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Photometric Accuracy of GSC 2.2.01

GSC-II Annual Meeting Barolo, Italy, 22-23 October 2001. Photometric Accuracy of GSC 2.2.01. Alessandro Spagna Osservatorio Astronomico di Torino. Summary. GOAL : check GSC 2.2.01 photometric accuracy and precision, by means of: Internal Tests: J, F plate-to-plate, J-F=const

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Photometric Accuracy of GSC 2.2.01

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  1. GSC-II Annual Meeting Barolo, Italy, 22-23 October 2001 Photometric Accuracy of GSC 2.2.01 Alessandro Spagna Osservatorio Astronomico di Torino

  2. Summary • GOAL: check GSC 2.2.01 photometric accuracy and precision, by means of: • Internal Tests: J, F plate-to-plate, J-F=const • Semi-internal tests: J, F GSC2 vs. GSCPC2 • External tests: against photometric catalogs (M67, DMS, EIS, etc.) • Comparisons against starcounts from Galaxy models

  3. Introduction Photographic Density Intensity Density Saturation Stellar PSF Intensity Sky background Sky background

  4. Introduction Photographic Density Intensity Density Saturation Integrated density above threshold Stellar PSF Intensity Sky background Sky background GSC-II photometric parameter = Integrated photographic density above threshold (Dsky + 3sky )

  5. Introduction Examples of saturated stellar PSF’s

  6. Introduction GSPC2 stars Tycho stars

  7. Introduction GSPC2 stars Tycho stars

  8. Introduction

  9. Internal Test: plate-to-plate comparisons Residuals: JXJ443 - JXJ444 vs. J (red crosses)

  10. Internal Test: plate-to-plate comparisons Residuals: FXP443 - FXP444 vs. F (black dots)

  11. Internal Test: plate-to-plate comparisons RMS(m)=0.27 / 2 mag from the analysis of stellar objects from 67 GSC-II (CRA) plates by Pannunzio, Morbidelli et al. (2001, OATo Rept. 57/01). This is an upper limit for GSC 2.2 because it is based on all stars down to the plate limits.

  12. Internal Test: color  const Hypothesis: mean J-F  constant for stars over areas of several degrees towards intermediate and high galactic latitudes, without extinction gradients and peculiar stellar populations (e.g. open clusters)  Zero point check Mean J-F color for 7249 objects with F<16 mag, classified as stars on the ER364 and S364 plates

  13. Internal Test: color  const n x n regions of 1° x 1° 7° 7° RMS[ J-F ] = 0.056 mag which include =0.024 error on J-F

  14. Internal Test: color  const RMS[ J-F ] = 0.066  J-F =0.024 RMS[ J-F ] = 0.14  J-F =0.03 RMS[ J-F ] = 0.146  J-F =0.03

  15. Semi-Internal Test: GSC2.2 vs. GSPC2 • This is not a real external test because GSPC2 (single) stars were used as photometric calibrators of GSC2. This means that many systematic effects as minimized, such as: • Geometric systematic errors (zero points variations vs. x,y) • Differences of Color transformations (standard to GSC2 photometric system) • Therefore we can use GSPC2 to verify the internal consistency of GSC-2.2.01 photometry and analyze in more details other effects, such as: • Random and systematic error vs. magnitude • Photometric accuracy at low and high galactic latitudes • Photometric accuracy in the case of extended objects, as well as of deblended objects.

  16. Semi-Internal Test: GSC2.2 vs. GSPC2 GSCP2 vs. GSC2.2 (>0° b>30° ) - All objects (stars + nonstars)

  17. Semi-Internal Test: GSC2.2 vs. GSPC2 GSCP2 vs. GSC2.2 (>0° b>30° ) - Stellar objects only

  18. Semi-Internal Test: GSC2.2 vs. GSPC2 GSCP2 vs. GSC2.2 (>0° b>30° ) - Non Stellar objects, i.e. extended objects (e.g. galaxies) and unresolved binaries.

  19. Semi-Internal Test: GSC2.2 vs. GSPC2 GSCP2 vs. GSC2.2 (>0° b>30° ) - Multi Stellar objects, i.e. deblended objects classified as stars.

