function makecirc, f, a
on_error,2                      ;Return to sender if broken
;;; Extract relevent parameters from A
xo = a[0] ;x centroid
yo = a[1] ;y centroid
r  = a[2] ;radius

;;; How big is the input array?
sz = size(f, /dimen)
xsize = sz[0]
ysize = sz[1]

;;; Construct abscissa values and fan 'em out
x = indgen(xsize)
y = indgen(ysize)
x = rebin(x, xsize, ysize)
y = rebin(reform(y, 1, ysize), ysize, xsize)

;;; In or out?
w = where((x - xo)^2 + (y - yo)^2 le r^2, nw)
;;; A little trickery, make MASK the same size as F, but fill with
;;; zeros. A job for fltarra? Nah.
mask = f*0
;;; Insert 1's in the appropriate places
if nw gt 0 then mask[w] = 1 
return, mask
end