Overview of the Coordinate Systems Used by SXT Ver 1.0 (MDM) 20-Mar-95 This note is intended to describe the different coordinate systems and what corrections and adjustments have been applied to them. The different systems are: P SXT Pixels A Angular offsets from sun center as viewed from earth H Heliocentric coordinates C Carrington coordinates HS Heliocentric coorindates in string format (N12W23) HXT HXT pixels AP Radius and angle (OR) 2 angle sun centered spherical Other documents that could be useful /ys/sxt/doc/sxt_prep.txt - Shows some sample SXT_PREP calls, the parameter list, and the /ydb databases required for SXT_PREP to work /ys/sxt/doc/coor_systems.txt - This document /ys/sxt/doc/pointing_suncen.txt - Describes the SXT coordinate system and how the coordinates are saved internally There are a collection of routines to convert between the various coordinate systems. Some of them are: conv_a2h.pro conv_a2p.pro conv_h2a.pro conv_h2hs.pro conv_h2hxt.pro conv_h2p.pro conv_hs2h.pro conv_hxt2p.pro conv_p2a.pro conv_p2h.pro conv_p2hxt.pro conv_a2c.pro conv_c2a.pro conv_c2h.pro conv_c2p.pro conv_h2c.pro conv_p2ap.pro conv_p2c.pro Some of the routines are simply two calls to other routines, for example CONV_P2H calls CONV_P2A and then CONV_A2H. See Section 4.1 of the Yohkoh Analysis Guide Reference Guide for more details and for some figures. -------------------------------------------------------------------------------- P SXT Pixels -------------------------------------------------------------------------------- The SXT pixel coordinates are in Full Resolution Equivalent IDL pixels. The idiosyncrasy with the on-chip summation has been taken into account. The pixel location "2.0" specifies the CENTER of the third full resolution pixel (because counting starts at 0.0). The units are always full resolution pixels, regardless of the summation of the image. These are the coordinates used for CCDCOORD inputs for SXT_PREP. -------------------------------------------------------------------------------- A Angle from sun center in arcsec as viewed from earth -------------------------------------------------------------------------------- The angle for sun center is a rectangular coordinate system which is a simple rectangular grid with (0,0) at the center of the sun and aligned with solar north. The conversion from "P" to "A" involves determining the sun center pixel coordinate and the roll of the S/C and SXT. "A" does not care about the B-angle. The units are arcseconds. This is the coordinate used for the SUNCOORD inputs for SXT_PREP. If you want your field of view centered on the sun center, then enter "suncoord=[0,0]". If you wish to view the east limb near the equator, then enter "suncoord=[-960,0]". The default for /REG and SUNCOORD not defined, then it does something like: "total(gt_center(index, /angle), 2) / n_elements(index)" which is the average center location of all images. -------------------------------------------------------------------------------- H Heliocentric coordinates -------------------------------------------------------------------------------- The heliocentric coordinates are latitude and longitude on the sun. They are not rectangular. The conversion from "A" to "H" takes the B-angle into account. The units are degrees. This is the coordinate system used for the HELIO inputs for SXT_PREP. It is necessary to specify the date associated with the coordinate. The coordinates can be entered in string format or as a 2 element array. A sample call would be "helio='S12W23'" which is the same as "helio=[23,-12]". -------------------------------------------------------------------------------- C Carrington coordinates -------------------------------------------------------------------------------- The same as heliocentric coordinates (above) except that the longitude is given as Carrington longitude. The default is to return the carrington longitude in degrees (0 to 360 increasing westward). If the /DC switch is used, then the longitude is in decimal Carrington rotation with the fraction increasing eastward (ie: 1893.50 says that it is half way through rotation number 1893). -------------------------------------------------------------------------------- HXT HXT pixels -------------------------------------------------------------------------------- The HXT pixel coordinates are a simple conversion from the SXT pixel coordinates. The S/C pointing, roll, and B-angle have no affect on the results. -------------------------------------------------------------------------------- AP Radius and angle (OR) 2 angle sun centered spherical -------------------------------------------------------------------------------- The "AP" routines can return the radial offset from sun center in either arcseconds or solar radii units (0.0 to 1.0) and the position angle measured counter-clockwise from the solar north pole. In place of the radius coordinate the AP routine can optionally return the angle between the point(s) of interest and the axis defined by the line passing through the center of the sun and the center of the projected disk of the sun (0 to 90 degrees).