SExtractor Draft - METU Astrophysics
SExtractor Draft - METU Astrophysics SExtractor Draft - METU Astrophysics
32Transformation to J2000 or B1950 is done without taking into account proper motions, whichare obviously unknown for the detected objects. In both cases, epoch 2000.0 is assumed.Here is a list of catalog parameters currently supporting angular coordinates:Image parameters World parameters Angular parametersX IMAGE, Y IMAGE X WORLD, Y WORLD ALPHA SKY, DELTA SKYALPHA J2000, DELTA J2000ALPHA B1950, DELTA B1950XPEAK IMAGE, YPEAK IMAGE XPEAK WORLD, YPEAK WORLD ALPHAPEAK SKY, DELTAPEAK SKYALPHAPEAK J2000, DELTAPEAK J2000ALPHAPEAK B1950, DELTAPEAK B1950TBW9.2.2 Use of the FITS keywords for astrometryTBW9.3 Cross-identification within SExtractorSExtractor allows one to perform on-line cross-identification of each detection with an ASCIIlist. Although the cross-identification algorithm is not very sophisticated — it works in pixelcoordinatesonly —, it is particularly convenient for assessing SExtractor performances, on imagesimulations from instance. Configuration parameters related to cross-identification are prefixedwith ASSOC.9.3.1 The ASSOC listThe ASSOC process is initiated by requesting in the parameter file at least one of the ASSOCcatalog parameters: VECTOR ASSOC and NUMBER ASSOC. Then SExtractor looks for an ASCII file(let’s call it the ASSOC list) whose file name has to be specified by the ASSOC NAME configurationkeyword. The ASSOC list must contain columns of numbers separated by spaces or tabs. Eachline describes a source that will enter the cross-identification process. Lines with zero characters,or beginning with “#” (for comments) are ignored. This means you may use any ASCII cataloggenerated by a previous SExtractor run as an ASSOC list.To perform the cross-identification, SExtractor needs to know what are the columns that containthe x and y coordinates 16 in the ASSOC list. These shall be specified using the ASSOC PARAMSconfiguration parameter. The syntax is: “ASSOC PARAMS c x ,c y [,c Z ]”, where c x and c y are thepositions of the columns containing the x and y coordinates (the first column has position 1).c Z (optional) specifies an extra column containing some “Z” parameter that may be used forcontrolling or weighting the ASSOC process. Z will typically be a flux estimate. c Z is required ifASSOC TYPE is MIN, MAX, MEAN or MAG MEAN (see below).9.3.2 Controlling the ASSOC processTwo configuration parameters control the ASSOC process. The first one, ASSOC RADIUS, accepts adecimal number which represents the maximum distance (in pixels) one should have between the16 The x and y coordinates must comply with the FITS (and SExtractor) convention: by definition, the centerof the first pixel in the image array has pixel-coordinates (1.0,1.0).
33barycenter of the current SExtractor detection and an ASSOC-list member to consider a match.This number must of course account for positional uncertainties in both catalogs. In most cases,a value of a few pixels will do just fine. The second configuration parameter, ASSOC TYPE, acceptsa keyword as argument and selects the kind of identification procedure one wants to operate:• FIRST: this is the simplest way of performing a cross-identification. It does not requirethe c Z column in ASSOC PARAMS. The first geometrical match encountered while scanningthe ASSOC list is retained as the actual match. This can used for catalogs with low spatialdensity.• NEAREST: this option does not require the c Z column in ASSOC PARAMS. The match isperformed with the ASSOC-list member the closest (in position) to the current detection,provided that it lies within the ASSOC RADIUS.• SUM: all parameters issued from ASSOC-list members which geometrically match the currentdetection are summed. c Z is not required.• MAG SUM: all parameters c i issued from ASSOC-list members which geometrically match thecurrent detection are combined using the following law: −2.5 log( ∑ i 10−0.4c i. This optionallows one to sum flux contributions from magnitude data. c Z is not required.• MIN: among all geometrical matches, retains the ASSOC-list member which has the smallestZ parameter.• MAX: among all geometrical matches, retains the ASSOC-list member which has the largestZ parameter.• MEAN: all parameters issued from ASSOC-list members which geometrically match the currentdetection are weighted-averaged, using the Z parameter as the weight.