In this paper we present preliminary results from the measurement and analysis of imag es of the ecliptic zoneobtained with MegaCam at Canada-France-Hawaii Telescope (CFHT) located a   t Mauna Kea(Hawaii ). We areevaluating the feasibili ty of constructing an astrometric and photometric cataloguewith all the a   stronomicalsources identified on those imag es. We comm  ent and discuss the criteria a  nd procedures for the identificationof sources, the photometric reduction and their class ification as s tellar or extendedobjectsEn este trabajo presentamos resultados preliminares de la medición y análisis de imágenes de la zona ecliptical obtenidas con la Cámara MegaCam instalada en el Canada-France-Hawaii Telescope (CFHT) ubicado en MaunaKea(Hawaii ). Se está evaluando la posibildad de construir un catálogo astrométrico y fotométrico de las fuentes astronómicas identificadas en las mencionadas imágenes. En esta presentación se comentan y discuten los criterios empleados y procedimientos aplicados para la identificación, reducción fotométrica y clasificación de las fuentes astronómicas como estelares o extendidasFil: Calderon, Jesus Humberto. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; ArgentinaFil: Bustos Fierro, Iván Heriberto. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; Argentin

Bustos Fierro, Iván Heriberto

Calderon, Jesus Humberto

Revista Mexicana de Astronomía y Astrofísica Serie de Conferencias

English

CONICET Digital

© 2014: Instituto de Astronomía, UNAM - Astronomía Dinámica en Latino América (ADeLA-2012)Ed. C. Allen, F. Arias, & R. OrellanaRevMexAA (Serie de Conferencias), 43, 43–46 (2013)MEASUREMENT OF CFHT IMAGES I. IDENTIFICATION, PHOTOMETRYAND CLASSIFICATIONJ. H. Caldero´n1,2 and I. H. Bustos Fierro1RESUMENEn este trabajo presentamos resultados preliminares de la medicio´n y ana´lisis de ima´genes de la zona eclipticalobtenidas con la Ca´mara MegaCam instalada en el Canada-France-Hawaii Telescope (CFHT) ubicado en MaunaKea (Hawaii). Se esta´ evaluando la posibilidad de construir un cata´logo astrome´trico y fotome´trico de lasfuentes astrono´micas identificadas en las mencionadas ima´genes. En esta presentacio´n se comentan y discutenlos criterios empleados y procedimientos aplicados para la identificacio´n, reduccio´n fotome´trica y clasificacio´nde las fuentes astrono´micas como estelares o extendidas.ABSTRACTIn this paper we present preliminary results from the measurement and analysis of images of the ecliptic zoneobtained with MegaCam at Canada-France-Hawaii Telescope (CFHT) located at Mauna Kea (Hawaii). We areevaluating the feasibility of constructing an astrometric and photometric catalogue with all the astronomicalsources identified on those images. We comment and discuss the criteria and procedures for the identificationof sources, the photometric reduction and their classification as stellar or extended objects.Key Words: astrometry — methods: data analysis1. INTRODUCTIONIn the last year we have started a collaborationwith the Laboratory SYRTE (Syste`mes de Re´fe´renceTemps-Espace, UM8630 of CNRS) at Paris Ob-servatory for the exploitation of 1700 wide-fieldframes taken at the Canada-France-Hawaii Tele-scope (CFHT), each one covering a 1◦×1◦ area witha mosaic of 36 CCD chips. These frames covering alarge band of the sky along the ecliptic are alreadyavailable as part of the CFHT Legacy Survey-VeryWide (CFHTLS-VW). The ultimate objective is theconstruction of a celestial catalogue called MEGA-CLIP much deeper and accurate than other exist-ing catalogues in the region covered. For the samezone of the sky it will contain much more objectsthan these catalogues, as well as the output cata-logue from GAIA (scheduled around 2017). More-over the MEGACLIP catalogue, thanks to its highdensity and its accuracy with respect to the existingcatalogues, will enable to determine