Source Extractor Research Articles

THE EXTENDED VIRGO CLUSTER CATALOG

We present a new catalog of galaxies in the wider region of the Virgo cluster, based on the Sloan Digital Sky Survey (SDSS) Data Release 7. The Extended Virgo Cluster Catalog (EVCC) covers an area of 725 deg^2 or 60.1 Mpc^2. It is 5.2 times larger than the footprint of the classical Virgo Cluster Catalog (VCC) and reaches out to 3.5 times the virial radius of the Virgo cluster. We selected 1324 spectroscopically targeted galaxies with radial velocities less than 3000 kms^-1. In addition, 265 galaxies that have been missed in the SDSS spectroscopic survey but have available redshifts in the NASA Extragalactic Database are also included. Our selection process secured a total of 1589 galaxies of which 676 galaxies are not included in the VCC. The certain and possible cluster members are defined by means of redshift comparison with a cluster infall model. We employed two independent and complementary galaxy classification schemes: the traditional morphological classification based on the visual inspection of optical images and a characterization of galaxies from their spectroscopic features. SDSS u, g, r, i, and z passband photometry of all EVCC galaxies was performed using Source Extractor. We compare the EVCC galaxies with the VCC in terms of morphology, spatial distribution, and luminosity function. The EVCC defines a comprehensive galaxy sample covering a wider range in galaxy density that is significantly different from the inner region of the Virgo cluster. It will be the foundation for forthcoming galaxy evolution studies in the extended Virgo cluster region, complementing ongoing and planned Virgo cluster surveys at various wavelengths.

HerMES: point source catalogues from Herschel-SPIRE observations II★

The Herschel Multi-tiered Extragalactic Survey (HerMES) is the largest Guaranteed Time Key Programme on the Herschel Space Observatory. With a wedding cake survey strategy, it consists of nested fields with varying depth and area totalling ~380 deg^2. In this paper, we present deep point source catalogues extracted from Herschel-SPIRE observations of all HerMES fields, except for the later addition of the 270 deg^2 HeLMS field. These catalogues constitute the second Data Release (DR2) made in October 2013. A subset of these catalogues, which consists of bright sources extracted from Herschel-SPIRE observations completed by May 1, 2010 (covering ~ 74 deg^2) were released earlier in the first extensive Data Release (DR1) in March 2012. Two different methods are used to generate the point source catalogues, the SUSSEXtractor (SXT) point source extractor used in two earlier data releases (EDR and EDR2) and a new source detection and photometry method. The latter combines an iterative source detection algorithm, StarFinder (SF), and a De-blended SPIRE Photometry (DESPHOT) algorithm. We use end-to-end Herschel-SPIRE simulations with realistic number counts and clustering properties to characterise basic properties of the point source catalogues, such as the completeness, reliability, photometric and positional accuracy. Over 500, 000 catalogue entries in HerMES fields (except HeLMS) are released to the public through the HeDAM website (http://hedam.oamp.fr/herMES).

ATLAS 1.4 GHz Data Release 2 - I. Observations of the CDF-S and ELAIS-S1 fields and methods for constructing differential number counts

This is the first of two papers describing the second data release (DR2) of the Australia Telescope Large Area Survey (ATLAS) at 1.4 GHz, which comprises deep wide-field observations in total intensity, linear polarization, and circular polarization over the Chandra Deep Field-South and European Large Area Infrared Space Observatory Survey-South 1 regions. DR2 improves upon the first data release by maintaining consistent data reductions across the two regions, including polarization analysis, and including differential number counts in total intensity and linear polarization. Typical DR2 sensitivities across the mosaicked multi-pointing images are 30 uJy/beam at approximately 12"x6" resolution over a combined area of 6.4 square degrees. In this paper we present detailed descriptions of our data reduction and analysis procedures, including corrections for instrumental effects such as positional variations in image sensitivity, bandwidth smearing with a non-circular beam, and polarization leakage, and application of the BLOBCAT source extractor. We present the DR2 images and catalogues of components (discrete regions of radio emission) and sources (groups of physically associated radio components). We describe new analytic methods to account for resolution bias and Eddington bias when constructing differential number counts of radio components.

