Photometric Uniformity Research Articles

The Dark Energy Survey Data Release 2

Abstract We present the second public data release of the Dark Energy Survey, DES DR2, based on optical/near-infrared imaging by the Dark Energy Camera mounted on the 4 m Blanco telescope at Cerro Tololo Inter-American Observatory in Chile. DES DR2 consists of reduced single-epoch and coadded images, a source catalog derived from coadded images, and associated data products assembled from 6 yr of DES science operations. This release includes data from the DES wide-area survey covering ∼5000 deg2 of the southern Galactic cap in five broad photometric bands, grizY. DES DR2 has a median delivered point-spread function FWHM of g = 1.11″, r = 0.95″, i = 0.88″, z = 0.83″, and Y = 0.″90, photometric uniformity with a standard deviation of < 3 mmag with respect to Gaia DR2 G band, a photometric accuracy of ∼11 mmag, and a median internal astrometric precision of ∼27 mas. The median coadded catalog depth for a 1.″95 diameter aperture at signal-to-noise ratio = 10 is g = 24.7, r = 24.4, i = 23.8, z = 23.1, and Y = 21.7 mag. DES DR2 includes ∼691 million distinct astronomical objects detected in 10,169 coadded image tiles of size 0.534 deg2 produced from 76,217 single-epoch images. After a basic quality selection, benchmark galaxy and stellar samples contain 543 million and 145 million objects, respectively. These data are accessible through several interfaces, including interactive image visualization tools, web-based query clients, image cutout servers, and Jupyter notebooks. DES DR2 constitutes the largest photometric data set to date at the achieved depth and photometric precision.

Dark Energy Survey Year 3 Results: Photometric Data Set for Cosmology

Abstract We describe the Dark Energy Survey (DES) photometric data set assembled from the first three years of science operations to support DES Year 3 cosmologic analyses, and provide usage notes aimed at the broad astrophysics community. Y3 GOLD improves on previous releases from DES, Y1 GOLD, and Data Release 1 (DES DR1), presenting an expanded and curated data set that incorporates algorithmic developments in image detrending and processing, photometric calibration, and object classification. Y3 GOLD comprises nearly 5000 deg2 of grizY imaging in the south Galactic cap, including nearly 390 million objects, with depth reaching a signal-to-noise ratio ∼10 for extended objects up to i AB ∼ 23.0, and top-of-the-atmosphere photometric uniformity <3 mmag. Compared to DR1, photometric residuals with respect to Gaia are reduced by 50%, and per-object chromatic corrections are introduced. Y3 GOLD augments DES DR1 with simultaneous fits to multi-epoch photometry for more robust galactic color measurements and corresponding photometric redshift estimates. Y3 GOLD features improved morphological star–galaxy classification with efficiency >98% and purity >99% for galaxies with 19 < i AB < 22.5. Additionally, it includes per-object quality information, and accompanying maps of the footprint coverage, masked regions, imaging depth, survey conditions, and astrophysical foregrounds that are used to select the cosmologic analysis samples.

Research on three-dimensional model reconstruction of slope erosion based on sequence images

To solve the problems of incomplete 2D display soil erosion, difficult visual expression, poor detail simulation of eroding slope, complex modeling technology, and high cost, a three-dimensional model reconstruction method for slope erosion based on sequence images is proposed. The method mainly uses structure from motion (SFM) and multi-view stereo (MVS) technology. First, the image sequence is feature matched using the scale-invariant feature transform algorithm for images captured by ordinary cameras. The random sampling and consensus algorithm is used to remove the false matching point generated by the nearest neighbor. Next, using the SFM method, the camera matrix and three-dimensional point coordinates are solved iteratively, and the nonlinear optimization is realized by bundle adjustment to distribute the errors evenly and improve the accuracy of the model. Finally, based on the patch-based multi-view stereo algorithm, the sparse point cloud generated by SFM is used as the seed patch to diffuse under local photometric uniformity and global visibility constraints to reconstruct the dense point cloud and establish the 3D model. To verify the reliability of the experimental methods, this paper takes the red soil erosion slope in Chenggong District of Kunming, Yunnan, China as the experimental object and realizes the rapid reconstruction of the three-dimensional model of the erosion gully on the slope. Multiple groups of adjacent eroded gully images are taken to verify the model, which involves method and accuracy of model. The results show that the modeling method has a good effect on soil modeling. The error from the control points is small and the relative accuracy is as high as 94.20%, 98.01%, etc. As a result, the control points can provide a reference for corrosion monitoring research for its reliable results.