GAIA’s coor-dinates during its operational mission by giving thecoordinates of reference stars with very good accu-racy.1Observatorio Astrono´mico, UNC, Laprida 854,X5000BGR, Co´rdoba, Argentina (ivanbf, calderon@oac.uncor.edu).2Consejo Nacional de Investigaciones Cient´ıficas yTe´cnicas, Argentina.2. IMAGES DESCRIPTIONThe wide-field imager MegaCam of CFHT pro-duces an image formed by 36 CCD frames of 2048×4612 13.5-microns square pixels, covering a full 1◦×1◦ field of view with a resolution of 0.187′′ per pixelto properly sample the 0.7′′ median seeing offered bythe CFHT at Mauna Kea.Images are part of the CFHTLS-VW covering alarge fraction of the ecliptic plane within a band of±2◦. Images were taken with the filters (g′/r′/i′) ofthe AB photometric system.Each image has a total of 340 megapixels hav-ing 16 bits digital resolution. Frames have a nomi-nal separation of 70 µm in East-West direction and430 µm in North-South direction. The image datafile is a unique fits file 649 Mby, each frame has34 Mby. Images are corrected for instrumental er-rors and they have rough astrometric and photomet-ric calibrations. These calibrations were not takeninto account in this work.We began analyzing field 739523p centered inRA=10h 00m 29s.03; DEC=2◦12′05′′ (band r′), ob-serving date 03/20/2004.3. VISUAL INSPECTION OF IMAGESWe performed a preliminary visual inspection ofthe field 739523p. Tycho-2 (Høg et al. 2000) andUCAC3 (Zacharias et al. 2010) stars were visually43© 2014: Instituto de Astronomía, UNAM - Astronomía Dinámica en Latino América (ADeLA-2012)Ed. C. Allen, F. Arias, & R. Orellana44 CALDERO´N & BUSTOS FIERROFig. 1. See text for description.recognized. All Tycho-2 and many UCAC3 stars aresaturated (Figure 1a). Their haloes and spikes af-fect objects around them (Figure 1b). CCDs inac-tive columns affect images of objects formed on themresulting in two objects instead of one (Figure 1c).Objects near the edge of CCD chips lose some pix-els and hence their measured centroids will not becorrect (Figure 1d). These last three issues must betaken into account because they will produce spu-rious detections. The FWHM measured for stellarimages in this field was ≈6 px (1.1′′), but smallervalues were measured in other fields.4. DETECTION AND CENTERING OFOBJECTSWe considered two possibilities for detection andcentering of possible astronomical sources. One ofthem was to use IRAF-DAOFIND (Stetson 1987)theother was to use SExtractor (Bertin & Arnouts1996). DAOFIND is based on the convolution withan elliptical Gaussian, which requires a priori fix-ing a value for FWHM of the PSF, a backgroundnoise, an axis ratio and a position angle. For this rea-son we adopted SExtractor that is a non-parametricalgorithm. It requires only fixing a minimum sig-nal/noise threshold of detection and a minimum areaof connected pixel. This amount represents the min-imum possible area corresponding to the image of atrue astronomical source and it is related to the mea-sured seeing on the image. SExtractor produces acatalogue of measured properties of detections: cen-Fig. 2. R.m.s deviation from average instrumental mag-nitude at different S/N detection levels.tral coordinates of centroids with their errors, in-strumental photometric magnitudes, shape parame-ters (FWHM, second order momenta, position an-gle, elipticity, elongation, etc.) and a classificationparameter (CLASS) useful for the classification ofdetections into different types of objects: stellar orextended.5. OPTIMAL SIGNAL TO NOISE DETECTIONTHRESHOLDWe observed a slight variation of errors of cen-troid coordinates according to the selected S/Nthreshold for detections. This will affect the resultsof astrometric reduction. We tried to find an opti-mal working S/N threshold, i.e. the threshold thatminimizes the errors. The optimal value found wasS/N=7. More details at this respect can be seenin