1. E. BertinAstro-WISE workshop 11/2005 1 Astrometry @ TERAPIX: SCAMP E.Bertin (IAP & Obs. de Paris/LERMA)

2. E. BertinAstro-WISE workshop 11/2005 2 Automatic astrometric and photometric calibration with SCAMP SCAMP and the TERAPIX software suite The CFHTLS Object centroiding Field centering The astrometric solution In practice Performance summary Forthcoming developments

3. E. BertinAstro-WISE workshop 11/2005 3 A complete image compositing system Source extraction Pixel weighting Astrometry Photometry Astrometry Photometry Warping and stacking Final images Exposures Defectix, Eye,Weightwatcher SExtractor SCAMP SWarp

4. E. BertinAstro-WISE workshop 11/2005 4 The CFHTLS Motivated the development of SCAMP >20TB of science image data Each individual MEGACAM exposure is a 1º mosaic of 36 x 9Mpixel CCD frames and contains about 10 4 -10 6 detections. 1500 sq.deg. covered in up to 5 bands In a given region of the sky anything from 1 to 2000 exposures can overlap Astrometric and photometric calibrations must accomodate various sky coverage strategies Scientific goals require an accurate relative positioning of images and photometry homogeneous at the % level

5. E. BertinAstro-WISE workshop 11/2005 5 Centroid measurements Calibration performed with SExtractor catalogs New centroiding procedure in SExtractor: –X_IMAGE, Y_IMAGE « isophotal » measurements are fast but their accuracy is poor –XWIN_IMAGE, YWIN_IMAGE Gaussian-weighted centroiding:. FWHM of the Gaussian set to twice the half-light radius Improves astrometric accuracy by ~3x on detections with high S/N. 5 iterations in average As accurate as PSF-fitting on background-noise limited images Works well with galaxies X_IMAGEXWIN_IMAGEXPSF_IMAGE

6. E. BertinAstro-WISE workshop 11/2005 6 Astrometry: pattern matching Requires a reference astrometric catalog (currently GSC, USNO, or UCAC, 2MASS or SDSS) and a first guess of field center coordinates, pixel scale and image orientation Two steps: –Find position angle and pixel scale using pairs of detections –Find coordinate shift using detections

7. E. BertinAstro-WISE workshop 11/2005 7 Astrometry: finding position angle and scale Match source pairs in the reference and extracted catalog in pos.angle-log(distance) space using bandpass-filtered cross-correlation (e.g. Kaiser et al. 1999) Must pay attention to folding on the scale axis In general, a 7’  15’ image can be correctly oriented and scaled within a 1sq.deg box.

8. E. BertinAstro-WISE workshop 11/2005 8 Astrometry: coordinate shift Match source pairs in the reference and extracted catalog in projected coordinate space using cross- correlation Possible image flip Modulation due to source clustering and catalog boundaries must be minimized with data-windowing and bandpass-filtering Typically, a 7’  15’ image can be correctly positioned within a box of a few sq.deg. Slight improvement by adding magnitude dependency.

9. E. BertinAstro-WISE workshop 11/2005 9 A global solution Astrometric distortions require a 3 rd order polynomial in projected coordinates ξ –20 free parameters per CCD –Naive approach: fit the distortion coefficients for each exposure using a reference catalog (GSC, USNO,…) Simple and fast but too sensitive to inaccuracies in the reference catalog.especially when a little more than 20 stars are cross-identified on a CCD. –Global solution: fit the distorsion coefficients by additionally minimizing the distances between overlapping detections. For every source s on overlapping exposures a and b minimize Manages to reach the theoretical instrumental accuracy Implemented in many current astrometric reduction packages (e.g. Deul 1995, Kaiser et al. 1999, Radovich 2002),

10. E. BertinAstro-WISE workshop 11/2005 10 A global solution MEGACAM: 36x20 = 720 free parameters per exposure –Quickly leads to impractically large normal equation matrices –Iterative approach necessary –Too many free parameters: robustness problems arise because of a lack of sources or confusion in some fields For a given instrument (and a given filter combination), one may assume that the distorsion pattern does not vary measurably over some period of time –We must allow the linear part of the distorsion pattern to vary globally from exposure to exposure because of differential atmospheric refraction and flexures –720. n instru + 6.( n exp - n instru ) free parameters –Requires an intermediary transformation to a common re-projection stereographic projection chosen because it maps disks to disks. Jacobians of the re-projections are involved

