Dark Energy Survey

DECam, short for the Dark Energy Camera, is a large camera built to replace the previous prime focus camera on the Victor M. Blanco Telescope. The camera consists of three major components: mechanics, optics, and CCDs. Mechanics The mechanics of the camera consists of a filter changer with an 8-filter capacity and shutter. There is also an optical barrel that supports 5 corrector lenses, the largest of which is 98 cm in diameter. These components are attached to the CCD focal plane which is cooled to with liquid nitrogen in order to reduce thermal noise in the CCDs. The focal plane is also kept in an extremely low vacuum of to prevent the formation of condensation on the sensors. The entire camera with lenses, filters, and CCDs weighs approximately 4 tons. When mounted at the prime focus it was supported with a hexapod system allowing for real time focal adjustment. similar to those used in the Sloan Digital Sky Survey (SDSS). This allows DES to obtain photometric redshift measurements to z≈1. DECam also contains five lenses acting as corrector optics to extend the telescope's field of view to a diameter of 2.2°, one of the widest fields of view available for ground-based optical and infrared imaging. CCDs The scientific sensor array on DECam is an array of 62 2048×4096 pixel back-illuminated CCDs totaling 520 megapixels; an additional 12 2048×2048 pixel CCDs (50 Mpx) are used for guiding the telescope, monitoring focus, and alignment. The full DECam focal plane contains 570 megapixels. The CCDs for DECam use high resistivity silicon manufactured by Dalsa and LBNL with 15×15 micron pixels. By comparison, the OmniVision Technologies back-illuminated CCD that was used in the iPhone 4 has a 1.75×1.75 micron pixel with 5 megapixels. The larger pixels allow DECam to collect more light per pixel, improving low light sensitivity which is desirable for an astronomical instrument. DECam's CCDs also have a 250-micron crystal depth; this is significantly larger than most consumer CCDs. The additional crystal depth increases the path length travelled by entering photons. This, in turn, increases the probability of interaction and allows the CCDs to have an increased sensitivity to lower energy photons, extending the wavelength range to 1050 nm. Scientifically this is important because it allows one to look for objects at a higher redshift, increasing statistical power in the studies mentioned above. When placed in the telescope's focal plane each pixel has a width of 0.27 on the sky, resulting in a total field of view of 3 square degrees. == Survey ==

DES imaged 5,000 square degrees of the southern sky in a footprint that overlaps with the South Pole Telescope and Stripe 82 (in large part avoiding the Milky Way). The survey took 758 observing nights spread over six annual sessions between August and February to complete, covering the survey footprint ten times in five photometric bands (g, r, i, z, and Y). The survey reached a depth of 24th magnitude in the i band over the entire survey area. Longer exposure times and faster observing cadence were made in five smaller patches totaling 30 square degrees to search for supernovae. First light was achieved on 12 September 2012; after a verification and testing period, scientific survey observations started in August 2013. The last observing session was completed on 9 January 2019. • The DESI Legacy Imaging Surveys (Legacy Surveys), as of data release 10, includes DECaLS, BASS and MzLS. It also incorporating additional DECam data, which means that it covers almost the entire extragalactic southern sky, including parts of the Magellanic Clouds. The purpose of the Legacy Surveys is to find targets for the Dark Energy Spectroscopic Instrument. • Dark Energy Camera Plane Survey (DECaPS), covers the Milky Way in the southern sky. camera CCD at the same scale.|left == Observing ==

Each year from August through February, observers will stay in dormitories on the mountain. During a weeklong period of work, observers sleep during the day and use the telescope and camera at night. There will be some DES members working at the telescope console to monitor operations while others are monitoring camera operations and data process. For the wide-area footprint observations, DES takes roughly two minutes for each new image: The exposures are typically 90 seconds long, with another 30 seconds for readout of the camera data and slewing to point the telescope at its next target. Despite the restrictions on each exposure, the team also need to consider different sky conditions for the observations, such as moonlight and cloud cover. In order to get better images, DES team use a computer algorithm called the "Observing Tactician" (ObsTac) to help with sequencing observations. It optimizes among different factors, such as the date and time, weather conditions, and the position of the moon. ObsTac automatically points the telescope in the best direction, and selects the exposure, using the best light filter. It also decides whether to take a wide-area or time-domain survey image, depending on whether or not the exposure will also be used for supernova searches. ==Results==

