity given the exposure to high levels of proton radiation and their ability to operate at low temperatures. Since a favored orbit is at the second Lagrangian point (L2), it is important to determine how long such Micro-Electrical Mechanical Systems (MEMS) would remain operational in the harsh L2 radiation environment, which primarily consists of solar protons and cosmic rays. To address this uncertainty, we have conducted DMD proton testing at the Lawrence Berkeley National Laboratory (LBNL) 88” Cyclotron. Three DMDs were irradiated with high-energy protons (20- 50MeV) with energies sufficient to penetrate the DMD package’s optical window and interact electrically with the device. After each irradiation step, an optical test procedure was used to validate the operability of each individual mirror on the DMD array. Each DMD was irradiated to a wide range of dosage levels and remained 100% operable up to a total dose of 30 krads. In addition, a few single event upsets were seen during each irradiation dose increment. To determine the minimal operating temperature of the DMDs, we placed a DMD in a liquid nitrogen dewar, and cooled it from room temperature to 130 K. During this test, the DMD was illuminated with a light source and monitored with a CCD camera. Additionally, the temperature was held constant at 173 K for 24 hours to test landing DMD patterns for long periods of time. There was no indication that extended periods of low temperature operation impact the DMD performance. Both of these results point to DMDs as a suitable candidate for future long duration space missions.
[The BigBOSS experiment is a redshift survey designed to map the large scale structure of the universe and probe the nature of dark energy. Using massively-multiplexed _ber spectroscopy over 14,000 deg2 of sky, the survey will deliver more than 20 million galaxy and quasar redshifts. The resulting three dimensional sky map will contain signatures from primordial baryon acoustic oscillations (BAO) that set a "standard ruler" distance scale. Using the BAO signature, BigBOSS will measure the cosmological distance scale to < 1% accuracy from 0.5<z<3.0, shedding new light on the expansion history and growth of large scale structure in the Universe at a time when dark energy began to dominate. In this work, we give an overview of the BigBOSS survey goals and methodology, focusing on measuring the [O II] λ3727 emission line doublet from star-forming galaxies. We detail a new spectral simulation tool used in generating BigBOSS observations for emission-line galaxy targets. We perform a trade study of the detected galaxy redshift distribution under two observational cases relative to the baseline survey and discuss the impact on the BigBOSS science goal.
The nature of Dark Energy can by constrained by the precise determination of super-novae distance moduli in ultraviolet
to near IR pass-bands. Space-based observations are required for these moduli to be measured with the scientifically
required photometric accuracies. Consequently, robust pass-band filters operable at cryogenic temperatures (120-140K)
are needed that have challenging performance attributes including high in-band transmission, low ripple, good out-ofband
rejection, and moderate band-edge slope. We describe the requirements and performance of dielectric multi-layer
filters with spectral profiles that are suitable for both achieving the science and for accurate calibration using plausible
on-orbit measurement systems.
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.
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.
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.
We present the baseline telescope design for the telescope for the SuperNova/Acceleration Probe (SNAP) space mission. SNAP’s purpose is to determine expansion history of the Universe by measuring the redshifts, magnitudes, and spectral classifications of thousands of supernovae with unprecedented accuracy. Discovering and measuring these supernovae demand both a wide optical field and a high sensitivity throughout the visible and near IR wavebands. We have adopted the annular-field three-mirror anastigmat (TMA) telescope configuration, whose classical aberrations (including chromatic) are zero. We show a preliminary optmechanical design that includes important features for stray light control and on-orbit adjustment and alignment of the optics. We briefly discuss stray light and tolerance issues, and present a preliminary wavefront error budget for the SNAP Telescope. We conclude by describing some of the design tasks being carried out during the current SNAP research and development phase.
The proposed SuperNova/Acceleration Probe (SNAP) mission will have a two-meter class telescope delivering diffraction-limited images to an instrumented 0.7 square degree field in the visible and near-infrared wavelength regime. The requirements for the instrument suite and the present configuration of the focal plane concept are presented. A two year R&D phase, largely supported by the Department of Energy, is just beginning. We describe the development activities that are taking place to advance our preparedness for mission proposal in the areas of detectors and electronics.
