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Great Circle Drift Scanning


The survey coordinate system tex2html_wrap_inline2905 is a spherical coordinate system with poles at tex2html_wrap_inline2907, tex2html_wrap_inline2909 and tex2html_wrap_inline2911, tex2html_wrap_inline2909. The survey equator is thus a great circle perpendicular to the J2000 celestial equator, intersecting it at tex2html_wrap_inline2915 and tex2html_wrap_inline2917. Lines of constant tex2html_wrap_inline2919 are great circles perpendicular to the survey equator and lines of constant tex2html_wrap_inline2921 are small circles parallel to the survey equator. tex2html_wrap_inline2923, tex2html_wrap_inline2925 is located at tex2html_wrap_inline2915, tex2html_wrap_inline2929, with tex2html_wrap_inline2919 increasing northward.

The survey area is divided into stripes, where each stripe is centered along a line of constant tex2html_wrap_inline2919, separated from the adjoining stripe(s) by tex2html_wrap_inline2935. Each drift scan tracks a survey stripe, offset by tex2html_wrap_inline2937 arcsec perpendicular to the stripe. Two scans (or ``strips''), one offset to the north and one to the south, are required to fill a stripe. The survey latitude tracked by stripe n is given by
in the Northern Galactic Hemisphere, and
for the three stripes in the Southern Galactic hemisphere. These stripes are superimposed on a Galactic extinction map in Figure 2 of york00.

The natural coordinate system to use for processing a given drift scan is the great circle coordinate system for that stripe, tex2html_wrap_inline2941, in which the equator of the coordinate system is the great circle tracked by the scan. This great circle is inclined by tex2html_wrap_inline2943 to the J2000 celestial equator, with an ascending node of tex2html_wrap_inline2945. tex2html_wrap_inline2947 at the ascending node. tex2html_wrap_inline2949 increases in the scan direction (east) and tex2html_wrap_inline2951 increases to the north. Each stripe has its own great circle coordinate system.

For reference, the equations to transform among the different coordinate systems are:


Michael Strauss
Thu Jan 30 11:15:34 EST 2003