the construction of the gsc2.2 catalog

Seminars on formation and evolution of the Galaxy Feb 12, 2002

The construction of the GSC2.2 Catalog

Mario G. Lattanzi

Osservatorio Astronomico di Torino


Basic Facts


After seven years of hard work, dedication, andunabated faith of more than 25 scientists in theUSA and Europe, the Space Telescope ScienceInstitute and the Osservatorio Astronomico di

Torino have released an all-sky magnitude-limitedsubset of the GSC-II Data Base.


GSC2.2.01 exported from the COMPASS Data Base

Page = Federated Database

= Database

= Container

= Class/Basic Object

Region Database

SourceIndex

RegionCatalogContainers

(for each plate definedarea within region)

ReferenceCatalogSource (for each reference catalog)

Plate Database

(for each plate)

[ COMPASS Federated Database ]

(for each of 32768 regions)

PlateSource

RegionInfoContainer PlateInfoContainer

Approximately 1 Billion distinct objects found to date


The GSC-II Data Base (called COMPASS) lists998,402,801 unique entries, i.e., all of the objects found

down to the faintest objects measurable on thephotographic glass plates. Each object (entry) listedfeatures at least 3 magnitudes and several positionstaken at different epochs for a total amount of datanearing 4 Tera-bytes (i.e., 4000 billions of bytes). Atthese levels, the GSC-II is the largest map of the skyever built, a 500-fold increase in information over its

predecessor GSC-I…Quite a beginning for catalogastronomy in the new millenium!


The version of the GSC II released, named GSC2.2, containspositions, classifications, and magnitudes in two bandpasses for435,457,355 objects. The GC2.2 is used to support operations ofground-based (GEMINI and VLT) and space-borne (HST, XMM)telescopes, and is accessible to the scientific community and thegeneral public via the World Wide Web.

It is based on 1” resolution scans of the second generationphotographic sky survey plates from the Palomar and UK Schmidttelescopes in F and J bandpasses (the Digitized Sky Survey 2).The magnitude limits are 18.5 in the F-red band and 19.5 in theJ -blue band, the astrometric and photometric errors are ~ 0.3”and 0.2 mag, respectively.


Single-plate calibrations

•Astrometry: polynomial fits + residual maps (mask) to

TYCHO2 stars (onto the ICRS reference system) [>Smart]

•Photometry: fits to a (B)VR CCD sequences to 18-19 mag

(GSPC2) transformed to natural bandpasses (J and F)

[GSPC2>Bucciarelli]

•Classification: 30 ranked features + oblique decision trees

+ voting [>Smart]


Single-plate photometric calibrations


Intensity

Sky background

Intensity

DensitySaturation

Photographic

Density

Sky background

Stellar PSF


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


Examples of saturated stellar PSF’s


Tycho stars

GSPC2 stars


GSC2.2 Photographic photometry: bandpasses


GSC2.2 Photographic photometry: transformation to plate natural bands


Photographic photometry

Photometric system of GSC-II

Band Survey Emulsion+Filter Lambda_eff FWHM (Angstrom)J385 POSS-II J DEC>0 IIIaJ+GG385 4660 1095F610 POSS-II F > 0 IIIaF+RG610 6540 580I9 POSS-II N > 0 IV-N+RG9 8060 1100J395 SERC J DEC<0 IIIaJ+GG395 4635 1080F590 SES F < 0 IIIaF+OG590 6450 720I715 UKST IR <0 IV-N+RG715 8020 1170


The Catalog


Catalog extraction rules

•Only plates included in the GSC2.2 plate list considered (POSS II: 897 XJxxx + 897 XFxxx fields; 606 SERC-J, Sxxx, 216 SERC-EJ, Erxxx, and the red material, XSxxx and GRxxx, short exp., plates)

•Objects brighter than: J<19.5 OR F<18.5

•Objects classed as defect on only one plate discarded

•Astrometry: export the position of that entry of a given object closest to the entry’s plate center (tolerance set to 0.25 deg). Brake ties by selecting entry with the most recent epoch

•Photometry: for each bandpass, take the value of the entry closest to the plate center


A view of the GSC2.2.01


GSC 2.2.01 Global counts

Hemisphere No. of stars No. of objectsNorth (0 ) 75,300,243 175,255,067South (<0 ) 104,085,736 280,596,171Total 179,385,978 455,851,238

