... Surveys

http://ledas-www.star.le.ac.uk/DSSimage/

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...$\sim$0.05 mas/pixel

http://cxc.harvard.edu/cal/Hrma/optaxis/platescale/ geom_public.html

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... observation

The best-fitting position in the 1999 observation has a 0.242 arcsec deviation from that inferred by Hui & Becker (2006) which corrected the offset by simply using the calculator provided by the Chandra Aspect team http://cxc.harvard.edu/cal/ASPECT/ fix_offset/fix_offset.cgi

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