Re: Precise centroid calculation and WCS-SIP fitting

[Mohammad Akhlaghi]

Dear Rafael,

The centroids can be found using MakeCatalog, either in pixel 
coordinates (first link below) or WCS coordinates (second link):

https://www.gnu.org/software/gnuastro/manual/html_node/Position-measurements-in-pixels.html

https://www.gnu.org/software/gnuastro/manual/html_node/Position-measurements-in-WCS.html

MakeCatalog needs two input images for its measurements: a "values" 
image (this is usually the scientific image and is usually in floating 
point type) and a "label" image (that has an integer type and each group 
of pixels that are meant to be measured together are given the same 
integer identifier). This is fully described in the documentation of 
MakeCatalog:

https://www.gnu.org/software/gnuastro/manual/html_node/MakeCatalog.html

The usual way is that you first run NoiseChisel to detect signal, then 
you run Segment to identify separate peaks within the detections and 
finally you run MakeCatalog with your reqested measurements like below:

astnoisechisel image.fits --output=nc.fits
astsegment nc.fits --output=seg.fits
astmkcatalog seg.fits --clumpscat --ids --ra --dec --x --y \
             --output=catalog.fits

If the peaks are important for you and not the diffuse regions, you want 
the 'CLUMPS' catalog (second HDU of 'catalog.fits' in the example 
above), not the 'OBJECTS' catalog (first HDU of 'catalog.fits'):

asttable catalog.fits -hCLUMPS

But of course, as described at the beginning of the MakeCatalog 
documentation above, you can build your labeled images in any way you 
prefer.

There is a full/complete tutorial on how to optimize NoiseChisel and 
Segment and work with the output catalogs in several scenarios:

https://www.gnu.org/software/gnuastro/manual/html_node/General-program-usage-tutorial.html

I hope this helps,
Cheers,
Mohammad