• MAG MEAN: all parameters issued from ASSOC-list members which geometrically match thecurrent detection are weighted-averaged, using 10 −0.4Z as the weight. This option is usefulfor weighting catalog sources with magnitudes.9.3.3 Output from ASSOCNow that we have described the cross-identification process, let’s see how informations comingfrom the matching with the ASSOC list are propagated to the output SExtractor catalog.The output of ASSOC data in SExtractor catalog is done through the VECTOR ASSOC() catalogparameter. VECTOR ASSOC() is a vector, each element of which refers to a column from theinput ASSOC list. VECTOR ASSOC() contains either ASSOC-list member data from the best match(if ASSOC TYPE is FIRST, NEAREST, MIN or MAX), or a combination of ASSOC-list member data(if ASSOC TYPE is MEAN, MAG MEAN, SUM or MAG SUM). If no match has been found, it just containszeros. The NUMBER ASSOC contains the number of ASSOC-list members that geometricallymatch the current SExtractor detection, and obviously, if different from zero, indicates thatVECTOR ASSOC() has a meaningful content.The ASSOC DATA configuration parameter is used to tell SExtractor to which column refers eachelement of VECTOR ASSOC(). The syntax of ASSOC DATA is similar to that of ASSOC PARAMS:“ASSOC DATA c 1 ,c 2 ,c 3 ,...” where the c i are the column positions in the ASSOC list. The specialcase “ASSOC DATA 0” tells SExtractor to propagate all columns from the ASSOC file to the outputcatalog.
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33barycenter of the current <strong>SExtractor</strong> detection and an ASSOC-list member to consider a match.This number must of course account for positional uncertainties in both catalogs. In most cases,a value of a few pixels will do just fine. The second configuration parameter, ASSOC TYPE, acceptsa keyword as argument and selects the kind of identification procedure one wants to operate:• FIRST: this is the simplest way of performing a cross-identification. It does not requirethe c Z column in ASSOC PARAMS. The first geometrical match encountered while scanningthe ASSOC list is retained as the actual match. This can used for catalogs with low spatialdensity.• NEAREST: this option does not require the c Z column in ASSOC PARAMS. The match isperformed with the ASSOC-list member the closest (in position) to the current detection,provided that it lies within the ASSOC RADIUS.• SUM: all parameters issued from ASSOC-list members which geometrically match the currentdetection are summed. c Z is not required.• MAG SUM: all parameters c i issued from ASSOC-list members which geometrically match thecurrent detection are combined using the following law: −2.5 log( ∑ i 10−0.4c i. This optionallows one to sum flux contributions from magnitude data. c Z is not required.• MIN: among all geometrical matches, retains the ASSOC-list member which has the smallestZ parameter.• MAX: among all geometrical matches, retains the ASSOC-list member which has the largestZ parameter.• MEAN: all parameters issued from ASSOC-list members which geometrically match the currentdetection are weighted-averaged, using the Z parameter as the weight.• MAG MEAN: all parameters issued from ASSOC-list members which geometrically match thecurrent detection are weighted-averaged, using 10 −0.4Z as the weight. This option is usefulfor weighting catalog sources with magnitudes.9.3.3 Output from ASSOCNow that we have described the cross-identification process, let’s see how informations comingfrom the matching with the ASSOC list are propagated to the output <strong>SExtractor</strong> catalog.The output of ASSOC data in <strong>SExtractor</strong> catalog is done through the VECTOR ASSOC() catalogparameter. VECTOR ASSOC() is a vector, each element of which refers to a column from theinput ASSOC list. VECTOR ASSOC() contains either ASSOC-list member data from the best match(if ASSOC TYPE is FIRST, NEAREST, MIN or MAX), or a combination of ASSOC-list member data(if ASSOC TYPE is MEAN, MAG MEAN, SUM or MAG SUM). If no match has been found, it just containszeros. The NUMBER ASSOC contains the number of ASSOC-list members that geometricallymatch the current <strong>SExtractor</strong> detection, and obviously, if different from zero, indicates thatVECTOR ASSOC() has a meaningful content.The ASSOC DATA configuration parameter is used to tell <strong>SExtractor</strong> to which column refers eachelement of VECTOR ASSOC(). The syntax of ASSOC DATA is similar to that of ASSOC PARAMS:“ASSOC DATA c 1 ,c 2 ,c 3 ,...” where the c i are the column positions in the ASSOC list. The specialcase “ASSOC DATA 0” tells <strong>SExtractor</strong> to propagate all columns from the ASSOC file to the outputcatalog.
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