Extractors for Circuit Sources

We obtain the first deterministic extractors for sources generated (or sampled) by small circuits of bounded depth. Our main results are (1) we extract $k (k/nd)^{O(1)}$ bits with exponentially small error from $n$-bit sources of min-entropy $k$ that are generated by functions $f : \{0, 1\}^\ell \to \{0, 1\}^n$, where each output bit depends on $\le d$ input bits. In particular, we extract from $\mathrm{NC}^0$ sources, corresponding to $d = O(1)$; (2) we extract $k (k/n^{1+\gamma})^{O(1)}$ bits with superpolynomially small error from $n$-bit sources of min-entropy $k$ that are generated by $\mathrm{poly}(n)$-size $\mathrm{AC}^0$ circuits, for any $\gamma > 0$. As our starting point, we revisit the connection by Trevisan and Vadhan [IEEE Symposium on Foundations of Computer Science, IEEE Computer Society, Los Alamitos, CA, 2000, pp. 32--42] between circuit lower bounds and extractors for sources generated by circuits. We note that such extractors (with very weak parameters) are equivalent to lower bounds f...

New Constructions of WOM Codes Using the Wozencraft Ensemble

In this paper, we give several new constructions of write-once-memory (WOM) codes. The novelty in our constructions is the use of the so-called Wozencraft ensemble of linear codes. Specifically, we obtain the following results. We give an explicit construction of a two-write WOM code that approaches capacity, over the binary alphabet. More formally, for every ϵ > 0, 0 <; p <; 1, and n=(1/ϵ)O(1/pϵ), we give a construction of a two-write WOM code of length n and capacity H(p)+1-p-ϵ. Since the capacity of a two-write WOM code is maxp (H(p)+1-p), we get a code that is ϵ-close to capacity. Furthermore, encoding and decoding can be done in time O(n2 ·poly (logn)) and time O(n ·poly (logn)), respectively, and in logarithmic space. In addition, we exhibit an explicit randomized encoding scheme of a two-write capacity-achieving WOM code of block length polynomial in 1/ϵ (again, ϵ is the gap to capacity), with a polynomial time encoding and decoding. We obtain a new encoding scheme for three-write WOM codes over the binary alphabet. Our scheme achieves rate 1.809-ϵ, when the block length is exp(1/ϵ). This gives a better rate than what could be achieved using previous techniques. We highlight a connection to linear seeded extractors for bit-fixing sources. In particular, we show that obtaining such an extractor with seed length O(logn) can lead to improved parameters for two-write WOM codes. We then give an application of existing constructions of extractors to the problem of designing encoding schemes for memory with defects.

INVESTIGATING THE CORE MORPHOLOGY-SEYFERT CLASS RELATIONSHIP WITHHUBBLE SPACE TELESCOPEARCHIVAL IMAGES OF LOCAL SEYFERT GALAXIES

The Unified Model of Active Galactic Nuclei (AGN) has provided a successful explanation for the observed diversity of AGN in the local Universe. However, recent analysis of multi-wavelength spectral and image data suggests that the Unified Model is only a partial theory of AGN, and may need to be augmented to remain consistent with all observations. Recent studies using high spatial resolution ground- and space-based observations of local AGN show that Seyfert class and the "core" (r<~1 kpc) host-galaxy morphology are correlated. Currently, this relationship has only been established qualitatively, by visual inspection of the core morphologies of low redshift (z<0.035) Seyfert host galaxies (Malkan et al. 1998). We re-establish this empirical relationship in Hubble Space Telescope (HST) optical imaging by visual inspection of a catalog of 85 local (D<63 Mpc) Seyfert galaxies. We also attempt to re-establish the core morphology-Seyfert class relationship using an automated, non-parametric technique that combines both existing classification parameters methods (the adapted CAS, G-M20), and a new method which implements the Source Extractor software for feature detection in unsharp-mask images. This new method is designed explicitly to detect dust features in the images. We use our automated approach to classify the morphology of the AGN cores and determine that Sy2 galaxies visually appear, on average, to have more dust features than Sy1. With the exception of this "dustiness" however, we do not measure a strong correlation between the dust morphology and the Seyfert class of the host galaxy using quantitative techniques. We discuss the implications of these results in the context of the Unified Model.

