diff --git a/rust/lib.rs b/rust/lib.rs
index fe85af587..cd3905b55 100644
--- a/rust/lib.rs
+++ b/rust/lib.rs
@@ -1,5 +1,5 @@
-use ndarray::{Array, Array2, ArrayView1, Axis, Ix2};
-use numpy::{IntoPyArray, PyArray2, PyReadonlyArray1};
+use ndarray::{ArrayView1, ArrayViewMut2, Axis};
+use numpy::{PyArray, PyArray2, PyReadonlyArray1};
 use pyo3::{pymodule, types::PyModule, PyResult, Python};
 
 #[inline]
@@ -23,8 +23,7 @@ fn bls_to_ants_2_147_483_648(bl: &u64) -> [u64; 2] {
 const BLS_2_147_483_648: u64 = 2_u64.pow(16) + 2_u64.pow(22);
 const BLS_2048: u64 = 2_u64.pow(16);
 
-fn _baseline_to_antnums(bls_array: ArrayView1<u64>) -> Array2<u64> {
-    let nbls = bls_array.len();
+fn _baseline_to_antnums(bls_array: ArrayView1<u64>, mut bls_out: ArrayViewMut2<u64>) {
     let bls_min = bls_array.fold(bls_array[0], |x, y| x.min(*y));
 
     let bl_fn = if bls_min >= BLS_2_147_483_648 {
@@ -34,7 +33,6 @@ fn _baseline_to_antnums(bls_array: ArrayView1<u64>) -> Array2<u64> {
     } else {
         bls_to_ants_256
     };
-    let mut bls_out = Array::<u64, Ix2>::zeros((2, nbls));
     bls_out
         .axis_iter_mut(Axis(1))
         .zip(bls_array)
@@ -43,8 +41,6 @@ fn _baseline_to_antnums(bls_array: ArrayView1<u64>) -> Array2<u64> {
             ant_array[0] = ants[0];
             ant_array[1] = ants[1];
         });
-
-    bls_out
 }
 
 #[pymodule]
@@ -54,14 +50,17 @@ fn _utils_rs<'py>(_py: Python<'py>, m: &'py PyModule) -> PyResult<()> {
         py: Python<'py>,
         bls: PyReadonlyArray1<'py, u64>,
     ) -> &'py PyArray2<u64> {
-        _baseline_to_antnums(bls.as_array()).into_pyarray(py)
+        let nbls = bls.shape()[0];
+        let bls_out = unsafe { PyArray::new(py, (2, nbls), false) };
+        _baseline_to_antnums(bls.as_array(), unsafe { bls_out.as_array_mut() });
+        bls_out
     }
     Ok(())
 }
 
 #[cfg(test)]
 mod test {
-    use ndarray::Array;
+    use ndarray::{Array, Ix2};
 
     use super::*;
 
@@ -80,7 +79,10 @@ mod test {
         )
         .unwrap();
 
-        let antnums = _baseline_to_antnums((&bls).into());
+        let nbls = bls.len();
+        let mut antnums = Array::<u64, Ix2>::zeros((2, nbls));
+
+        _baseline_to_antnums((&bls).into(), (&mut antnums).into());
         assert_eq!(ants, antnums)
     }
 
@@ -99,7 +101,10 @@ mod test {
         )
         .unwrap();
 
-        let antnums = _baseline_to_antnums((&bls).into());
+        let nbls = bls.len();
+        let mut antnums = Array::<u64, Ix2>::zeros((2, nbls));
+
+        _baseline_to_antnums((&bls).into(), (&mut antnums).into());
         assert_eq!(ants, antnums)
     }
 
@@ -118,7 +123,10 @@ mod test {
         )
         .unwrap();
 
-        let antnums = _baseline_to_antnums((&bls).into());
+        let nbls = bls.len();
+        let mut antnums = Array::<u64, Ix2>::zeros((2, nbls));
+
+        _baseline_to_antnums((&bls).into(), (&mut antnums).into());
         assert_eq!(ants, antnums)
     }
 }