Bustos Fierro & Caldero´n (2013). We found thatnot only errors in centroids of coordinates were mini-mized at S/N=7 but also errors in instrumental mag-nitudes have their minima as can be seen in Figure 2.We have also considered how detection thresholdaffects the number of detected sources. We observedthat sources with CLASS ≥ 0.7 have stellar appear-ance, therefore sources with CLASS ≥ 0.7 will here-after be considered as stars. Results for field 739523pcan be seen in Table 1. Finally we adopted S/N=7 toobtain data for calibrations and S/N=3 to producea deep enough catalogue. Note that the increase ofthe number of non-stars is larger than that of thenumber of stars when decreasing S/N.6. PRELIMINARY PHOTOMETRICCALIBRATIONWe performed a preliminary photometric cali-bration in order to have an estimation of the lim-iting magnitude of the images. We used as refer-ence data for calibration the magnitudes from the© 2014: Instituto de Astronomía, UNAM - Astronomía Dinámica en Latino América (ADeLA-2012)Ed. C. Allen, F. Arias, & R. OrellanaMEASUREMENT OF CFHT IMAGES I 45TABLE 1NUMBER OF DETECTED OBJECTS ATDIFFERENT S/N THRESHOLDSS/N Stars Non-stars All3 4709 14220 189294 4106 10623 147295 3695 8648 123436 3385 7227 106127 3169 6247 94168 2971 5565 85369 2823 4933 775610 2687 4444 7131Fig. 3. R.m.s deviation from average instrumental mag-nitude at different S/N detection levels.UCAC3 catalogue. We are aware that the UCAC3magnitudes are not photometric standards but theywere useful as a starting point. We apologize tophotometrists. For the relation between UCAC3magnitudes (mUCAC3) and instrumental magnitudes(mInst) we proposed a linear fit: mUCAC3 = a ×mInst + b.We have applied an iterative method in order toimprove the correlation coefficient of the fit. Result-ing limiting magnitudes can be seen in the histogramin Figure 3.We also made a comparison of the number of de-tected objects and limiting magnitude with the cat-alogue USNO-B1 (Monet et al. 2003). See Figure 4.7. CLASSIFICATIONWe analyzed the results of automatic SExtrac-tor classification. The parameter CLASS providedby SExtractor has values between 0 and 1. Low val-ues of CLASS indicate extended appearance objects,high values indicate objects of stellar appearance.Fig. 4. R.m.s deviation from average instrumental mag-nitude at different S/N detection levels.Fig. 5. See text for description.The distributions of CLASS on the same field attwo different S/N detection thresholds are shown inFigure 5. As seen in the histograms there is a con-tinuous range of values without evident separationbetween stellar and extended objects.© 2014: Instituto de Astronomía, UNAM - Astronomía Dinámica en Latino América (ADeLA-2012)Ed. C. Allen, F. Arias, & R. Orellana46 CALDERO´N & BUSTOS FIERROFig. 6. See text for description.From visual inspection of a random sample con-sisting of 20% of detections we found that there aresome extended sources with CLASS ≥ 0.7 and somestellar-like sources with CLASS < 0.7. Informationabout how CLASS is calculated by SExtractor is notavailable (or we cannot access it); thus we were notable to attack this matter using CLASS only. Weperformed statistic diagrams for classification tak-ing into account the shape parameters provided bySExtractor, namely elongation, FWHM, position an-gle and the size of the minor axis. We also noticedan increase in the ellipticity of stellar images whenincreasing distance to the center of the field.8. SPURIOUS DETECTIONSWe consider as spurious those detections whoseimage does not correspond to an astronomical sourceas well as those which correspond to distorted im-ages of astronomical sources, as seen in Figure 1.From another analysis of the sample mentioned in§ 7 we found a number of spurious detections with awide range of values in CLASS and in shape param-eters. Figure 6 shows some examples, two of themresulting from the