Andy,
You probably want to use SETSKY rather than WCSADD.
David
On 12/07/07, Andy Gibb <[log in to unmask]> wrote:
> I have a question or two about WCSADD...
>
> In the SCUBA-2 pipeline we want to align our images in a given SKY
> coordinate frame, but the input data are in focal plane coords. My
> current approach is to use KAPPA creframe to create a 1x1 NDF, and
> then use wcsadd to add WCS info based on the SkyRef attribute of the
> input data. My questions regard the pixel dimensions/scale.
>
> We want an output image that has a inter-pixel distance of 5.8 (say)
> arcsec. But if I create WCS info in the following way, the pixel scale
> (as reported by ndftrace and measured in gaia) is 5.8*cos(dec). We
> don't want that: we want the pixel scale to be 5.8 arcsec. Put another
> way, we want delta_RA to be 5.8 arcsec, not delta_RA = 0.38666 s.
>
> wcsadd frame=grid domain=sky frmtype=skyframe maptype=linear \
> attrs=system=$cosys tr=[$x0,$dx,0,$y0,0,$dy] accept
>
> where $x0, $y0 is the SkyRef position and $dx, $dy are the X,Y pixel
> scales which I've set to be the same. Knowing the above behaviour I
> can kludge the X pixel scale by dividing by cos(dec) but this seems
> rather, well, kludgy. ($cosys is the output coord system, eg ICRS.)
>
> Any advice on how to get what I want? Am I missing something obvious?
>
> Cheers,
> Andy
>
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