Cosmology Dark Energy Group published several papers presenting their results for cosmology. Most of these cosmology results coming from its first-year data and the third-year data. Their results for cosmology were concluded with a Multi-Probe Methodology, which mainly combine the data from Galaxy-Galaxy Lensing, different shape of weak lensing, cosmic shear, galaxy clustering and photometric data set. For the first-year data collected by DES, Dark Energy Survey Group showed the Cosmological Constraints results from Galaxy Clustering and Weak Lensing results and cosmic shear measurement. With Galaxy Clustering and Weak Lensing results, S_8=\sigma_8(\Omega_m/0.3)^{0.5}= 0.773_{-0.020}^{+0.026} and \Omega_m= 0.267_{-0.017}^{+0.030} for ΛCDM, S_8= 0.782_{-0.024}^{+0.036}, \Omega_m= 0.284_{-0.030}^{+0.033} and \omega= -0.82_{-0.20}^{+0.21} at 68% confidence limits for ωCMD. Combine the most significant measurements of cosmic shear in a galaxy survey, Dark Energy Survey Group showed that \sigma_8(\Omega_m/0.3)^{0.5}= 0.782_{-0.027}^{+0.027} at 68% confidence limits and \sigma_8(\Omega_m/0.3)^{0.5}= 0.777_{-0.038}^{+0.036} for ΛCDM with \omega= -0.95_{-0.36}^{+0.33}. Other cosmological analyses from first year data showed a derivation and validation of redshift distribution estimates and their uncertainties for the galaxies used as weak lensing sources. The DES team also published a paper summarize all the Photometric Data Set for Cosmology for their first-year data. For the third-year data collected by DES, they updated the Cosmological Constraints to \sigma_8(\Omega_m/0.3)^{0.5}= 0.759_{-0.025}^{+0.023} for the ΛCDM model with the new cosmic shear measurements. From third-year data of Galaxy Clustering and Weak Lensing results, DES updated the Cosmological Constraints to S_8=\sigma_8(\Omega_m/0.3)^{0.5}= 0.776_{-0.017}^{+0.017} and \Omega_m= 0.339_{-0.031}^{+0.032} in ΛCDM at 68% confidence limits, S_8=\sigma_8(\Omega_m/0.3)^{0.5}= 0.775_{-0.024}^{+0.026}, \Omega_m= 0.352_{-0.041}^{+0.035} and \omega= -0.98_{-0.20}^{+0.32} in at 68% confidence limits. Similarly, the DES team published their third-year observations for photometric data set for cosmology comprising nearly 5000 deg2 of imaging in the south Galactic cap, including nearly 390 million objects, with depth reaching S/N ~ 10 for extended objects up to i_{AB} ~ 23.0, and top-of-the-atmosphere photometric uniformity 2 with and a typical redshift uncertainty of 0.03(1+z). From their statistics, they combine the likelihoods derived from angular correlations and spherical harmonics to constrain the ratio of comoving angular diameter distance D_m(Z_eff = 0.835)/r_d=18.92\pm0.51 at the effective redshift of our sample to the sound horizon scale at the drag epoch. Type Ia supernova observations In May 2019, Dark Energy Survey team published their first cosmology results using Type Ia supernovae. The supernova data was from DES-SN3YR. The Dark Energy Survey team found Ωm = 0.331 ± 0.038 with a flat ΛCDM model and Ωm = 0.321 ± 0.018, w = −0.978 ± 0.059 with a flat model. Analyzing the same data from DES-SN3YR, they also found a new current Hubble constant, H_0= 67.1 \pm1.3\,\mathrm{km\,s^{-1}\,Mpc^{-1}}. This result has an excellent agreement with the Hubble constant measurement from Planck Satellite Collaboration in 2018. In June 2019, there a follow-up paper was published by DES team discussing the systematic uncertainties, and validation of using the supernovae to measure the cosmology results mentioned before. The team also published their photometric pipeline and light curve data in another paper published in the same month. Minor planets Several minor planets were discovered by DeCam in the course of The Dark Energy Survey, including high-inclination trans-Neptunian objects (TNOs). : The MPC has assigned the IAU code W84 for DeCam's observations of small Solar System bodies. As of October 2019, the MPC inconsistently credits the discovery of nine numbered minor planets, all of them trans-Neptunian objects, to either "DeCam" or "Dark Energy Survey". The list does not contain any unnumbered minor planets potentially discovered by DeCam, as discovery credits are only given upon a body's numbering, which in turn depends on a sufficiently secure orbit determination. == Gallery ==

File:Dark Energy Survey deep field image.jpg|alt=Dark Energy Survey deep field image| File:Dark Energy Survey - distant galaxies (14958324522).jpg|The large spiral galaxy in the center of this image is roughly 385 million light-years from Earth. File:Dark Energy Survey - Fornax cluster (14958323932).jpg|The three large objects in this image captured by the Dark Energy Camera are galaxies in the nearby Fornax cluster, roughly 65 million light-years from Earth. File:Dark Energy Survey - galaxy NGC 1398 (14935678916).jpg|Dark Energy Survey - galaxy NGC 1398 == See also ==