The SuperNova/Acceleration Probe (SNAP) will measure precisely the cosmological expansion history over both the acceleration and deceleration epochs and thereby constrain the nature of the dark energy that dominates our universe today. The SNAP focal plane contains equal areas of optical CCDs and NIR sensors and an integral field spectrograph. Having over 150 million pixels and a field-of-view of 0.34 square degrees, the SNAP NIR system will be the largest yet constructed. With sensitivity in the range 0.9-1.7 μm, it will detect Type Ia supernovae between z = 1 and 1.7 and will provide follow-up precision photometry for all supernovae. HgCdTe technology, with a cut-off tuned to 1.7 μm, will permit passive cooling at 140 K while maintaining noise below zodiacal levels. By dithering to remove the effects of intrapixel variations and by careful attention to other instrumental effects, we expect to control relative photometric accuracy below a few hundredths of a magnitude. Because SNAP continuously revisits the same fields we will be able to achieve outstanding statistical precision on the photometry of reference stars in these fields, allowing precise monitoring of our detectors. The capabilities of the NIR system for broadening the science reach of SNAP are discussed.
A well-adapted spectrograph concept has been developed for the SNAP (SuperNova/Acceleration Probe) experiment. The goal is to ensure proper identification of Type Ia supernovae and to standardize the magnitude of each candidate by determining explosion parameters. 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 very 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.
The proposed SuperNova/Acceleration Probe (SNAP) mission will have a two-meter class telescope delivering diffraction-limited images to an instrumented 0.7 square-degree field sensitive in the visible and near-infrared wavelength regime. We describe the requirements for the instrument suite and the evolution of the focal plane design to the present concept in which all the instrumentation -- visible and near-infrared imagers, spectrograph, and star guiders -- share one common focal plane.
The Supernova / Acceleration Probe (SNAP) is a proposed space-borne observatory that will survey the sky with a wide-field optical/near-infrared (NIR) imager. The images produced by SNAP will have an unprecedented combination of depth, solid-angle, angular resolution, and temporal sampling. For 16 months each, two 7.5 square-degree fields will be observed every four days to a magnitude depth of AB=27.7 in each of the SNAP filters, spanning 3500-17000Å. Co-adding images over all epochs will give AB=30.3 per filter. In addition, a 300 square-degree field will be surveyed to AB=28 per filter, with no repeated temporal sampling. Although the survey strategy is tailored for supernova and weak gravitational lensing observations, the resulting data will support a broad range of auxiliary science programs.
The SuperNova/Acceleration Probe (SNAP) mission will require a two-meter class telescope delivering diffraction limited images spanning a one degree field in the visible and near infrared wavelength regime. This requirement, equivalent to nearly one billion pixel resolution, places stringent demands on its optical system in terms of field flatness, image quality, and freedom from chromatic aberration. We discuss the advantages of annular-field three-mirror anastigmat (TMA) telescopes for applications such as SNAP, and describe the features of the specific optical configuration that we have baselined for the SNAP mission. We discuss the mechanical design and choice of materials for the telescope. Then we present detailed ray traces and diffraction calculations for our baseline optical design. We briefly discuss stray light and tolerance issues, and present a preliminary wavefront error budget for the SNAP Telescope. We conclude by describing some of tasks to be carried out during the upcoming SNAP research and development phase.
The SuperNova / Acceleration Probe (SNAP) is a space-based experiment to measure the expansion history of the Universe and study both its dark energy and the dark matter. The experiment is motivated by the startling discovery that the expansion of the Universe is accelerating. A 0.7~square-degree imager comprised of 36 large format fully-depleted n-type CCD's sharing a focal plane with 36 HgCdTe detectors forms the heart of SNAP, allowing discovery and lightcurve measurements simultaneously for many supernovae. The imager and a high-efficiency low-resolution integral field spectrograph are coupled to a 2-m three mirror anastigmat wide-field telescope, which will be placed in a high-earth orbit. The SNAP mission can obtain high-signal-to-noise calibrated light-curves and spectra for over 2000 Type Ia supernovae at redshifts between z = 0.1 and 1.7. The resulting data set can not only determine the amount of dark energy with high precision, but test the nature of the dark energy by examining its equation of state. In particular, dark energy due to a cosmological constant can be differentiated from alternatives such as "quintessence", by measuring the dark energy's equation of state to an accuracy of ± 0.05, and by studying its time dependence.