Testing Accuracy and Precision of Existing Photometry Algorithms on Moving Targets

ABSTRACTPrevious studies determining which astronomical photometry software is best suited for a particular dataset are usually focused on speed, source classification, and/or meeting a sensitivity requirement. For faint objects in particular, the priority is given to maximizing signal-to-noise ratio. Photometry of moving targets offers additional challenges (1) to aperture photometry because background object contamination varies from image to image, and (2) to routines that build a PSF model from point sources in the image because trailed field stars do not perfectly represent the PSF of the untrailed target. Here, we present the results of testing several photometry algorithms (tphot, DAOPHOT, DoPHOT, APT, and multiple techniques within Source Extractor and IRAF’s PHOT) on data for a faint, slow-moving solar system object with a known light curve. We find that the newly-developed tphot software most accurately and precisely reproduces the object’s true light curve, with particular advantages in centroiding, exclusion of contaminants from the target’s flux, and fitting flux in the wings of the point-spread function.

Extractors for Polynomial Sources over Fields of Constant Order and Small Characteristic

A polynomial source of randomness over F n is a random variable X = f(Z) where f is a polynomial map and Z is a random variable distributed uniformly over F r for some integer r. The three main parameters of interest associated with a polynomial source are the order q of the field, the (total) degree D of the map f , and the base-q logarithm of the size of the range of f over inputs in F r , denoted by k. For simplicity we call X a (q; D; k)-source.

The Millennium Run Observatory: first light

Simulations of galaxy evolution aim to capture our current understanding as well as to make predictions for testing by future experiments. Simulations and observations are often compared in an indirect fashion: physical quantities are estimated from the observational data and compared to models. However, many applications can benefit from a more direct approach, where the observing process is also simulated, so that the models are seen fully from the observer’s perspective. To facilitate this, we have developed the Millennium Run Observatory (MRObs), a theoretical virtual observatory which uses virtual telescopes to ‘observe’ semi-analytic galaxy formation simulations based on the suite of Millennium Run (MR) dark matter simulations. The MRObs produces data that can be processed and analysed using the standard observational software packages developed for real observations. At present, we produce images in 40 filters covering the rest-frame ultraviolet to infrared for two stellar population synthesis models, for three different models of absorption by the intergalactic medium, and in two cosmologies (Wilkinson Microwave Anisotropy Probe year 1 and 7). Galaxy distributions for a large number of mock light cones can be ‘observed’ using models of major ground- and space-based telescopes. The data include light cone catalogues linked to structural properties of galaxies, pre-observation model images, mock telescope images and Source Extractor products that can all be traced back to the higher level dark matter, semi-analytic galaxy and light cone catalogues available in the MR data base. Here, we describe our methods and announce a first public release of simulated observations that emulate a large number of extragalactic surveys [e.g. Sloan Digital Sky Survey, Canada-France-Hawaii Telescope Legacy Survey (CFHT-LS), Great Observatories Origins Deep Survey (GOODS), GOODS/Early Release Science (ERS), Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS) and Hubble Ultra Deep Field (HUDF)]. The MRObs browser, an online tool, further facilitates exploration of the simulated data. We demonstrate the benefits of a direct approach through a number of example applications: (1) deep galaxy number counts in the CANDELS survey; (2) observed properties of galaxy clusters; (3) structural parameters of galaxies; and (4) identification of dropout galaxies. The MRObs enhances the range of questions that can be asked of semi-analytic models, allowing observers and theorists to work towards each other with virtually complete freedom of where to meet.

