Confused about terminal output for RA and Dec versus values in WCS header file

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Aaron Lancaster

Mar 28, 2024, 3:55:36 PMMar 28
to astrometry
Hello all, 

I have noticed something unusual that has me confused about the output of the solve-field command. When I run the command in my Ubuntu terminal, I get my expected output which includes the RA and Dec of the field center, like so: 

Field center: (RA, Dec) = (187.178070, 56.716414) deg.

(this is for apod4 in the demo files btw)

But when I open the WCS header file generated by the solve, apod4.wcs, I don't see these values anywhere. Instead, I see:

CRVAL1  =        174.256985357 / RA  of reference point      
CRVAL2  =        54.5148885864 / DEC of reference point

I originally assumed that the reference point was the field center-- but it looks like the coordinates of the reference point are given in the WCS header as:

CRPIX1  =        304.683069865 / X reference pixel
CRPIX2  =        102.908372879 / Y reference pixel

Is there a reason why this is chosen as the reference point and why the RA and Dec are given for this point instead? And is there a simple way to find the RA and Dec of the field center from these values? (The image height and width are IMAGEH= 507 pixels and IMAGEW= 719 pixels).

Thanks in advance for your help understanding this.


Dustin Lang

Mar 28, 2024, 3:57:46 PMMar 28
to Aaron Lancaster, astrometry
Yes, by default the reference point is chosen to be the center of the "mini-constellation" of stars we matched from your image to the reference catalog.
If you add the solve-field flag "--crpix-center" it will move that reference point to the center of the image.


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