11. E. BertinAstro-WISE workshop 11/2005 11 Astrometry: observing strategies 

12. E. BertinAstro-WISE workshop 11/2005 12 Astrometry: improving accuracy For large time intervals between exposures, one may leave source proper motions and parallaxes as free parameters (e.g. Eichhorn & Russell 1976) Differential Chromatic Refraction –Atmospheric –Chromatic aberrations Intrinsic sources of astrometric errors –Variability of the intra-pixel response profile from pixel to pixel Mostly affect IR arrays On modern CCDs, repeatability of centroiding with properly sampled stars is ~ 1/300 th of a pixel over the array (e.g. Yano et al. 2004) –Step-and-repeat pixel size error on some large CCDs (Shaklan & Pravdo 1994): typically 0.5  m (a few hundredth of a pixel) each 512 or 1024 pixel Equivalent to a few mas MEGACAM CCDs do not have this problem

13. E. BertinAstro-WISE workshop 11/2005 13 Differential Chromatic Refraction For a star with spectral index , observed at zenithal distance z in a filter of bandwidth w (in microns) centered on wavelength 0 (in arcsec): For CFHTLS, w  0.1  m –At z=45 deg, ∆z varies from ~20mas (z band) to ~150mas (u band). Most all-sky catalogs are not corrected for DCR! Correction for differential chromatic refraction available in SCAMP –2 different photometric instruments are required at least –Available in the merged catalog ∆  and ∆  as a function of u-g at airmass ~1.5

14. E. BertinAstro-WISE workshop 11/2005 14 Proper motions Proper motion estimates –At least 2 different epochs are required –Available in the merged catalog Comparison with USNO-B1 proper motions: 20 D3 exposures in r over a period of 15 months

15. E. BertinAstro-WISE workshop 11/2005 15 In practice Command line: scamp *.cat When read by SCAMP, SExtractor binary catalogs are automatically organized –By astrometric context (FITS keyword list) –By photometric context (FITS keyword list) –By group of fields on the sky (initial WCS information) –Specific handling of mosaics An astrometric reference catalog is automatically downloaded from the CDS around each group of fields –CDSclient (F.Ochsenbein) –2MASS, GSC, UCAC, USNO catalogs –Can be stored and retrieved locally Output: –The solutions can be computed for most WCS projections (WCSlib by M.Calabretta) –Astrometric and photometric solutions written as “pseudo-FITS” headers –A merged catalog (with proper motions) –PlPlot “Check-plots” (PNG,JPEG,PS,…) –XML summary file with statistics –XSL style-sheet for humans

16. E. BertinAstro-WISE workshop 11/2005 16 “Check-plots”

17. E. BertinAstro-WISE workshop 11/2005 17 Differential Chromatic Refraction For a star with spectral index , observed at zenithal distance z in a filter of bandwidth w (in microns) centered on wavelength 0 (in arcsec): For CFHTLS, w  0.1  m –At z=45 deg, ∆z varies from ~20mas (z band) to ~150mas (u band). Most all-sky catalogs are not corrected for DCR! Correction for differential chromatic refraction available in SCAMP –2 different photometric instruments are required at least –Available in the merged catalog ∆  and ∆  as a function of u-g at airmass ~1.5

18. E. BertinAstro-WISE workshop 11/2005 18 Proper motions Proper motion estimates –At least 2 different epochs are required –Available in the merged catalog Comparison with USNO-B1 proper motions: 20 D3 exposures in r over a period of 15 months

19. E. BertinAstro-WISE workshop 11/2005 19 Current Performance Speed –Based on FFTW, BLAS/LAPack –Multi-threaded –Efficient cross-identification engine (millions of detections/s) –Full processing typically takes 4s / MEGACAM field (36 CCDs) on a quadri- Opteron@2.4 GHz –Memory footprint: 200 bytes / detection Accuracy –Star field image simulations: chi2/d.o.f. close to 1 –MEGACAM: internal pairwise residuals down to ~6 mas rms for uncrowded, high S/N sources in selected CFHTLS exposures. 20mas rms is the more typical value –There seems to be a large-scale (~10’) distorsion component variable from exposure to exposure at the level of a few mas RMS “Anomalous refraction”?

20. E. BertinAstro-WISE workshop 11/2005 20 Other instruments: single array Planet Tautenbug (Schmidt)

21. E. BertinAstro-WISE workshop 11/2005 21 Other instruments: mosaics WIRCAM+2MASSCFH12k

22. E. BertinAstro-WISE workshop 11/2005 22 The (near) future Public release December’05 –Does photometric calibration, too JDocumentation is on the way Upgrade to the latest version of WCSlib –Compliancy with the most recent WCS prescriptions for describing distorsion patterns Astrometric calibration web- service –Distributed TERAPIX web-service engine (J.-C. Malapert)

23. E. BertinAstro-WISE workshop 11/2005 23 terapix.iap.fr