code: def get_hstwcs(filename,hdulist,extnum):
      
      hdrwcs = wcsutil.HSTWCS(hdulist,ext=extnum)
      hdrwcs.filename = filename
      hdrwcs.expname = hdulist[extnum].header['expname']
      hdrwcs.extver = hdulist[extnum].header['extver']
  
      return hdrwcs
docs: Return the HSTWCS object for a given chip.