haloes of bright stars, and two ofthem of unknown origin. In every case the center ofthe image is the position of the centroid provided bySExtractor, and the width is five times the largestbetween FWHM and major axis.9. CONCLUSIONSVisual inspection of the field showed images ofa number of astronomical sources that are affectedby distortions of instrumental origin (edges and coldcolumns) and by the glare of bright stars. In addi-tion we observed a number of detections that do notcorrespond to any astronomical source.SExtractor resulted to be more adequate thanDAOFIND to perform detections in these images.It allows to identify objects that are not necessarilystars. It also provides many parameters useful todescribe and classify the detections.The best photometric accuracy with SExtractoris achieved with the detection threshold at S/N=7but the number of detections is about half the num-ber at S/N=3. Therefore we propose to performthe astrometric and photometric calibrations withS/N=7 and to construct a deeper catalogue withS/N=3.From the preliminary photometric reduction wecould estimate a limiting magnitude ∼23–24, two ormore magnitudes fainter than USNO-B1 and the ex-pected limit of GAIA.We found that the parameter CLASS is a goodstarting point for stellar/extended object classifica-tion, but it is not enough. Further analysis, takinginto account other parameters and techniques, is inprocess. Similar problems occur with the identifica-tion of spurious detections.This work is based on observations obtained withMegaPrime/MegaCam, a joint project of CFHT andCEA/DAPNIA, at the Canada-France-Hawaii Tele-scope (CFHT) which is operated by the NationalResearch Council (NRC) of Canada, the Institut Na-tional des Science de l’Univers of the Centre Nationalde la Recherche Scientifique (CNRS) of France, andthe University of Hawaii. This work is based in parton data products produced at TERAPIX and theCanadian Astronomy Data Centre as part of theCanada-France-Hawaii Telescope Legacy Survey, acollaborative project of NRC and CNRS.This research has made use of the VizieR cata-logue access tool, CDS, Strasbourg, France.The authors wish to thank Dr. Jean Souchayhis kind invitation to be part of the MEGACLIPproject.REFERENCESBertin, E., & Arnouts, S. 1996, A&AS, 117,393Bustos Fierro, I. H., & Caldero´n, J. H. 2013, RevMexAA(SC), 43, 47Høg, E., et al. 2000, A&A, 355, 27Monet, D. G., et al. 2003, AJ, 125, 984Stetson, P. B. 1987, PASP, 99, 191Zacharias, N., et al. 2010, AJ, 139, 2184

Measurement of CFHT Images I: Identification, Photometry and Classification

LAReferencia - Red Federada de Repositorios Institucionales de Publicaciones Científicas Latinoamericanas

© 2014: Instituto de Astronomía, UNAM - Astronomía Dinámica en Latino América (ADeLA-2012)Ed. C. Allen, F. Arias, & R. OrellanaRevMexAA (Serie de Conferencias), 43, 43–46 (2013)MEASUREMENT OF CFHT IMAGES I. IDENTIFICATION, PHOTOMETRYAND CLASSIFICATIONJ. H. Calderón1,2 and I. H. Bustos Fierro1RESUMENEn este trabajo presentamos resultados preliminares de la medición y análisis de imágenes de la zona eclipticalobtenidas con la Cámara MegaCam instalada en el Canada-France-Hawaii Telescope (CFHT) ubicado en MaunaKea (Hawaii). Se está evaluando la posibilidad de construir un catálogo astrométrico y fotométrico de lasfuentes astronómicas identificadas en las mencionadas imágenes. En esta presentación se comentan y discutenlos criterios empleados y procedimientos aplicados para la identificación, reducción fotométrica y clasificaciónde las fuentes astronómicas como estelares o extendidas.ABSTRACTIn this paper we present preliminary results from the measurement and analysis of images of the ecliptic zoneobtained with MegaCam at Canada-France-Hawaii Telescope (CFHT) located at Mauna Kea (Hawaii). We areevaluating the feasibility of constructing an astrometric and photometric catalogue