Galactic coordinates No. of objects No. of starsLow Latitudes (b<20 ) 356,425,331 118,556,811High latitudes (b20 ) 99,425,907 60,829,167Total 455,851,238 179,385,978


The GSC2 Export Catalog: specifications

•Positions: <0.”5 absolute (average across one plate); better than 0.”2 relative (over a 0.5 deg field); 0.”15 rms deviation from ICRS (average over all plates)

•Magnitudes: precision to better than 0.2 mag for stars between 12 and 18

mag; accuracy <0.15 mag

•Classification: 95% accuracy within 2 mag from plate limit

•Completeness: V=18 minimum


Global Error Analysis


Astrometric errors

External Comparisons to:

•UCAC1

Sky coverage: Southern hemisphere (-90 deg/-15 deg)

Mag limit = 16.0, precision: better than 0.1 arcsec per coordinate)

•2MASS

Approaching all-sky coverage. Mag limit close to that of GSC2.2; position to 0.2 arcsec (per coordinate)

Internal comparisons: on average relative error well within the 0.2 arcsec requirement.


Method:

• Statistical tests on the residuals (also called catalog-to-catalog differences)>>>

d=(GSC2.2.01 - external catalog).

Ex.: a realization of the statistic d could be declination differences >> {di = DECi,gsc2 - DECi,ext.cat. }

• The hypothesis put to test is that the residuals follow (within the chosen confidence level) the expected distribution: if the values from the two catalogs are Gaussian distributed >> d is also Gaussian distributed.


TWO TESTS: Chi-square, Kolmogorov-Smirnov

Sample mean and variance are computed before the application of the tests. Outliers are eliminated from the sample to avoid biased

values.

Chi-square: need to group data to form histogram >> depends on number of bins.

Chi-sq statistic

,0

202

i

i

ii

np

npn


where ni is the number of differences in bin i-th, and

2

2

2

2

0 ,}2

exp{2

1x

d

xd

i

i

i

dss

p

the integral is evaluated using the error function.


K-S Test: do not need to group data (> reliable even with small samples)

K-S statistic

,)()(max xFxSD nn

where

x xerf

dss

XF2

)(5.0}

2exp{

2

1)(

2

2

Xx

xxxnr

xx

xS

n

rrn

)(

)1()(

)1(

1

/

0

)( and


Confidence levels set to 95% for both tests: from the distributions of the Chi-square and K-S statistics we calculate the quantiles (the values) for which the probabilities of the two statistics to be above those values are 0.05.

Finally, if the values of 2 and Dn calculated from the samples are smaller than the quantiles the tests are satisfied with the 95% confidence level.


Astrometric Error (external): UCAC1


Astrometric Error (external): UCAC1, Chi2 and K-S tests(Large sample, no mag and no plate-based selection)

Both tests fail!….But,…..


Astrometric Error (external): UCAC1 , Chi2 and K-S tests(Selected sample, mag range 16-16.5, no plate-based selection)

K-S tests passed, and Chi2 also well behaved!!


Astrometric Error (external): 2MASS


Astrometric Error (external): 2MASS , Chi2 and K-S tests(Large sample, no mag and no plate-based selection)


Astrometric Error (external): Conclusions

Overall, the residuals appear well behaved, although simple statistical tests fail.

As expected, this indicates the presence of magnitude and plate-based effects. Indeed, the same tests are consistently passed when limited to small magnitude bins or residuals from the same plate. This, in turn, is further proof that the single plate calibrations do obey gaussian statistics.


Photometric Errors

•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


Photometric Errors: internal test

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


Internal photometric errors: plate-to-plate comparison statistics

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.


Photometric Errors - 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)(Spagna)


Photometric Errors - Semi-Internal Test: GSC2.2 vs. GSPC2

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


Photometry. 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: 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

(Spagna)


Classification Errors


Catalog Completeness (1/4)

All stars, J mag

Jcompl = 17.3

Toward the galactic center (l,b~0.5 deg)


Catalog Completeness

galactic center (l,b~0.5 deg)

All stars, F mag

Fcompl = 15.8




All obj.s, J mag

Jcompl = 18.4




All obj.s, F mag

Fcompl = 18.0


Comparison to SDSS


Contributions for this presentation from:

B. Bucciarelli

R. Drimmel

R. Smart

A. Spagna


THE END

the construction of the gsc2.2 catalog

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