[[PageOutline]]

= Outline =

This page outlines an investigation into the cross-matching used by the Durham group to generate the 'true positives' and 'false positives' data subsets. Durham used TOPCAT to perform the cross-match, and presented the results as FITS files. Our aim is to reproduce the same results as Durham then experiment with different cross-matching parameters. 

= Coverage =

As can be seen from the plot below, Sloan stripe 82 does not fully cover the SAS footprint. This needs to be taken into account when performing crossmatching and determining false positives.

[[Image(coverage.gif, 600px)]]

= Durham-supplied data =

Durham supplied the following files (FITS binary table format) for analysis.

|| '''File''' || '''Rows''' || '''Comments''' ||
|| footprint-stripe82.fits|| 449,254 || Sloan Stripe 82 master file||
|| footprint-g-all-psphot.fits.bz2 || 282,604|| all IPP g filter||
|| footprint-r-all-psphot.fits.bz2|| 278,280 || all IPP r filter||
|| footprint-i-all-psphot.fits.bz2|| 320,655 || all IPP i filter||
|| footprint-z-all-psphot.fits.bz2|| 258,130|| all IPP z filter||
|| footprint-y-all-psphot.fits.bz2|| 175,578 || all IPP y filter||
|| footprint-g-not-matched-psphot.fits.bz2 || 171,973 || all IPP false detections in g ||
|| footprint-r-not-matched-psphot.fits.bz2|| 67,455 || all IPP false detections in r ||
|| footprint-i-not-matched-psphot.fits.bz2|| 59,973 || all IPP false detections in i ||
|| footprint-y-not-matched-psphot.fits.bz2|| 46,729 || all IPP false detections in z ||
|| footprint-z-not-matched-psphot.fits.bz2|| 51,391|| all IPP false detections in y ||


= Durham cross-matching =

Durham used TOPCAT to perform the cross-matching. They used the following settings:

 * 'Algorithm' : Sky
 * 'Max Error' : 1.0 arcsec
 * 'Match Selection' : Best Match Only

It is thought (by Nigel Metcalfe) that Peter Draper cut the Sloan data on BINNED1 = true. I applied this cut, and it only removed 428 sourced out of a total of 449,254.
 
= Tiger-team cross-matching =

We attempt to first reproduce the Durham results, then experiment with different cross-matching constraints.
== Attempts to reproduce Durham results ==

Using TOPCAT with the same data and same settings as Durham, while also first removing all IPP detections above outwith Strip82, we are able produce the the same results (we cut the Sloan data using BINNED1 = true, as above).

Results of all cross-matching are shown on the spreadsheet [https://docs.google.com/spreadsheet/pub?hl=en_GB&hl=en_GB&key=0AgWTz2sVkmIIdDVqMWdwR1cwMTVtc3lKMm1XbVJKTWc&single=true&gid=0&output=html here]

== Cross-matching with different error radii ==

[[Image(chart1.png)]]