with all the astronomicalsources identified on those images. We comment and discuss the criteria and procedures for the identificationof sources, the photometric reduction and their classification as stellar or extended objects.Key Words: astrometry — methods: data analysis1. INTRODUCTIONIn the last year we have started a collaborationwith the Laboratory SYRTE (Systèmes de RéférenceTemps-Espace, UM8630 of CNRS) at Paris Ob-servatory for the exploitation of 1700 wide-fieldframes taken at the Canada-France-Hawaii Tele-scope (CFHT), each one covering a 1◦×1◦ area witha mosaic of 36 CCD chips. These frames covering alarge band of the sky along the ecliptic are alreadyavailable as part of the CFHT Legacy Survey-VeryWide (CFHTLS-VW). The ultimate objective is theconstruction of a celestial catalogue called MEGA-CLIP much deeper and accurate than other exist-ing catalogues in the region covered. For the samezone of the sky it will contain much more objectsthan these catalogues, as well as the output cata-logue from GAIA (scheduled around 2017). More-over the MEGACLIP catalogue, thanks to its highdensity and its accuracy with respect to the existingcatalogues, will enable to determine GAIA’s coor-dinates during its operational mission by giving thecoordinates of reference stars with very good accu-racy.1Observatorio Astronómico, UNC, Laprida 854,X5000BGR, Córdoba, Argentina (ivanbf, calderon@oac.uncor.edu).2Consejo Nacional de Investigaciones Cient́ıficas yTécnicas, Argentina.2. IMAGES DESCRIPTIONThe wide-field imager MegaCam of CFHT pro-duces an image formed by 36 CCD frames of 2048×4612 13.5-microns square pixels, covering a full 1◦ ×1◦ field of view with a resolution of 0.187′′ per pixelto properly sample the 0.7′′ median seeing offered bythe CFHT at Mauna Kea.Images are part of the CFHTLS-VW covering alarge fraction of the ecliptic plane within a band of±2◦. Images were taken with the filters (g′/r′/i′) ofthe AB photometric system.Each image has a total of 340 megapixels hav-ing 16 bits digital resolution. Frames have a nomi-nal separation of 70 µm in East-West direction and430 µm in North-South direction. The image datafile is a unique fits file 649 Mby, each frame has34 Mby. Images are corrected for instrumental er-rors and they have rough astrometric and photomet-ric calibrations. These calibrations were not takeninto account in this work.We began analyzing field 739523p centered inRA=10h 00m 29s.03; DEC=2◦12′05′′ (band r′), ob-serving date 03/20/2004.3. VISUAL INSPECTION OF IMAGESWe performed a preliminary visual inspection ofthe field 739523p. Tycho-2 (Høg et al. 2000) andUCAC3 (Zacharias et al. 2010) stars were visually43© 2014: Instituto de Astronomía, UNAM - Astronomía Dinámica en Latino América (ADeLA-2012)Ed. C. Allen, F. Arias, & R. Orellana44 CALDERÓN & BUSTOS FIERROFig. 1. See text for description.recognized. All Tycho-2 and many UCAC3 stars aresaturated (Figure 1a). Their haloes and spikes af-fect objects around them (Figure 1b). CCDs inac-tive columns affect images of objects formed on themresulting in two objects instead of one (Figure 1c).Objects near the edge of CCD chips lose some pix-els and hence their measured centroids will not becorrect (Figure 1d). These last three issues must betaken into account because they will produce spu-rious detections. The FWHM measured for stellarimages in this field was ≈6 px (1.1′′), but smallervalues were measured in other fields.4. DETECTION AND CENTERING OFOBJECTSWe considered two possibilities for detection andcentering of possible astronomical sources. One ofthem was to use IRAF-DAOFIND (Stetson 1987)theother was to use SExtractor (Bertin & Arnouts1996). DAOFIND is based on the convolution withan elliptical Gaussian, which requires a priori fix-ing a value for FWHM of the PSF, a backgroundnoise, an axis ratio and a position angle. For this rea-son we adopted