The SuperCOSMOS all-sky galaxy catalogue

We describe the construction of an all-sky galaxy catalogue, using SuperCOSMOS scans of Schmidt photographic plates from the UKST and POSS2 surveys. The photographic photometry is calibrated using SDSS data, with results that are linear to 2% or better. All-sky photometric uniformity is achieved by matching plate overlaps and also by requiring homogeneity in optical-to-2MASS colours, yielding zero points that are uniform to 0.03 mag. or better. The typical AB depths achieved are B_J<21, R_F<19.5 and I_N<18.5, with little difference between hemispheres. In practice, the I_N plates are shallower than the B_J & R_F plates, so for most purposes we advocate the use of a catalogue selected in these two latter bands. At high Galactic latitudes, this catalogue is approximately 90% complete with 5% stellar contamination; we quantify how the quality degrades towards the Galactic plane. At low latitudes, there are many spurious galaxy candidates resulting from stellar blends: these approximately match the surface density of true galaxies at |b|=30 deg. Above this latitude, the catalogue limited in B_J & R_F contains in total about 20 million galaxy candidates, of which 75% are real. This contamination can be removed, and the sky coverage extended, by matching with additional datasets. This SuperCOSMOS catalogue has been matched with 2MASS and with WISE, yielding quasi-allsky samples of respectively 1.5 million and 18.5 million galaxies, to median redshifts of 0.08 and 0.20. This legacy dataset thus continues to offer a valuable resource for large-angle cosmological investigations.

THE DISCOVERY OF PULSATING HOT SUBDWARFS IN NGC 2808

We present the results of a Hubble Space Telescope program to search for pulsating hot subdwarfs in the core of NGC 2808. These observations were motivated by the recent discovery of such stars in the outskirts of omega Cen. Both NGC 2808 and omega Cen are massive globular clusters exhibiting complex stellar populations and large numbers of extreme horizontal branch stars. Our far-UV photometric monitoring of over 100 hot evolved stars has revealed six pulsating subdwarfs with periods ranging from 85 to 149 s and UV amplitudes of 2.0 to 6.8%. In the UV color-magnitude diagram of NGC 2808, all six of these stars lie immediately below the canonical horizontal branch, a region populated by the subluminous "blue-hook" stars. For three of these six pulsators, we also have low-resolution far-UV spectroscopy that is sufficient to broadly constrain their atmospheric abundances and effective temperatures. Curiously, and in contrast to the omega Cen pulsators, the NGC 2808 pulsators do not exhibit the spectroscopic or photometric uniformity one might expect from a well-defined instability strip, although they all fall within a narrow band (0.2 mag) of far-UV luminosity.

The United Kingdom Infrared Telescope Infrared Deep Sky Survey First Data Release

The First Data Release (DR1) of the UKIRT Infrared Deep Sky Survey (UKIDSS) took place on 2006 July 21. UKIDSS is a set of five large near-infrared surveys, covering a complementary range of areas, depths, and Galactic latitudes. DR1 is the first large release of survey-quality data from UKIDSS and includes 320 sq degs of multicolour data to (Vega) K=18, complete (depending on the survey) in three to five bands from the set ZYJHK, together with 4 sq degs of deep JK data to an average depth K=21. In addition the release includes a similar quantity of data with incomplete filter coverage. In JHK, in regions of low extinction, the photometric uniformity of the calibration is better than 0.02 mag. in each band. The accuracy of the calibration in ZY remains to be quantified, and the same is true of JHK in regions of high extinction. The median image FWHM across the dataset is 0.82 arcsec. We describe changes since the Early Data Release in the implementation, pipeline and calibration, quality control, and archive procedures. We provide maps of the areas surveyed, and summarise the contents of each of the five surveys in terms of filters, areas, and depths. DR1 marks completion of 7 per cent of the UKIDSS 7-year goals.

