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Data Overview

The SuperCOSMOS data held in the SSA primarily originate from scans of the UK Schmidt and Palomar POSS II blue, red and near-IR sky surveys. The ESO Schmidt R (dec < -17.5) and Palomar POSS-I E (dec > -17.5) surveys have also been scanned and provide an early (1st) epoch red measurement. Further details on the surveys, the scanning process and the raw parameters extracted can be found under the SuperCOSMOS Sky Survey (SSS) pages. The SSA and SSS are based on the same underlying data with the main differences arising in the construction of the SSA merged source table.

The SSA is housed in a relational database running on Microsoft SQL Server 2005. Data are stored in tables which are inter-linked via reference ID numbers. In addition to the astronomical object catalogues these tables also contain information on the plates that were scanned, survey field centres and calibration coefficients. Most user science queries will only need to access the Source table or to a lesser extent the Detection table (see tables).

The following sections summarize the structure and content of the SSA database. Further details can be found using the schema browser.

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SSA - Tables

The database housing the SSA contains two main tables that hold the object catalogues extracted by SuperCOSMOS, namely Detection and Source.

The Detection table contains individual plate detections for every scanned plate in the SSA. Each scanned plate in the SSA results in a pixel map. This map was processed using standard software (known as Image Analysis Mode, or IAM, software) to produce a list of parameterised detections on each plate. This table contains all the attributes for each detected object. The attribute set consists of the standard 32 IAM parameters plus calibrated photometry, spatial index quantities and ID numbers.

Source contains merged sources for every field in the SSA. Each field within the SSA is covered by four plates in passbands B, R and I with R being covered twice at different times. This results in four-plate multi-colour, multi-epoch data which are merged into a single source catalogue for general science exploitation. This table contains the associated merged records created from the objects in Detection, along with a full astrometric solution (including proper motions) computed from the available position measures. The most useful subset of image morphological descriptors are also propagated into this table for ease of use. Many of the parameter names in Source are appended with the letters B, R1, R2 or I this indicates which survey the parameter refers to.

B = UK Schmidt (dec < +2.5) or Palomar-II (dec > +2.5) blue (Bj) survey
R1 = ESO Schmidt (dec < -17.5) or Palomar POSS-I E (dec > -17.5) red survey
R2 = UK Schmidt (dec < +2.5) or Palomar-II (dec > +2.5) red survey
I = UK Schmidt (dec < +2.5) or Palomar-II (dec > +2.5) near-infrared survey

Source and Detection are linked via the parameter sourceID common to both and the parameters objIDB, objIDR1, objIDR2 and objIDI in Source which reference parameter objID in Detection.

SSA - Source merging

In the original SSS, source pairing is done "on-the-fly" using a series of pair index pointers between any two given plate datasets. This yields complete record association out to the maximum pair radius (6 arcsec) but sometimes results in the same slave detection being paired with more than one master image and other anomalous effects, especially in crowded regions. In the SSA, a pre-merged source table (Source) has been created from the individual plate detections (Detection) using those original pair indices, but multiple pairings have been removed to create a self-consistent dataset. The procedure for purging anomalous pairs uses the full-blown astrometric solution information in cases where an image is paired to more than one other image: the lowest chi2 value is used to indicate the pair set that is most likely to be correct. Where no chi2 attribute is available (i.e. when only two detections are involved: one master detection, and a choice of 2 or more possible corresponding slave detections) the choice for the most likely correct association is based on absolute proximity (the nearest slave is chosen). Table Source is also seamless in the sense that multiple detections of the same objects in plate overlap regions are purged (on the basis of the measurement that is nearest the respective plate centre is the one retained). Additionally, any detections that touch measurement boundaries (i.e. have attribute quality>65535), any detections that are parent images (i.e. have attribute blend<0), and any images that are in regions of a plate likely to be affected by step-wedges and/or plate labels (i.e. have bit 7 of attribute quality set) are also purged. Hence, table Source is the best-efforts seamless, merged survey catalogue that should be the first point of querying for most general science applications.

The SSS uses the ESO/SRC field system (see table FieldSystem) as a means of curating the sky survey data. In the Declination zone -17.5 < Dec < +90.0, POSS-I E plates have been used as first epoch R measures since there are no ESO-R plates in this region. The SSS procedure for incorporating those plate measures, which are on a different set of field centres to the ESO/SRC, is to "remosaic" the POSS-I E data into a set of ESO/SRC fields and then merge them in to the survey dataset in the same way as ESO-R plates south of the Dec=-17.5 boundary. This is all done transparently to the user, but results in some subtle features in the region in question (for example, do not be surprised to see a handful of different plate numbers for first epoch R detections if you select a sample of these first epoch R detections - that is to say remosaiced POSS-I E detections - from, say, ESO/SRC field 823 at RA=0h, Dec=0.0). Note that the southern ESO/SRC field system was reflected into the northern hemisphere for the second generation POSS2 survey, and the SSA uses this.

SSA - Astrometry

All SSA global astrometry is tied to the Hipparcos-Tycho reference frame via the Tycho-2 and ACT catalogues. The UKJ and UKR surveys are all reduced with respect to Tycho-2; for the UKI, ESO-R and POSS-I E surveys, the plate reductions are split roughly 50/50 between Tycho-2 and the ACT (to see which reference catalogue was used for a given survey plate you can query table Plate for attribute starCat). The plate reduction methodology is described in The SuperCOSMOS Sky Survey - III. Astrometry. Briefly, a stiff (linear) plate model is used between measured xy and reference catalogue RA,Dec in the tangent plane along with Schmidt r/tan(r) radial and "swirl mask" non-linear distortion corrections. This procedure has been tested against the ICRF defining sources, and astrometry is globally good to between 0.2 and 0.3 arcsec in either co-ordinate. Some low latitude or otherwise crowded fields have reference star residuals significantly larger than this indicating poorer global astrometry. As a general indication of the accuracy of a given plate reduction, select attributes astResidX, astResidY from table Plate.

