Handling celestial coordinates

• insert spacecraft pointing in image header
• compute detector pointing (or insert in image header)
• convert pixels to sky coordinates
• convert sky to pixel coordinates
• overlay a sky grid

command [mandatory {detnam pixsize} {ra dec roll} {alpha beta gamma}

• Spacecraft pointing

  There is no program to deal with this. The following is a possible procedure.
  • select an appropriate stable attitude interval (discretion of the user) into which to perform all accumulations
  • accumulate (in such interval) an attitude time profile for each of the quantity zra, zdec, xdec and ydec
  • take the mean value of each of the four quantities
  • compute the roll angle as 90-atan( sin(xdec)/sin*ydec) )
  • insert the mean of zra, zdec and the roll angle in the image header in the designated keywords using commands :

    header_edit [ file RA_SCZ   value(s) D
    header_edit [ file DEC_SCZ  value(s) D
    header_edit [ file ROLL_SCZ value(s) D

• Detector pointing

  The detector pointing coordinates can be computed from the spacecraft pointing coordinates (present in the last plotted image header or supplied by the user) and applying the misalignments for the relevant detector, using commands

  detpointing [Compute {detnam pixsize} {ra dec roll} {alpha beta gamma}

  These values can be stored to header keywords RA_PNT, DEC_PNT and ROLL_PNT of the last plotted image using the following commands (if they are already in the header and one wants to change them, use Update instead of Insert mode.

  detpointing [Insert {detnam pixsize} {ra dec roll} {alpha beta gamma}

• Pixels to sky conversion

  To convert pixel coordinates into celestial coordinates (i.e. to measure a target's position) one can use the following command (in the second form use the mouse cursor in conjunction with an image plot to point at the wished pixel position) :

  pixtosky [x y    {detnam pixsize} {ra dec roll} {alpha beta gamma}
  pixtosky [CURSOR {detnam pixsize} {ra dec roll} {alpha beta gamma}

  The x y coordinates are assumed in linearized pixels, and are converted to mm internally. One may force them to be interpreted as unlinearized pixels or mm using the xyunit variable, but its usage is meaningful only if use of the plotted image is disabled.

• Sky to pixel conversion

  To convert celestial coordinates into pixels one can use the following command (in the second form use the mouse cursor in conjunction with an image plot to point at the wished pixel position) :

  pixtosky [ra dec {detnam pixsize} {ra dec roll} {alpha beta gamma}

  The coordinates ra dec must be given in decimal degrees (0-360 and -90 to +90 resp.).
  If an image is currently displayed and overlay is enabled a marker will also be plotted at the computed pixel position.

• Sky grid

  The following command is used to overlay a coordinate grid onto the displayed image.

  skygrid {detnam pixsize} {ra dec roll} {alpha beta gamma}

  The grid will have a spacing of 10 arcmin in either RA and Dec, except that close to the poles the RA spacing may be changed as indicated by the program. The main RA and Dec axes (passing for the point at integral degree coordinates closest to the centre) will be plotted usually in white (or current datapen) while the other grid lines will be colour coded as follows :
  • red : declinations south of (less than) the central one
  • green : declinations north of (greater than) the central one
  • yellow : RAs west of (less than) the central one
  • cyan : RAs east of (greater than) the central one

  In addition the North and East directions will be marked (but the marks may fall out of the plot). Note also that behaviour of this program is not guaranteed close to the poles or to the zero meridian.