Alignment of an Image to a Reference Image

 

Often an input image (with any world coordinate system) has to be aligned to a reference image (in general X-ray image with general EXSAS header). This can be done if a set of reference stars is known in the input image.

ALIGN/SCAN_IMAGE stars numbers [inpima] [refima] [outtab]

 

• stars : name of table containing R.A.(2000) (R*4 column :RA_DEG) and Decl(2000) (R*4 column :DEC_DEG) coordinates of reference stars (contained at least in part) in the field of view of the reference (e.g. optical/radio) image
• numbers : number,sequence_numbers of stars (from table stars) to be used for the alignment. An example is : numbers = 5,23,32,14,6,2 (5 stars with sequence numbers 23,32,14,6 and 2)
• inpima : name of (loaded) image to be aligned. The image has to be loaded in that sense, that the stars have higher intensity (white) than the background (black).
• refima : name of image onto which inpima will be aligned. refima must have the general EXSAS header and the special EXSAS descriptors SKY_CEN_X, SKY_CEN_Y, and SKY_PIX_SIZE.
• outtab : name of output table created by the alignment. The default is align.tbl. The user normally does not have to make use of this table.

Note:Star positions are determined by a Gaussian fit. The user is requested to click on the stars specified by their sequence numbers in the parameter numbers. The size of the cursor box can be adjusted with the arrow keys. The aligned result image is written to inpima_relin.bdf.

Example:

ALIGN/SCAN_IMAGE mystars 5,10,12,15,18,22 optical m33

Align an image optical.bdf to a reference image m33.bdf. Select the 5 stars with sequence numbers 10,12,15,18,22 from the table mystars.tbl by cursor on the displayed image. The 5 stars have to be put in the cursor box for the sequence given in the parameter numbers.