Include WCS coordinates in a FITS image

Question

I have a 2D numpy array 'ZEA_N_sky' initially created as

n=128
ZEA_N_sky=np.zeros((n,n))

and later some values were assigned to every pixel of this array. I can plot this array as FITS image (that can be opened by ds9) using the following code, however I wish to see the WCS coordinates as well. That is, when I roll my mouse cursor over the image, I should be able to see the gal-long, gal-lat; Right-Asc, Dec , etc. How can I achieve that ? Do i need to do it manually or there is some header() trick ?

The documentation given using WCS package and use all_pix2world() does not seem to work (rather I dont seem to understand it). If somebody can help me please with the code? Thanks !

from astropy.io import fits
import numpy as np
from astropy import wcs
from astropy.table import Table

out_file_name = 'FITS_image.fits'

hdr = fits.Header()
hdr['Projection'] = "ZEA"
hdr['nd_size'] = str(nd_size)
hdr['SCALE']="nd_size/2"
fits.writeto(out_file_name, ZEA_N_sky, hdr,clobber=True)

---

I have made some progress. This is the bit of code I am using right now using which I do obtain the WCS parameters, but the value of longitude that I am getting on my FITS image is wrong ! I think I am making mistake in the ZEA projection parameters values.

NSGP=1 #Parameter for North side projection, -1 for south side

w = wcs.WCS(naxis=2)
w.wcs.crpix = [SCALE,SCALE] # SCALE is half the value of my pixel range in the image. n/2=128/2=64
w.wcs.cdelt = np.array([-NSGP*90.0/float(SCALE) * 0.90032, NSGP*90.0/float(SCALE) * 0.90032]) # increments in degrees per pixel
w.wcs.crval = [NSGP*90.0, NSGP*90.0] #RA and dec values in hours and degrees    
w.wcs.ctype = ["GLON-ZEA", "GLAT-ZEA"]
#w.wcs.set_pv([(180, NSGP, float(SCALE))])

out_file_name = 'N_Mateu_ZEA_stream_mask.fits'
# Now, write out the WCS object as a FITS header
header = w.to_header()
hdu = fits.PrimaryHDU(ZEA_N_sky,header=header)
hdu.writeto(out_file_name, clobber=True)

Answer 1

Try making a WCS object with the correct WCS parameters for your image, and then call header = wcs.to_header , and store the image data and header in the FITS file. See example here .

Usually you don't create WCS or headers yourself, it's something you read from FITS files. In that case you do wcs = WCS(header) to create a WCS object from the FITS header. See example here .