Set alignment (CCD_SET) coordinates

You may notice that in the last example, a column headed ``CCD_SET domain'' has appeared in the output, with the entry ``PIXEL'' for each of the new Set members. This indicates that as well as the Set header, each file has had a new coordinate system attached to its WCS component. This new coordinate system added by MAKESET always has the Domain (name) ``CCD_SET'', and in this case is a copy of the PIXEL-Domain coordinate system which existed in the input image. When MAKESET adds a CCD_SET coordinate system like this, it is declaring that all the members of the same Set are known to be correctly aligned in these coordinates. All the CCDPACK programs concerned with registration will recognise a CCD_SET coordinate system and assume that the alignment of all the images in a given Set is correct in these coordinates and is not to be tampered with. Thus, it is only a good idea to add a CCD_SET coordinate system if it really does represent an accurate alignment of images within the Set and not one which, for instance, you might want to improve at a later date by object matching. When MODE=SPLIT, it is assumed that the PIXEL coordinates of the original image do represent this sort of rock-solid alignment, since the Set members have been hewn out of the same data array. With the other modes of MAKESET Set alignment coordinates are not added by default, but if the alignment of the Set is known, it can be added by setting the ADDWCS parameter. In this case a copy of the Current attached coordinate system of each image is used for the CCD_SET alignment coordinates.

So suppose that you have a directory full of HDS container files each holding the data frames from one observation. Each has an attached coordinate system in its WCS component, with the Domain ``FOCAL'' which gives its exact position on the focal plane of the instrument -- this may have been added by the observing system, and it is known to be accurate. First ensure that the FOCAL coordinates are the Current coordinates of all the frames:

% wcsedit '*' current focal

and then add the Set headers with the ADDWCS parameter set true:

% makeset '*' mode=container addwcs=yes accept

    MAKESET
    =======
  3 input NDFs accessed using parameter IN

 Set name  Set index   NDF name                            CCD_SET domain
 --------  ---------   --------                            --------------

 f10001:
               1       f10001.I1                           FOCAL  
               2       f10001.I2                           FOCAL  
               3       f10001.I3                           FOCAL

As you can see, the FOCAL coordinate system is listed as the one copied to make the new one. Note that since the new CCD_SET coordinate system is a copy of the FOCAL one, it will remain attached to the images even if the FOCAL one is erased or modified.

Variations on these methods of adding Set headers, as well as the possibility of controlling Set formation from FITS headers, are discussed in the description of MAKESET in the appendix.

If you need to add Set alignment coordinates to some files after MAKESET has already been run, you can just attach a new coordinate system with the name CCD_SET, using WCSEDIT or in some other way. Note however that a CCD_SET-domain coordinate system will only be treated as special when the images in question have valid Set headers.