Galaxy And Mass Assembly (GAMA): Structural Investigation of Galaxies via Model Analysis

We present single-S\'ersic two-dimensional model fits to 167,600 galaxies modelled independently in the ugrizYJHK bandpasses using reprocessed Sloan Digital Sky Survey Data Release Seven (SDSS DR7) and UKIRT Infrared Deep Sky Survey Large Area Survey (UKIDSS-LAS) imaging data available from the GAMA database. In order to facilitate this study we developed SIGMA, an R wrapper around several contemporary astronomy software packages including Source Extractor, PSF Extractor and GALFIT 3. SIGMA produces realistic 2D model fits to galaxies, employing automatic adaptive background subtraction and empirical PSF measurements on the fly for each galaxy in GAMA. Using these results, we define a common coverage area across the three GAMA regions containing 138,269 galaxies. We provide S\'ersic magnitudes truncated at 10 re which show good agreement with SDSS Petrosian and GAMA photometry for low S\'ersic index systems (n < 4), and much improved photometry for high S\'ersic index systems (n > 4), recovering as much as \Delta m = 0.5 magnitudes in the r band. We employ a K band S\'ersic index/u - r colour relation to delineate the massive (n > ~2) early-type galaxies (ETGs) from the late-type galaxies (LTGs). The mean S\'ersic index of these ETGs shows a smooth variation with wavelength, increasing by 30% from g through K. LTGs exhibit a more extreme change in S\'ersic index, increasing by 52% across the same range. In addition, ETGs and LTGs exhibit a 38% and 25% decrease respectively in half-light radius from g through K. These trends are shown to arise due to the effects of dust attenuation and stellar population/metallicity gradients within galaxy populations.

Invertible Extractors and Wiretap Protocols

A wiretap protocol is a pair of randomized encoding and decoding functions such that knowledge of a bounded fraction of the encoding of a message reveals essentially no information about the message, while knowledge of the entire encoding reveals the message using the decoder. In this paper, the notion of efficiently invertible extractors is studied and it is shown that a wiretap protocol can be constructed from such an extractor. Then, invertible extractors for symbol-fixing, affine, and general sources are constructed and used to create wiretap protocols with asymptotically optimal trade-offs between their rate (ratio of the length of the message versus its encoding) and resilience (ratio of the observed positions of the encoding and the length of the encoding). The results are further applied to create wiretap protocols for challenging communication problems, such as active intruders who change portions of the encoding, network coding, and intruders observing arbitrary Boolean functions of the encoding.

Affine Dispersers from Subspace Polynomials

Dispersers and extractors for affine sources of dimension $d$ in $\mathbb{F}_2^m$---where $\mathbb{F}_2$ denotes the finite field of size $2$---are functions $f: \mathbb{F}_2^m \rightarrow \mathbb{F}_2$ that behave pseudorandomly when their domain is restricted to any particular affine space $S \subseteq \mathbb{F}_2^m$ of dimension at least $d$. For dispersers, “pseudorandom behavior” means that $f$ is nonconstant over $S$, i.e., $\{ f(s) \: \middle| \: s\in S\}=\mathbb{F}_2$. For extractors, it means that $f(s)$ is distributed almost uniformly over $\mathbb{F}_2$ when $s$ is distributed uniformly over $S$. Dispersers and extractors for affine sources have been considered in the context of deterministic extraction of randomness from structured sources of imperfect randomness. Previously, explicit constructions of affine dispersers were known for every $d = \Omega(m)$ (due to [B. Barak et al., in Proceedings of the ACM Symposium on Theory of Computing, ACM, New York, 2005]), and explicit affine extractors for the same dimensions were obtained by [J. Bourgain, Geom. Funct. Anan., 17 (2007), pp. 33--57]. The main result of this paper is an efficient deterministic construction of affine dispersers for sublinear dimension $d = O(m^{4/5})$. We also construct analogous objects over $\mathbb{F}_p$ for prime $p$. Additional results include a new and simple affine extractor for dimension $d>2m/5$ and a simple disperser for multiple independent affine sources. The main novelty in this paper lies in the method of proof, which makes use of classical algebraic objects called subspace polynomials. In contrast, the papers mentioned above relied on the sum-product theorem for finite fields and other recent results from additive combinatorics.