SExtractor that is a non-parametricalgorithm. It requires only fixing a minimum sig-nal/noise threshold of detection and a minimum areaof connected pixel. This amount represents the min-imum possible area corresponding to the image of atrue astronomical source and it is related to the mea-sured seeing on the image. SExtractor produces acatalogue of measured properties of detections: cen-Fig. 2. R.m.s deviation from average instrumental mag-nitude at different S/N detection levels.tral coordinates of centroids with their errors, in-strumental photometric magnitudes, shape parame-ters (FWHM, second order momenta, position an-gle, elipticity, elongation, etc.) and a classificationparameter (CLASS) useful for the classification ofdetections into different types of objects: stellar orextended.5. OPTIMAL SIGNAL TO NOISE DETECTIONTHRESHOLDWe observed a slight variation of errors of cen-troid coordinates according to the selected S/Nthreshold for detections. This will affect the resultsof astrometric reduction. We tried to find an opti-mal working S/N threshold, i.e. the threshold thatminimizes the errors. The optimal value found wasS/N=7. More details at this respect can be seenin Bustos Fierro & Calderón (2013). We found thatnot only errors in centroids of coordinates were mini-mized at S/N=7 but also errors in instrumental mag-nitudes have their minima as can be seen in Figure 2.We have also considered how detection thresholdaffects the number of detected sources. We observedthat sources with CLASS ≥ 0.7 have stellar appear-ance, therefore sources with CLASS ≥ 0.7 will here-after be considered as stars. Results for field 739523pcan be seen in Table 1. Finally we adopted S/N=7 toobtain data for calibrations and S/N=3 to producea deep enough catalogue. Note that the increase ofthe number of non-stars is larger than that of thenumber of stars when decreasing S/N.6. PRELIMINARY PHOTOMETRICCALIBRATIONWe performed a preliminary photometric cali-bration in order to have an estimation of the lim-iting magnitude of the images. We used as refer-ence data for calibration the magnitudes from the© 2014: Instituto de Astronomía, UNAM - Astronomía Dinámica en Latino América (ADeLA-2012)Ed. C. Allen, F. Arias, & R. OrellanaMEASUREMENT OF CFHT IMAGES I 45TABLE 1NUMBER OF DETECTED OBJECTS ATDIFFERENT S/N THRESHOLDSS/N Stars Non-stars All3 4709 14220 189294 4106 10623 147295 3695 8648 123436 3385 7227 106127 3169 6247 94168 2971 5565 85369 2823 4933 775610 2687 4444 7131Fig. 3. R.m.s deviation from average instrumental mag-nitude at different S/N detection levels.UCAC3 catalogue. We are aware that the UCAC3magnitudes are not photometric standards but theywere useful as a starting point. We apologize tophotometrists. For the relation between UCAC3magnitudes (mUCAC3) and instrumental magnitudes(mInst) we proposed a linear fit: mUCAC3 = a ×mInst + b.We have applied an iterative method in order toimprove the correlation coefficient of the fit. Result-ing limiting magnitudes can be seen in the histogramin Figure 3.We also made a comparison of the number of de-tected objects and limiting magnitude with the cat-alogue USNO-B1 (Monet et al. 2003). See Figure 4.7. CLASSIFICATIONWe analyzed the results of automatic SExtrac-tor classification. The parameter CLASS providedby SExtractor has values between 0 and 1. Low val-ues of CLASS indicate extended appearance objects,high values indicate objects of stellar appearance.Fig. 4. R.m.s deviation from average instrumental mag-nitude at different S/N detection levels.Fig. 5. See text for description.The distributions of CLASS on the same field attwo different S/N detection thresholds are shown inFigure 5. As seen in the histograms there is a con-tinuous range of values without evident separationbetween stellar and extended objects.