IDL does seem to like this loop and I'm not sure why. I am trying to find the median counts for an im image and plot. I get an error saying that median needs to be in the form of a float...? pro hst_1,file=file im=readfits(file) im=fltarr(1024,1024) for i=0,1023 do begin for j=0,1023 do begin med(i,j)=median(im(i,j)) im2=im-med plot,im2 endfor endfor end