diff --git a/nifty_gridder.cc b/nifty_gridder.cc
index e1c8b63b1aff8ed528e575d52360fdfc053be82d..4025f73aa01c2aa3afd6fe1f6d36b81bc67b7db3 100644
--- a/nifty_gridder.cc
+++ b/nifty_gridder.cc
@@ -1488,8 +1488,13 @@ template<typename T> pyarr_c<T> vis2dirty_wstack(const Baselines<T> &baselines,
 cout << "data w range: " << wmin << " to " << wmax << endl;
 
 //FIXME temporary
-double psz=gconf.Pixsize_x();
-  double dw = psz;// distance between two w planes, FIXME
+double dw;
+{
+  double x0 = -0.5*nx_dirty*psx,
+         y0 = -0.5*ny_dirty*psy;
+  double nmin = sqrt(1.-x0*x0-y0*y0)-1.;
+  dw = 1./(2*abs(nmin));
+}
 cout << "delta w: " << dw << endl;
 
   double w_eps=1e-7; // FIXME