from astropy.io import fits
import os
import glob

inp_dir = 'ukidss_full_images/UKIDSS_K'

files = glob.glob(os.path.join(inp_dir, '*.fit'))

for f in files:
  h = fits.open(f)
  for i in [1,2,3,4]:
     h[i].data = h[i].data - h[i].header['SKYLEVEL']
     h[i].header['EXP_TIME'] = h[0].header['EXP_TIME']
  h.writeto(f, overwrite=True)