Black box polynomial identity testing of generalized depth-3 arithmetic circuits with bounded top fan-in

In this paper we consider the problem of determining whether an unknown arithmetic circuit, for which we have oracle access, computes the identically zero polynomial. This problem is known as the black-box polynomial identity testing (PIT) problem. Our focus is on polynomials that can be written in the form $f(\bar x) = \sum\nolimits_{i = 1}^k {h_i (\bar x) \cdot g_i (\bar x)} $, where each h i is a polynomial that depends on only ρ linear functions, and each g i is a product of linear functions (when h i = 1, for each i, then we get the class of depth-3 circuits with k multiplication gates, also known as ΣΠΣ(k) circuits, but the general case is much richer). When maxi (deg(h i · g i )) = d we say that f is computable by a ΣΠΣ(k; d;ρ) circuit. We obtain the following results. A deterministic black-box identity testing algorithm for ΣΠΣ(k; d;ρ) circuits that runs in quasi-polynomial time (for ρ=polylog(n+d)). In particular this gives the first black-box quasi-polynomial time PIT algorithm for depth-3 circuits with k multiplication gates. A deterministic black-box identity testing algorithm for read-k ΣΠΣ circuits (depth-3 circuits where each variable appears at most k times) that runs in time $n^{2^{O(k^2 )} } $. In particular this gives a polynomial time algorithm for k=O(1). Our results give the first sub-exponential black-box PIT algorithm for circuits of depth higher than 2. Another way of stating our results is in terms of test sets for the underlying circuit model. A test set is a set of points such that if two circuits get the same values on every point of the set then they compute the same polynomial. Thus, our first result gives an explicit test set, of quasi-polynomial size, for ΣΠΣ(k; d;ρ) circuits (when ρ=polylog(n+d)). Our second result gives an explicit polynomial size test set for read-k depth-3 circuits. The proof technique involves a construction of a family of affine subspaces that have a rank-preserving property that is inspired by the construction of linear seeded extractors for affine sources of Gabizon and Raz [9], and a generalization of a theorem of [8] regarding the structure of identically zero depth-3 circuits with bounded top fan-in.

Extending the Canada-France Brown Dwarf Survey to the near infrared

We present the first results of the Canada-France Brown Dwarfs Survey-InfraRed, hereafter CFBDSIR, our near infrared extension to the optical wide field survey CFBDS. Our final objectives are to constrain ultracool atmosphere physics by finding a statistically significant sample of objects cooler than 600K and to explore the ultracool brown dwarf mass function building on a well defined sample of such objects. We identify candidates in CFHT/Wircam J and CFHT/MegaCam z ′ images using optimized psf-fitting within Source Extractor, and follow them up with pointed near infrared imaging with SOFI at NTT. We have so far analysed and followed up all candidates on the first 66 square degrees of the 280 square degrees survey. We identified 64 T dwarfs candidates with z ′− J > 3.5 and have confirmed 3 of them as ultracool brown dwarfs (later than T7 dwarfs and Y dwarfs candidates), and 14 of them as early and mid-T dwarfs based on their far red and NIR colours. We also present here the NIR spectra of one of these ultracool dwarfs, CFBDSIR1458 which confirms it as one of the coldest brown dwarf known, in the 500–600 K temperature range.

