4.8 Aligning Ground-Based Images with SXT

The software tools for aligning SXT images and other images is in a very early stage of development. The routine COAL_IMAGE along with some other tools can be used in a fairly manual method to co-align a pair of images.

The alignment between SXT images and other images can be performed by the following steps. The SXT images do not require steps 1 or 2 since the absolute pointing and roll are known for SXT images.

  
Figure: Coordinate definition for COAL_IMAGE

1. For the non-SXT image, determine the distance that the lower left pixel is from the center of the sun in pixel units (see Fig. 6). This can be accomplished in one of several methods.
   • If the image contains enough of the solar limb, then a fitting routine can be used to determine the coordinates of the center of the sun.
   • The spacecraft or observation site might know the absolute coordinates of the image, so the pixel address of the lower left corner can be easily derived.
   • Solar features can be manually co-registered with a full disk image, and the coordinates for the center of the sun can be derived from that.
2. Determine the roll of the image relative to solar north. Again there are several techniques to determine this value
   • The most common method is simple knowledge based on spacecraft attitude, star trackers, or ephemeris information.
   • It is possible to manually co-register the image with another image which has a known roll.
3. Run COAL_IMAGE to register one image to another image. The program will correct for:
   • translational offsets
   • roll
   • different pixel resolutions

• 4.8.1 Determining the Coordinates Using FIT_LIMB
• 4.8.2 Determining the Coordinates Using DSK_LOCB
• 4.8.3 Determining the Coordinates and/or Roll by Feature Correlation
• 4.8.4 Registering One Image to Another Image