Properties of five low-contrast open clusters in the third quadrant

We derive photometric, structural and dynamical evolution-related parameters of five as yet unstudied low-contrast open clusters located in the third quadrant using 2MASS data. The target clusters are Czernik 31, Czernik 32, Haffner 9, Haffner 11 and Trumpler 13. We apply a statistical field-star decontamination procedure to infer on the intrinsic colour–magnitude diagram (CMD) morphology which is critical for such low-contrast objects. Consequently, it became possible to derive accurate reddening, age, distance from the Sun and Galactocentric distance for the five clusters. In the structural and luminosity/mass-function analyses we apply a colour-magnitude filter which encompasses the cluster evolutionary CMD sequences and excludes stars with discrepant colours. Using this procedure we derive core and limiting radii, mass function (MF) slope, total mass, mass density and relaxation time. We derive ages in the range 140–1100 Myr, Galactocentric distances within 7.7–11.4 kpc, and total masses within 360–2900 $M_\odot$. Reflecting large-scale mass segregation, the MF slope in the core is significantly flatter than that in the halo of the five clusters. Although some of the present clusters are relatively younger than the Gyr-old clusters, they present evidence of advanced dynamical evolution. This kind of study has become possible because of the photometric uniformity and spatial coverage of 2MASS which allows a proper subtraction of the field-star contamination on the target CMDs. The present study indicates that low-contrast clusters can be studied with 2MASS, particularly after field-star subtraction, which is important since most of the unstudied open clusters belong to this class.

Perceptual photometric seamlessness in projection-based tiled displays

Arguably, the most vexing problem remaining for multi-projector displays is that of photometric (brightness) seamlessness within and across different projectors. Researchers have strived for &lt;i&gt;strict photometric uniformity&lt;/i&gt; that achieves identical response at every pixel of the display. However, this goal typically results in displays with severely compressed dynamic range and poor image quality. In this article, we show that strict photometric uniformity is not a requirement for achieving photometric seamlessness. We introduce a general goal for photometric seamlessness by defining it as an optimization problem, balancing &lt;i&gt;perceptual uniformity&lt;/i&gt; with &lt;i&gt;display quality&lt;/i&gt;. Based on this goal, we present a new method to achieve &lt;i&gt;perceptually seamless high quality displays&lt;/i&gt;. We first derive a model that describes the photometric response of projection-based displays. Then we estimate the model parameters and modify them using perception-driven criteria. Finally, we use the graphics hardware to reproject the image computed using the modified model parameters by manipulating only the projector inputs at interactive rates. Our method has been successfully demonstrated on three different practical display systems at Argonne National Laboratory, made of 2 × 2 array of four projectors, 2 × 3 array of six, projectors, and 3 × 5 array of fifteen projectors. Our approach is efficient, automatic and scalable---requiring only a digital camera and a photometer. To the best of our knowledge, this is the first approach and system that addresses the photometric variation problem from a perceptual stand point and generates truly seamless displays with high dynamic range.

The Clustering Dipole of the Local Universe from the Two Micron All Sky Survey

The unprecedented sky coverage and photometric uniformity of the Two Micron All Sky Survey (2MASS) provides a rich resource for investigating the galaxies populating the local universe. A full characterization of the large-scale clustering distribution is important for theoretical studies of structure formation. 2MASS offers an all-sky view of the local galaxy population at 2.15 μm, unbiased by young stellar light and minimally affected by dust. We use 2MASS to map the local distribution of galaxies, identifying the largest structures in the nearby universe. The inhomogeneity of these structures causes an acceleration on the Local Group of galaxies, which can be seen in the dipole of the cosmic microwave background (CMB). We find that the direction of the 2MASS clustering dipole is 16° from the CMB dipole, confirming that the local galaxy distribution accelerates the Local Group. From the magnitude of the dipole, we find a value of the linear bias parameter b = 1.06 ± 0.17 in the Ks band. Thus, the linear bias parameter of Ks-selected galaxies is similar to the bias parameter found in other wave bands.

The Life and Times of SagDeg

ASTROPHYSICS The Sagittarius dwarf elliptical galaxy (SagDeg) is on a collision course with the Milky Way and in the next few billion years will be tidally disrupted by its larger neighbor. The SagDeg galaxy has had a long history (about 10 billion years) of star formation; however, the details of SagDeg's stellar life are poorly understood. Using the photometric uniformity and precision of the Two Micron All Sky Survey, Cole has derived a new color-magnitude diagram of about 1400 red giant stars in the center of SagDeg that shows a large spread in the mean metallicity. A larger fraction of high-metallicity-to-low-metallicity stars was found, indicating a longer history of star formation than previously suggested. This enhanced stellar history is also surprising given SagDeg's current lack of gas. It is possible that SagDeg used to be a larger galaxy that is now in the last stages of tidal disruption or that it captured some Milky Way gas on a previous close passage. — LR Astrophys. J. , in press.