Deterministic extractors for small-space sources

We give polynomial-time, deterministic randomness extractors for sources generated in small space, where we model space s sources on { 0 , 1 } n as sources generated by width 2 s branching programs. Specifically, there is a constant η > 0 such that for any ζ > n − η , our algorithm extracts m = ( δ − ζ ) n bits that are exponentially close to uniform (in variation distance) from space s sources with min-entropy δn, where s = Ω ( ζ 3 n ) . Previously, nothing was known for δ ⩽ 1 / 2 , even for space 0. Our results are obtained by a reduction to the class of total-entropy independent sources. This model generalizes both the well-studied models of independent sources and symbol-fixing sources. These sources consist of a set of r independent smaller sources over { 0 , 1 } ℓ , where the total min-entropy over all the smaller sources is k. We give deterministic extractors for such sources when k is as small as polylog ( r ) , for small enough ℓ.

Web-scale extraction of structured data

A long-standing goal of Web research has been to construct a unified Web knowledge base. Information extraction techniques have shown good results on Web inputs, but even most domain-independent ones are not appropriate for Web-scale operation. In this paper we describe three recent extraction systems that can be operated on the entire Web (two of which come from Google Research). The TextRunner system focuses on raw natural language text, the WebTables system focuses on HTML-embedded tables, and the deep-web surfacing system focuses on "hidden" databases. The domain, expressiveness, and accuracy of extracted data can depend strongly on its source extractor; we describe differences in the characteristics of data produced by the three extractors. Finally, we discuss a series of unique data applications (some of which have already been prototyped) that are enabled by aggregating extractedWeb information.

Extractors for a Constant Number of Polynomially Small Min-Entropy Independent Sources

We consider the problem of randomness extraction from independent sources. We construct an extractor that can extract from a constant number of independent sources of length n, each of which have min-entropy $n^\gamma$, for an arbitrarily small constant $\gamma>0$. Our extractor is obtained by composing seeded extractors in simple ways. We introduce a new technique to condense independent somewhere-random sources which looks like a useful way to manipulate independent sources. Our techniques are different from those used in recent work [B. Barak, R. Impagliazzo, and A. Wigderson, SIAM J. Comput., 36 (2006), pp. 1095–1118; B. Barak, G. Kindler, R. Shaltiel, B. Sudakov, and A. Wigderson, Simulating independence: New constructions of condensers, Ramsey graphs, dispersers, and extractors, in Proceedings of the 37th Annual ACM Symposium on Theory of Computing, ACM, New York, 2005, pp. 1–10; R. Raz, Extractors with weak random seeds, in Proceedings of the 37th Annual ACM Symposium on Theory of Computing, ACM, New York, 2005, pp. 11–20; J. Bourgain, Int. J. Number Theory, 1 (2005), pp. 1–32] for this problem in the sense that they do not rely on any results from arithmetic combinatorics. Using an extractor of Bourgain's [Int. J. Number Theory, 1 (2005), pp. 1–32] as a black box, we obtain a new extractor for two independent block sources with few blocks, even when the min-entropy is as small as $\operatorname{polylog}(n)$. We also show how to modify the 2 source disperser for linear min-entropy of Barak et al. [Simulating independence: New constructions of condensers, Ramsey graphs, dispersers, and extractors, in Proceedings of the 37th Annual ACM Symposium on Theory of Computing, ACM, New York, 2005, pp. 1–10] and the three source extractor of Raz [Extractors with weak random seeds, in Proceedings of the 37th Annual ACM Symposium on Theory of Computing, ACM, New York, 2005, pp. 11–20] to get dispersers/extractors with exponentially small error and linear output length where previously both were constant.

