SExtractor Draft - METU Astrophysics
SExtractor Draft - METU Astrophysics SExtractor Draft - METU Astrophysics
34There are situations where it might be desirable to keep in the output SExtractor catalog onlythose detections that were matched with some ASSOC-list member. Such a feature is controlledby the ASSOCSELEC TYPE configuration parameter, which accepts one of the three followingkeywords:• ALL: keep all SExtractor detections, regardless of matching. This is the default.• MATCHED: keep only SExtractor detections that were matched with at least one ASSOC-listmember.• -MATCHED: keep only SExtractor detections that were not matched with any ASSOC-listmember.AcknowledgmentsReferences[1] Beard S.M., McGillivray H.T., Thanisch P.F., 1990, MNRAS 247, 311[2] Bertin E., E.y.E 1.1, User’s manual, 1997, Leiden[3] Bertin E., WeightWatcher 1.2, User’s manual, 1997, ESO[4] Bijaoui A., Dantel M., 1991, A&A 6, 51[5] Bijaoui A., Slezak E., Vandame B., 1998, in Astrophysics and Algorithms: a DIMACSWorkshop on Massive Astronomical Data Sets[6] Dalcanton J.J., Spergel D.N., Gunn J.E., Schmidt M., Schneider D.P., 1997, AJ, 114, 635[7] Das P.K., 1991, Optical Signal Processing, (Springer-Verlag)[8] Greisen E.W., Calabretta M., 1995, A.D.A.S.S. 4, 233 ()[9] Infante L., 1987, A&A 183, 177[10] Irwin M.J., 1985, MNRAS 214, 575[11] Jarvis J.J., Tyson J.A., 1981, AJ, 86, 476[12] Kaiser N., Squires G., Broadhurst T., 1995, ApJ, 449, 460[13] Kendall M., Stuart K., 1977, The Advanced Theory of Statistics, Vol. 1, (Charles Griffin& Co., London)[14] Kron R.G., 1980, ApJS 43, 305[15] Lutz R.K., 1979, The Computer Journal 23, 262[16] Moffat A.F.J., 1969,[17] Wells D.C., Greisen E.W., Harten R.H., 1981, A&AS 44, 363
35AAppendicesA.1 FAQ (Frequently Asked Questions)Fairly often, I am asked by users about the reason for some limitations or choices in the waythings are done in SExtractor. In this section, I try to justify them.Q: SExtractor supports WCS. So why isn’t it possible to have the ASSOC crossidentificationworking in α, δ (or any other world-coordinates)?A: The ASSOC list which is used for cross-identification can be very long (100,000 objects ormore). Performing an exhaustive cross-id in real-time can therefore be extremely slow, unlessthe ASSOC coordinates are sorted in some way beforehand. In pixel coordinates, such a sorting issimple and very efficient, as SExtractor works line-by-line; but it would be much more difficultin the general WCS context. This is why this hasn’t been implemented, considering it as beyondthe scope of SExtractor.Q: Why isn’t the detection threshold expressed in units of the background noisestandard deviation in the FILTERed image ?A: There are two reasons for this. First, it makes the threshold independent of the choice of aFILTER, which is a good thing. Second, having σ measured on the FILTERed image may havegiven un-informed users the wrong impression that increasing filtering systematically improvesthe detectability of any source, whereas it depends on scale.
- Page 1: 1DraftSExtractorv2.1.3User’s guid
- Page 4 and 5: 47 Weighting 227.1 Weight-map forma
- Page 6 and 7: 62.2 Obtaining SExtractorThe easies
- Page 8 and 9: 8BACK SIZE — integers (n ≤ 2) S
- Page 10 and 11: 10INTERP TYPE ALL keywords (n ≤ 2
- Page 12 and 13: 12VIGNET(15,15), yield arrays of nu
- Page 14 and 15: 14Segmentation in SExtractor is ach
- Page 16 and 17: 166.2 Filtering6.2.1 ConvolutionDet
- Page 18 and 19: 186.2.4 Image boundaries and bad pi
- Page 20 and 21: 206.4 DeblendingEach time an object
- Page 22 and 23: 22are generally useless, except per
- Page 24: 243. The CLEANing process (§??) ta
- Page 27 and 28: 279.1.3 Position of the peak: XPEAK
- Page 29 and 30: 29¡ ¡ ¢ £ ¤ ¥ ¦ § ¨ © ©
- Page 31 and 32: 31Positional errors are more diffic
- Page 33: 33barycenter of the current SExtrac
35AAppendicesA.1 FAQ (Frequently Asked Questions)Fairly often, I am asked by users about the reason for some limitations or choices in the waythings are done in <strong>SExtractor</strong>. In this section, I try to justify them.Q: <strong>SExtractor</strong> supports WCS. So why isn’t it possible to have the ASSOC crossidentificationworking in α, δ (or any other world-coordinates)?A: The ASSOC list which is used for cross-identification can be very long (100,000 objects ormore). Performing an exhaustive cross-id in real-time can therefore be extremely slow, unlessthe ASSOC coordinates are sorted in some way beforehand. In pixel coordinates, such a sorting issimple and very efficient, as <strong>SExtractor</strong> works line-by-line; but it would be much more difficultin the general WCS context. This is why this hasn’t been implemented, considering it as beyondthe scope of <strong>SExtractor</strong>.Q: Why isn’t the detection threshold expressed in units of the background noisestandard deviation in the FILTERed image ?A: There are two reasons for this. First, it makes the threshold independent of the choice of aFILTER, which is a good thing. Second, having σ measured on the FILTERed image may havegiven un-informed users the wrong impression that increasing filtering systematically improvesthe detectability of any source, whereas it depends on scale.
Change language