  Photometric observations of galaxies at submillimetre to millimetre
wavelengths (50 - 1000 GHz) are susceptible to spatial variations in both the
background CMB temperature and CIB emission that can be comparable to the flux
from the target galaxy. We quantify the residual uncertainty when background
emission inside a circular aperture is estimated by the mean flux in a
surrounding annular region, assumed to have no contribution from the source of
interest. We present simple formulae to calculate this uncertainty as a
function of wavelength and aperture size. Drawing on examples from the
literature, we illustrate the use of our formalism in practice and highlight
cases in which uncertainty in the background subtraction needs to be considered
in the error analysis. We make the code used to calculate the uncertainties
publicly available on the web.