The APM Bright Galaxy Catalogue

The APM Bright Galaxy Catalogue lists positions, magnitudes, shapes and morphological types for 14,681 galaxies brighter than $b_J$ magnitude 16.44 over a 4,180 square degree area of the southern sky. Galaxy and stellar images have been located from glass copy plates of the United Kingdom Schmidt Telescope (UKST) IIIaJ sky survey using the Automated Photographic Measuring (APM) facility in Cambridge, England. The majority of stellar images are rejected by the regularity of their image surface brightness profiles. Remaining images are inspected by eye on film copies of the survey material and classed as stellar, multiple stellar, galaxy, merger or noise. Galaxies are further classified as elliptical, lenticular, spiral, irregular or uncertain. The 180 survey fields are put onto a uniform photometric system by comparing the magnitudes of galaxies in the overlap regions between neighbouring plates. The magnitude zero-point, photometric uniformity and photographic saturation are checked with CCD photometry. Finally, the completeness and reliability of the catalogue is assessed using various internal tests and by comparing with several independently constructed galaxy catalogues.

The analytic and functional accuracy of a video densitometry system

Applications using radiotracers and quantitative film autoradiography are increasing dramatically in the neurosciences. Microcomputer-based image analyzing systems with video input have been developed to provide rapid quantification of autoradiographic images on relatively inexpensive systems. However, there has been some question as to whether such systems can reliably produce high levels of densitometric accuracy, especially when compared to mechanical scanners which are standard in research requiring extreme fidelity of measurement. We report methods and results from tests done to determine the analytical and functional accuracy of the Drexel Unix-based Microcomputer image Analysis System (DUMAS), which is a video densitometric system designed to provide quantitative data from autoradiograms. Analytic accuracy was determined by measuring photometric uniformity, the optical density transfer function, temporal stability, geometric uniformity, and flare. In addition, data are provided on the resolution of the system at several magnifications. Functional accuracy, the accuracy of the estimates of mean isotope concentrations in diverse neural structures, was determined by comparing the results obtained on the DUMAS system with the results from analyzing the same [ 14C]2-deoxyglucose images with two different Optronics P1000 systems. Our results show that, provided care is taken in the choice of a camera and a light source, the analytic accuracy of videodensitometry is high. Its functional accuracy is also high in that measurements of radioisotope concentrations in diverse neural structures made on the DUMAS system agree closely with the measurements from a properly adjusted Optronics P1000 system. The rapidity and economy of videodensitometry is not, therefore, obtained at the sacrifice of densitometric accuracy. Given adequately tested hardware and provided that suitable checks on instrument calibration and adjustment are made, the errors in autoradiographic quantification due to the image analyzing system itself are minor in comparison to other sources of error, including, as we show, variations in the user's delineation of the boundaries of neural structures.

Unique Optical System For Conversion Of Existing Photometers Into Scanning Microphotometers

There are many thousands of luminance photometers in the field, both telephotometers and microphotometers; however, very few of these have scanning microphotometric capability. This paper describes a unique optical system that can be field-interchanged with the standard objective lens, without requiring any modification of the photometer's internal optical system, to convert an existing photometer into a high-resolution scanning microphotometer. One embodiment, called the MicroScanner Spatial Scanner, will be described in detail. Used with any standard Pritchard Photometer, this scanner provides variable-speed scanning of a half-inch (12 mm) of object space with field coverage as fine as 0.0001 in. (0.0025 mm) or as broad as 0.39 in. (10 mm). The system features photometric uniformity within ±0.5% over the entire scan distance, freedom from polarization error, and a half-inch scan distance independent of choice of objective lens.

Photometry of the lunar surface

The photometry of the Moon gives us some information about the properties of the lunar surface. The photometric uniformity of the lunar surface as a scattering screen is determined by the shadow phenomena on small irregularities due to the dust layer covering the whole surface. A small component of light (< 10 %) exhibits the features of the luminescence excited by solar radiations.