import numpy
import astropy
from astropy.io import fits as pyfits

FitsFile = pyfits.open('M33.fits')

image = FitsFile[0].data
header =FitsFile[0].header

FitsFile.close()

newimage = numpy.zeros_like(image)

newimage[50:70, 160:230, 160:230] = 0.1

print(newimage[60, 170, 170])

newfitsfile = 'M33MockSecondaryVolume.fits'

pyfits.writeto(newfitsfile, newimage, header=header)