© 2014: Instituto de Astronomía, UNAM - Astronomía Dinámica en Latino América (ADeLA-2012)Ed. C. Allen, F. Arias, & R. Orellana46 CALDERÓN & BUSTOS FIERROFig. 6. See text for description.From visual inspection of a random sample con-sisting of 20% of detections we found that there aresome extended sources with CLASS ≥ 0.7 and somestellar-like sources with CLASS < 0.7. Informationabout how CLASS is calculated by SExtractor is notavailable (or we cannot access it); thus we were notable to attack this matter using CLASS only. Weperformed statistic diagrams for classification tak-ing into account the shape parameters provided bySExtractor, namely elongation, FWHM, position an-gle and the size of the minor axis. We also noticedan increase in the ellipticity of stellar images whenincreasing distance to the center of the field.8. SPURIOUS DETECTIONSWe consider as spurious those detections whoseimage does not correspond to an astronomical sourceas well as those which correspond to distorted im-ages of astronomical sources, as seen in Figure 1.From another analysis of the sample mentioned in§ 7 we found a number of spurious detections with awide range of values in CLASS and in shape param-eters. Figure 6 shows some examples, two of themresulting from the haloes of bright stars, and two ofthem of unknown origin. In every case the center ofthe image is the position of the centroid provided bySExtractor, and the width is five times the largestbetween FWHM and major axis.9. CONCLUSIONSVisual inspection of the field showed images ofa number of astronomical sources that are affectedby distortions of instrumental origin (edges and coldcolumns) and by the glare of bright stars. In addi-tion we observed a number of detections that do notcorrespond to any astronomical source.SExtractor resulted to be more adequate thanDAOFIND to perform detections in these images.It allows to identify objects that are not necessarilystars. It also provides many parameters useful todescribe and classify the detections.The best photometric accuracy with SExtractoris achieved with the detection threshold at S/N=7but the number of detections is about half the num-ber at S/N=3. Therefore we propose to performthe astrometric and photometric calibrations withS/N=7 and to construct a deeper catalogue withS/N=3.From the preliminary photometric reduction wecould estimate a limiting magnitude ∼23–24, two ormore magnitudes fainter than USNO-B1 and the ex-pected limit of GAIA.We found that the parameter CLASS is a goodstarting point for stellar/extended object classifica-tion, but it is not enough. Further analysis, takinginto account other parameters and techniques, is inprocess. Similar problems occur with the identifica-tion of spurious detections.This work is based on observations obtained withMegaPrime/MegaCam, a joint project of CFHT andCEA/DAPNIA, at the Canada-France-Hawaii Tele-scope (CFHT) which is operated by the NationalResearch Council (NRC) of Canada, the Institut Na-tional des Science de l’Univers of the Centre Nationalde la Recherche Scientifique (CNRS) of France, andthe University of Hawaii. This work is based in parton data products produced at TERAPIX and theCanadian Astronomy Data Centre as part of theCanada-France-Hawaii Telescope Legacy Survey, acollaborative project of NRC and CNRS.This research has made use of the VizieR cata-logue access tool, CDS, Strasbourg, France.The authors wish to thank Dr. Jean Souchayhis kind invitation to be part of the MEGACLIPproject.REFERENCESBertin, E., & Arnouts, S. 1996, A&AS, 117,393Bustos Fierro, I. H., & Calderón, J. H. 2013, RevMexAA(SC), 43, 47Høg, E., et al. 2000, A&A, 355, 27Monet, D. G., et al. 2003, AJ, 125, 984Stetson, P. B. 1987, PASP, 99, 191Zacharias, N., et al. 2010, AJ, 139, 2184

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Measurement of CFHT Images I: Identification, Photometry and Classification

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