SSA - Proper motion measures

Proper motions in the SSA are enhanced with respect to those originally available in the SSS (as reported in Paper III). New proper motions have been computed using all available position measures for a given merged source. The position error mapping algorithm reported in Paper III has been applied between the B, I and first epoch R datasets on a field by field with respect to the UK Schmidt or POSS2 R plate (depending on the hemisphere), providing a set of relative position measures at up to four different epochs for each merged source. Relative astrometric errors as a function of magnitude were computed from the set of mean positions for all stars in each field, and then a full astrometric solution performed for each merged source given the available position measures (an inverse-variance weighted least-squares fit). This full solution results in a zero-point position at a given epoch, estimates of proper motion in RA and Dec, formal errors on those quantities and a goodness-of-fit parameter ("reduced" chi-squared, ie. normalised per degree of freedom). These quantities are stored as an extended set of attributes in the merged source table Source as follows: ra, dec, epoch, sigRA, sigDec, muAcosD, muD, sigMuAcosD, sigMuD, chi2; the attribute Nplates indicates the number of position measures available for a given source astrometric solution (up to 4). Default values (-0.9999995e9) are present for attributes where a solution is not possible (Nplates=1) or where goodness-of-fit cannot be determined (Nplates=2). Note that because of the large variation in epoch separation for any given source (which can range from nearly 50 years, to just a few hours for objects paired between two plates exposed consecutively on the same observing night) there is a huge range in the proper motion errors; typically, an image that has four position measures over several decades will have proper motion errors of 5 to 10 mas/yr, whereas in some fields where the survey plates were obtained in quick succession proper motion errors may be many orders of magnitude larger. Before using the proper motions (e.g. proper motion correcting to a given epoch) the significance of the proper motion should be assessed with respect to the error and positions should be left uncorrected if the quoted motion is consistent with being zero (e.g. do not use proper motions if they are smaller than, say, 3-sigma). Note also that the default value for proper motions is the same as all other floating point attributes, i.e. very large and negative - needless to say, you should not proper motion correct positions by this number!

SSA - Magnitudes

Several photometric measures are available for each detection and merged source in the SSA; the most appropriate one to use depends on the individual science application. In general, you should use the attributes gCorMag and sCorMag for extended and point (galaxy and star) images respectively in Source. So, if you are interested in point sources, you should always use sCorMag, whereas if you are interested in extended source photometry, use gCorMag. If your science application requires both resolved and point source information, then you should use classMag. This is because the most accurate photometric measurement from the plates is dependent on image morphology and two separate calibrations are applied to resolved and point sources, but you must be aware that the image classifier is not perfect and sometimes classMag may not be the best photometric estimator for a given object. Note that when using R band photometry, you should always use R2 in preference to R1 since the former are higher signal-to-noise measurements and have a better calibration.

The SuperCOSMOS Sky Survey - II. Image detection, parameterisation, classification and photometry describes the calibration procedures, and the corrections applied to obtain the most accurate possible stellar magnitude scale. Note that colour-corrected stellar magnitudes are quoted in Source.sCorMag while uncorrected photometry is quoted in Detection.sMag. For extended sources, a new calibration has been applied for B, R2 and I that supersedes the old SSS one (but note that R1 has not been recalibrated - for extended sources, R1 is inconsistent with R2 as it has the old calibration and it should not be used). The new extended source calibration has been used and is documented for the final release of the 2dF Galaxy Redshift Survey, where some more details can be found. The old calibration is as supplied with the original online SSS and is documented in Paper II.

N.B. the galaxy photometry in B, R2 and I (i.e. that in gCorMag) is an AB magnitude scale, while the stellar magnitudes (sCorMag) are Vega-based.

SSA - Enhanced quality information

Finally, each detection in the SSS comes with a bitwise "quality flag" used to note certain conditions that arose during pixel analysis and/or post-processing which may affect the reliability of the attributes for that image. In Storkey et al. (2004) we describe a new method of analysing the parameterised images within the SSS to detect image artifacts resulting from plate defects such as satellite trails, aeroplane trails, scratches, bright stellar halos and diffraction spikes. The SSA includes the results of runs of this procedure over all plates except the POSS1 "E" (i.e. R1) plates, and the results are flagged using bits 11, 13 and 14 of the quality flag. Attributes associated with the running of the procedure (including a posterior probability of the liklihood that the spurious nature of the image has been correctly identified) are also stored in table JunkDetection. The newly updated bitwise quality flag information (after Table 2 of Paper II) is as follows:

Event Bit set Increment Severity Comments
Orientation calculation failed 0 1 Information Image perfectly round
Ellipticity calculation failed 1 2 Information Image perfectly straight
Image too multiple for deblending 24Warning Image split into too many fragments
Bright image 416Information Image has pixels brighter than highest areal profile level
Large image 664Warning Image has area greater than maximum specified for deblending
Possible bad image 7128Warning Image is in step-wedge/label region
Image near very bright star 101024Warning Image may be spurious due to bright star artifact
Halo artifact 112048Strong warning Image is likely spurious result of bright star halo
Diffraction spike artifact 138192Strong warning Image is likely spurious result of a bright star diffraction spike
Track artifact 1416384Strong warning Image is likely spurious result of satellite/'plane/scratch track
Image touches boundary 1665536Severe defect Image pixels partially missing

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