diff --git a/libsharp/sharp_geomhelpers.c b/libsharp/sharp_geomhelpers.c
index e39d7e9..cd088b3 100644
--- a/libsharp/sharp_geomhelpers.c
+++ b/libsharp/sharp_geomhelpers.c
@@ -34,14 +34,14 @@
 #include "sharp_geomhelpers.h"
 #include "c_utils.h"
 #include "ls_fft.h"
+#include <stdio.h>
 
-void sharp_make_weighted_healpix_geom_info (int nside, int stride,
-  const double *weight, sharp_geom_info **geom_info)
+void sharp_make_subset_healpix_geom_info (int nside, int stride, int nrings,
+  const int *rings, const double *weight, sharp_geom_info **geom_info)
   {
   const double pi=3.141592653589793238462643383279502884197;
   ptrdiff_t npix=(ptrdiff_t)nside*nside*12;
   ptrdiff_t ncap=2*(ptrdiff_t)nside*(nside-1);
-  int nrings=4*nside-1;
 
   double *theta=RALLOC(double,nrings);
   double *weight_=RALLOC(double,nrings);
@@ -49,9 +49,10 @@ void sharp_make_weighted_healpix_geom_info (int nside, int stride,
   double *phi0=RALLOC(double,nrings);
   ptrdiff_t *ofs=RALLOC(ptrdiff_t,nrings);
   int *stride_=RALLOC(int,nrings);
+  ptrdiff_t curofs=0, checkofs; /* checkofs used for assertion introduced when adding rings arg */
   for (int m=0; m<nrings; ++m)
     {
-    int ring=m+1;
+    int ring = (rings==NULL)? (m+1) : rings[m];
     ptrdiff_t northring = (ring>2*nside) ? 4*nside-ring : ring;
     stride_[m] = stride;
     if (northring < nside)
@@ -59,7 +60,7 @@ void sharp_make_weighted_healpix_geom_info (int nside, int stride,
       theta[m] = 2*asin(northring/(sqrt(6.)*nside));
       nph[m] = 4*northring;
       phi0[m] = pi/nph[m];
-      ofs[m] = 2*northring*(northring-1)*stride;
+      checkofs = 2*northring*(northring-1)*stride;
       }
     else
       {
@@ -71,14 +72,21 @@ void sharp_make_weighted_healpix_geom_info (int nside, int stride,
         phi0[m] = 0;
       else
         phi0[m] = pi/nph[m];
-      ofs[m] = (ncap + (northring-nside)*nph[m])*stride;
+      checkofs = (ncap + (northring-nside)*nph[m])*stride;
+      ofs[m] = curofs;
       }
     if (northring != ring) /* southern hemisphere */
       {
       theta[m] = pi-theta[m];
-      ofs[m] = (npix - nph[m])*stride - ofs[m];
+      checkofs = (npix - nph[m])*stride - checkofs;
+      ofs[m] = curofs;
       }
     weight_[m]=4.*pi/npix*((weight==NULL) ? 1. : weight[northring-1]);
+    if (rings==NULL) {
+        UTIL_ASSERT(curofs==checkofs, "Bug in computing ofs[m]");
+    }
+    ofs[m] = curofs;
+    curofs+=nph[m];
     }
 
   sharp_make_geom_info (nrings, nph, ofs, stride_, phi0, theta, weight_,
@@ -92,6 +100,12 @@ void sharp_make_weighted_healpix_geom_info (int nside, int stride,
   DEALLOC(stride_);
   }
 
+void sharp_make_weighted_healpix_geom_info (int nside, int stride,
+  const double *weight, sharp_geom_info **geom_info)
+  {
+  sharp_make_subset_healpix_geom_info(nside, stride, 4 * nside - 1, NULL, weight, geom_info);
+  }
+
 static inline double one_minus_x2 (double x)
   { return (fabs(x)>0.1) ? (1.+x)*(1.-x) : 1.-x*x; }
 
diff --git a/libsharp/sharp_geomhelpers.h b/libsharp/sharp_geomhelpers.h
index 8f80d9b..b7f98c4 100644
--- a/libsharp/sharp_geomhelpers.h
+++ b/libsharp/sharp_geomhelpers.h
@@ -38,6 +38,18 @@
 extern "C" {
 #endif
 
+/*! Creates a geometry information describing a HEALPix map with an
+    Nside parameter \a nside. \a weight contains the relative ring
+    weights and must have \a 2*nside entries. The rings array contains
+    the indices of the rings, with 1 being the first ring at the north
+    pole; if NULL then we take them to be sequential. Pass 4 * nside - 1
+    as nrings and NULL to rings to get the full HEALPix grid.
+    \note if \a weight is a null pointer, all weights are assumed to be 1.
+    \note if \a rings is a null pointer, take all rings
+    \ingroup geominfogroup */
+void sharp_make_subset_healpix_geom_info (int nside, int stride, int nrings,
+  const int *rings, const double *weight, sharp_geom_info **geom_info);
+
 /*! Creates a geometry information describing a HEALPix map with an
     Nside parameter \a nside. \a weight contains the relative ring
     weights and must have \a 2*nside entries.