CODEWITHSUNDEEP
pythonphysicsastronomyastropy
Khyati Malhan
Khyati Malhan

Reputation: 11

Include WCS coordinates in a FITS image

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)

Upvotes: 0

Views: 3434

Answers (1)

Christoph
Christoph

Reputation: 2940

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.

Upvotes: 1

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