Deterministic extractors for affine sources over large fields

An (n,k)-affine source over a finite field $$\mathbb{F}$$is a random variable X = (X1,..., Xn) ∈ $$\mathbb{F}^n$$, which is uniformly distributed over an (unknown) k-dimensional affine subspace of $$\mathbb{F}^n$$. We show how to (deterministically) extract practically all the randomness from affine sources, for any field of size larger than nc (where c is a large enough constant). Our main results are as follows: (For arbitraryk): For any n,k and any $$\mathbb{F}$$of size larger than n20, we give an explicit construction for a function D : $$\mathbb{F}^n $$→ $$\mathbb{F}^{k - 1}$$, such that for any (n,k)-affine source X over $$\mathbb{F}$$, the distribution of D(X) is ∊-close to uniform, where ∊ is polynomially small in |$$\mathbb{F}$$|. (Fork=1): For any n and any $$\mathbb{F}$$ of size larger than nc, we give an explicit construction for a function D: $$\mathbb{F}^n \to \{ 0,1\} ^{(1 - \delta )log_2 |\mathbb{F}|}$$, such that for any (n, 1)-affine source X over $$\mathbb{F}$$, the distribution of D(X) is ∊-close to uniform, where ∊ is polynomially small in |$$\mathbb{F}$$|. Here, δ>0 is an arbitrary small constant, and c is a constant depending on δ.

Finding ultracool brown dwarfs with MegaCam on CFHT: method and first results

Aims. We present the first results of a wide field survey for cool brown dwarfs with the MegaCam camera on the CFHT telescope, the Canada-France Brown Dwarf Survey, hereafter CFBDS. Our objectives are to find ultracool brown dwarfs and to constrain the field-brown dwarf mass function thanks to a larger sample of L and T dwarfs. Methods. We identify candidates in CFHT/MegaCam iand zimages using optimised psf-fitting within Source Extractor, and follow them up with pointed near-infrared imaging on several telescopes. Results. We have so far analysed over 350 square degrees and found 770 brown dwarf candidates brighter than z � = 22.5. We cur- rently have J-band photometry for 220 of these candidates, which confirms 37% as potential L or T dwarfs. Some are among the red- dest and farthest brown dwarfs currently known, including an independent identification of the recently published ULAS J003402.77- 005206.7 and the discovery of a second brown dwarf later than T8, CFBDS J005910.83-011401.3. Infrared spectra of three T dwarf candidates confirm their nature, and validate the selection process. Conclusions. The completed survey will discover ∼100 T dwarfs and ∼500 L dwarfs or M dwarfs later than M8, approximately doubling the number of currently known brown dwarfs. The resulting sample will have a very well-defined selection function, and will therefore produce a very clean luminosity function.

In Pursuit of LSST Science Requirements: A Comparison of Photometry Algorithms

ABSTRACTWe have developed an end-to-end photometric data-processing pipeline to compare current photometric algorithms commonly used on ground-based imaging data. This test bed is exceedingly adaptable and enables us to perform many research and development tasks, including image subtraction and co-addition, object detection and measurements, the production of photometric catalogs, and the creation and stocking of database tables with time-series information. This testing has been undertaken to evaluate existing photometry algorithms for consideration by a next-generation image-processing pipeline for the Large Synoptic Survey Telescope (LSST). We outline the results of our tests for four packages: the Sloan Digital Sky Survey’s Photo package, DAOPHOT and ALLFRAME, DOPHOT, and two versions of Source Extractor (SExtractor). The ability of these algorithms to perform point-source photometry, astrometry, shape measurements, and star-galaxy separation and to measure objects at low signal-to-noise ratio is quantified. We also perform a detailed crowded-field comparison of DAOPHOT and ALLFRAME, and profile the speed and memory requirements in detail for SExtractor. We find that both DAOPHOT and Photo are able to perform aperture photometry to high enough precision to meet LSST’s science requirements, and less adequately at PSF-fitting photometry. Photo performs the best at simultaneous point- and extended-source shape and brightness measurements. SExtractor is the fastest algorithm, and recent upgrades in the software yield high-quality centroid and shape measurements with little bias toward faint magnitudes. ALLFRAME yields the best photometric results in crowded fields.