  20. Semi-Internal Test: GSC2.2 vs. GSPC2 GSCP2 vs. GSC2.2: >0°, b>30° (low crowding) GSCP2 vs. GSC2.2: >0°, -15 ° <b<+15° (galactic plane)

  21. Semi-Internal Test: GSC2.2 vs. GSPC2 Global accuracy and precision as a function of the magnitude: GSCP2 vs. GSC2.2 (>0°, b>30°, stars only ) - Mean and RMS of F and J plus 2 error-bars

  22. External Test: M67 (l=216°, b=+32°) M67. CMD F610 vs. J385-F610 for about 1000 GSC2 stars within a 0.5° x 0.5° region, plus isochrone (5 Gyr, Z=0.019, m-M=9.6) from Girardi et al. (2000, A&AS, 141, 371)

  23. External Test: M67 (l=216°, b=+32°) All objects J = -0.07 RMS=0.11 F = -0.01 RMS=0.13 Stars only J = -0.06 RMS=0.10 F = 0.00 RMS=0.10 CCD data (0.5° x 0.5°) from Montgomery et al. (1993, AJ, 106, 181)

  24. External Test: NGC 2506 (l=230°, b=+10°) All objects J = -0.27 RMS=0.22 F = -0.07 RMS=0.21 Stars only J = -0.16 RMS=0.16 F = 0.00 RMS=0.16 All objects CCD data (0.5° x 0.5°) from EIS (Pre-Flames Momany, Y. et al. 2001, A&A, in press)

  25. External Test: NGC 2506 All objects J = -0.27 RMS=0.22 F = -0.07 RMS=0.21 Stars only J = -0.16 RMS=0.16 F = 0.00 RMS=0.16 Stellar objects CCD data (0.5° x 0.5°) from EIS (Pre-Flames Momany, Y. et al. 2001, A&A, in press)

  26. External Test: GSC2.2 vs. DMS All objects J = -0.06 RMS=0.10 F = +0.01 RMS=0.17 Stars only J = -0.05 RMS=0.09 F = 0.02 RMS=0.15 Stellar objects CCD data (0.83 sq-deg in six fields) from DMS (Deep Multicolor Survey; Osmer et al, 1998, ApJS, 119, 189)

  27. External Test: GSC2.2 vs. SDSS FGSC2 vs rSDSS and J vs. gSDSS - from R.White

  28. External Test: GSC2.2 vs. SDSS FGSC2- r SDSS vs r SDSS and JGSC2 -g SDSS vs. g SDSS - from R.White

  29. External Test: GSC2.2 vs. APS galaxies Galaxies. Photometric residuals EAPS - FGSC2vs. F for about 18000 galaxies towards NGP. Photometric calibrations produce galaxies systematically brighter. APS scans of POSS-I plates (Odewahn & Aldering, AJ 1995, 110, 1009)

  30. Distribution of GSC2.2 counts NGP SGP All sky distribution of GSC 2.2.01 objects brighter than F<18.5

  31. Comparison against Galaxy models NGP Solid Histogram=GSC2.2 starcounts; dotted histogram=GSC2.2 counts; Solid line=starcounts predicted by Galaxy Model (Mendez & van Altena, 1997, A&A)

  32. Conclusions • Photometric accuracy (stars): 0.05-0.2 mag, zero point variations as a function of position and magnitude (from internal and external tests). Systematic internal offset for bright blue magnitudes (J=+0.07 mag for J<15 ) • Photometric precision (stars): 0.10-0.20 mag, based on RMS vs. external catalogs (EIS, DMS, …) • Best precision for stars of JF15 mag; 30-50% degradation down to the magnitude limit of GSC 2.2 (from GSPC-2 comparisons) • Extended objects. Large systematic error (>1 mag) as a function of the magnitude (decreasing with mag) for galaxies and diffuse objects. • Blends. Internal precision of de-blended multiple stars is almost a factor 2 worse than that of single stars. (Note that towards the galactic plane >50% of the stars are resolved or unresolved blends.) • The exported photometric error is a “worst case” value. It is 3-5 times overestimated with respect to the precision of typical stars.

  33. Conclusions GSC 2.2 provides 2 color photometry for about 1/2 billion objects,and an accuracy better than <0.2 mag is attained for point-like objects, significantly better than the photometry of USNO catalog, which at the moment is the only comparable catalog in terms of number of objects. GSC 2.2 photometry appears suitable as source of reference stars needed for telescope operations. In these cases, a check of the object flags which specify the quality of the photometric solution (interpolated/extrapolated solutions, de-deblended status, etc.) is recommended. GSC 2.2 seems also suitable for scientific studies, taking into account the limits deriving from the presence of systematic errors.

  34. Future improvements • Test infrared IV-N magnitudes (for GSC 2.3) • Recalibrate photometry of extended objects (D-to-I model) • Correct geometric systematics (plate “flat fielding”) • Improve calibrating model and reduce magnitude systematics of both interpolated objects and extrapolated stars, fainter than GSPC2 calibrators down to the plate limits • Improve estimation of photometric errors

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