diff --git a/README.md b/README.md
index 72bc74418fb39ca3c127d183f7bb1115ecd11a91..b7c589b51d4bf2a21d9350c816abd945eaf962fb 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,8 @@ healpix_fermi_intensity_skymap.cc
    Gaussian-smoothed energy bins and  energy-integrated values are  also output.
    Spherical-harmonic maps A_lm are output, by energy ranges in columns.
 
+   For many purposes it is the counts and exposure maps separately which are required, for model fitting and statistical analysis. However for displaying images and comparing with other skymaps e.g. radio or gas, the visual display is useful. It can be viewed and manipulated with Aladin from CDS Strasbourg.
+
    The two input files are arguments, the first is divided by the second, with interpolation of the second in energy, according to the energies.
    The exposure need not have the same HealPix order as the counts; if different, the exposure is rebinned to the order of counts before division.
    The pixel solid angle (uniform = 4pi/number of pixels in HealPix) is used to convert units to intensity per sr.