4.2.1 Definition of the Coordinate Systems ``P'', ``A'', ``H'' and ``C''

There are three common coordinate systems which can be used when analyzing the Yohkoh data.

P

signifies ``pixel''. This is the full resolution IDL pixel address when using SXT images. This means that (0,0) is in the lower left corner of the image, which is the south-east corner. The upper right corner is (1023, 1023). The coordinates are rectangular.

A

is a rectangular coordinate system relative to sun center as viewed from the earth in units of arcseconds. A point on the farthest edge on the east limb will have the value (-900, 0). The B-angle is not taken into consideration, so a (0,0) value will not be on the equator unless the B-angle is zero.

H

signifies the heliocentric coordinates. The solar B-angle is taken into consideration when calculating this value. These coordinates are spherical.

C

signifies the Carrington longitude

HS

signifies a string representation of heliocentric coordinates (``N12W23'' for example)

HXT

signifies the HXT pixel address in 126 arcsec units.

The first three of these conventions have a two element array for each point described. If there are N points, then the array will be 2xN. The values stored in (0,*) are the position in the east/west direction (longitude for heliocentric coordinates), with east being negative. The values in (1,*) are in the north/south direction (longitude for heliocentric coordinates), with south being negative.