[daip] coordinates in AIPS

[Anna Bartkiewicz]

Dear Eric,

Thank you for your explanations. I understand more now.
Will try to figure out how to manage to work on figures.
Greetings,

Anna
>
> Perhaps you have not understood.
>
> Someone has run astrometric software on your image and fit parameters to the 
> coordinates.  The CDi_j are the result of that fit - along with the CRPIXi, 
> CRVALi, CTYPEi, etc.  The pixels in your image are not on a "simple" 
> projective geometry, although they are not terribly far from one (the usual 
> optical telescope -TAN).  They are warped so as to be on a rectangular grid 
> but even that is twisted.  The amount of the warping
> becomes several pixels by the edges of your image.
>
> There is a software library WCSLIB that can handle this complexity.
> Unfortunately, AIPS has never been overhauled to replace all the careful 
> coordinate work we invented ahead of WCSLIB to use the more advanced library. 
> Other software has been revised to use WCSLIB - public packages such as ds9 
> for example or iraf.  Try using one of these to read in your image - perhaps 
> they have an image regridding routine that will take your skewed image and 
> recompute it onto a more regular projective grid.  AIPS has such a task 
> (OHGEO, HGEOM) but it is not smart enough to handle your input image.
>
> Eric Greisen
>