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.

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 method

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.

Attempts to replicate the Durham method

Using TOPCAT with the same data and same settings as Durham, we get the following:

	Durham	Us with Durham settings
Total IPP detections	1,315,247
True positives	917,726	268,044
False positives	397,521	1,047,203