def psfScale(D, wavelength, pixSize):
  """
  Return the PSF scale appropriate for the required pixel size, wavelength and telescope diameter
  The aperture is padded by this amount; resultant pix scale is lambda/D/psf_scale, so for instance full frame 256 pix
  for 3.5 m at 532 nm is 256*5.32e-7/3.5/3 = 2.67 arcsec for psf_scale = 3
  
  Args:
      D (real): telescope diameter in m
      wavelength (real): wavelength in Angstrom
      pixSize (real): pixel size in arcsec
  
  Returns:
      real: psf scale
  """
  DInCm = D * 100.0
  wavelengthInCm = wavelength * 1e-8
  return 206265.0 * wavelengthInCm / (DInCm * pixSize)