Photometric Accuracy of GSC 2.2.01

GSC-II Annual Meeting

Barolo, Italy, 22-23 October 2001

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

• Semi-internal tests: J, F GSC2 vs. GSCPC2

• External tests: against photometric catalogs (M67, DMS, EIS, etc.)

• Comparisons against starcounts from Galaxy models

Introduction

Intensity

Sky background

Intensity

DensitySaturation

Photographic

Density

Sky background

Stellar PSF

Introduction

Intensity

Sky background

Intensity

DensitySaturation

Photographic

Density

Integrated density above threshold

Sky background

GSC-II photometric parameter =

Integrated photographic density

above threshold (Dsky + 3 sky )

Stellar PSF

Introduction

Examples of saturated stellar PSF’s

Introduction

GSPC2 stars

Tycho stars

Introduction

Tycho stars

GSPC2 stars

Introduction

Internal Test: plate-to-plate comparisons

Residuals: JXJ443 - JXJ444 vs. J (red crosses)

Internal Test: plate-to-plate comparisons

Residuals: FXP443 - FXP444 vs. F (black dots)

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.

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

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

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

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.

Semi-Internal Test: GSC2.2 vs. GSPC2

GSCP2 vs. GSC2.2 (>0° b>30° ) - All objects (stars + nonstars)

Semi-Internal Test: GSC2.2 vs. GSPC2

GSCP2 vs. GSC2.2 (>0° b>30° ) - Stellar objects only

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.

Semi-Internal Test: GSC2.2 vs. GSPC2

GSCP2 vs. GSC2.2 (>0° b>30° ) - Multi Stellar objects, i.e. deblended objects classified as stars.

Semi-Internal Test: GSC2.2 vs. GSPC2

n.ro (red)objects

F(mag)

RMS[F](mag)

n.ro (blue)objects

J(mag)

RMS[J](mag)

Stars 8298 +0.002 0.089 7080 -0.035 0.099

Multiple-stars 451 -0.013 0.123 423 -0.023 0.166

Non-stars 1877 -0.262 0.275 1360 -0.403 0.394

All objects 9788 -0.018 0.114 8050 -0.020 0.123

n.ro (red)objects

F(mag)

RMS[F](mag)

n.ro (blue)objects

J(mag)

RMS[J](mag)

Stars 12874 +0.042 0.109 11869 +0.035 0.125

Multiple-stars 3495 +0.046 0.167 3323 +0.063 0.191

Non-stars 19926 -0.071 0.199 14119 -0.083 0.218

All objects 32365 -0.020 0.168 25692 -0.019 0.181

GSCP2 vs. GSC2.2: >0°, b>30° (low crowding)

GSCP2 vs. GSC2.2: >0°, -15 ° <b<+15° (galactic plane)

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

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)

External Test: M67 (l=216°, b=+32°)

CCD data (0.5° x 0.5°) from Montgomery et al. (1993, AJ, 106, 181)

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

External Test: NGC 2506 (l=230°, b=+10°)

CCD data (0.5° x 0.5°) from EIS (Pre-Flames

Momany, Y. et al. 2001, A&A, in press)

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

External Test: NGC 2506

CCD data (0.5° x 0.5°) from EIS (Pre-Flames

Momany, Y. et al. 2001, A&A, in press)

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

External Test: GSC2.2 vs. DMS

CCD data (0.83 sq-deg in six fields) from DMS (Deep Multicolor Survey; Osmer et al, 1998, ApJS, 119, 189)

Stellar objects

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

External Test: GSC2.2 vs. SDSS

FGSC2 vs rSDSS and J vs. gSDSS - from R.White

External Test: GSC2.2 vs. SDSS

FGSC2- r SDSS vs r SDSS and JGSC2 -g SDSS vs. g SDSS - from R.White

External Test: GSC2.2 vs. APS galaxies

Galaxies. Photometric residuals EAPS - FGSC2 vs. 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)

Distribution of GSC2.2 counts

NGP SGP

All sky distribution of GSC 2.2.01 objects brighter than F<18.5

NGP

Comparison against Galaxy models

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

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 JF15 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.

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.

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