diff --git a/src/bin/flat/tube/metric.rs b/src/bin/flat/tube/metric.rs
index 8bd5724..96c6fe2 100644
--- a/src/bin/flat/tube/metric.rs
+++ b/src/bin/flat/tube/metric.rs
@@ -51,8 +51,12 @@ impl Metric for Tube {
 
 #[cfg(test)]
 mod test {
+	use approx::assert_abs_diff_eq;
 	use glam::{Vec2, vec2};
+	use itertools_num::linspace;
 	use crate::riemann::{Decomp2, Metric};
+	use crate::tube::Space;
+	use crate::types::Ray;
 	use super::Tube;
 
 	#[test]
@@ -81,4 +85,118 @@ mod test {
 			}
 		}
 	}
+
+	#[test]
+	fn test_accelerator() {
+		let space = Space {
+			tube: Tube {
+				inner_radius: 30.0,
+				outer_radius: 50.0,
+				internal_halflength: 100.0,
+				external_halflength: 300.0,
+			},
+			objs: vec![],
+		};
+		let ε = 1e-3;
+		let off = 10.0;
+		let steps = 1024;
+		for ax in [-30.0 + ε, -25.0, -3.0, 17.0, 30.0 - ε] {
+			for bx in [0.0, ε, 1.0, 7.0, 30.0 - ε] {
+				let a = vec2(ax, -(space.tube.external_halflength + off));
+				let b = vec2(bx, space.tube.external_halflength + off);
+				let Δ = vec2(bx - ax, 2.0 * (space.tube.internal_halflength + off));
+				let dir = Δ / (steps as f32);
+				let traced = space.trace_iter(Ray { pos: a, dir }).nth(steps).unwrap();
+				assert_abs_diff_eq!(traced.pos.x, b.x, epsilon=1.0e-2);
+				assert_abs_diff_eq!(traced.pos.y, b.y, epsilon=1.0e1);
+				assert_abs_diff_eq!(traced.dir.x, dir.x, epsilon=1.0e-3);
+				assert_abs_diff_eq!(traced.dir.y, dir.y, epsilon=1.0e-2);
+			}
+		}
+	}
+
+	#[test]
+	#[ignore]
+	fn test_accelerator_slow() {
+		let space = Space {
+			tube: Tube {
+				inner_radius: 30.0,
+				outer_radius: 50.0,
+				internal_halflength: 100.0,
+				external_halflength: 300.0,
+			},
+			objs: vec![],
+		};
+		let ε = 1e-3;
+		let off = 10.0;
+		let steps = 4096;
+		for ax in linspace(-space.tube.inner_radius + ε, space.tube.inner_radius - ε, 20) {
+			for bx in linspace(-space.tube.inner_radius + ε, space.tube.inner_radius - ε, 20) {
+				let a = vec2(ax, -(space.tube.external_halflength + off));
+				let b = vec2(bx, space.tube.external_halflength + off);
+				let Δ = vec2(bx - ax, 2.0 * (space.tube.internal_halflength + off));
+				let dir = Δ / (steps as f32);
+				let traced = space.trace_iter(Ray { pos: a, dir }).nth(steps).unwrap();
+				assert_abs_diff_eq!(traced.pos.x, b.x, epsilon=1.0e-2);
+				assert_abs_diff_eq!(traced.pos.y, b.y, epsilon=1.0e0);
+				assert_abs_diff_eq!(traced.dir.x, dir.x, epsilon=1.0e-3);
+				assert_abs_diff_eq!(traced.dir.y, dir.y, epsilon=1.0e-3);
+			}
+		}
+	}
+
+	#[test]
+	#[ignore] // TODO make the metric smooth so that this passes
+	fn test_accelerator_inner_edge() {
+		let space = Space {
+			tube: Tube {
+				inner_radius: 30.0,
+				outer_radius: 50.0,
+				internal_halflength: 100.0,
+				external_halflength: 300.0,
+			},
+			objs: vec![],
+		};
+		let ε = 1e-3;
+		let off = 10.0;
+		let steps = 4096;
+		for x in [space.tube.inner_radius - ε, space.tube.inner_radius + ε] {
+			let a = vec2(x, -(space.tube.external_halflength + off));
+			let b = vec2(x, space.tube.external_halflength + off);
+			let Δ = vec2(0.0, 2.0 * (space.tube.internal_halflength + off));
+			let dir = Δ / (steps as f32);
+			let traced = space.trace_iter(Ray { pos: a, dir }).nth(steps).unwrap();
+			assert_abs_diff_eq!(traced.pos.x, b.x, epsilon=1.0e0);
+			assert_abs_diff_eq!(traced.pos.y, b.y, epsilon=1.0e0);
+			assert_abs_diff_eq!(traced.dir.x, dir.x, epsilon=1.0e-2);
+			assert_abs_diff_eq!(traced.dir.y, dir.y, epsilon=1.0e-2);
+		}
+	}
+
+	#[test]
+	fn test_accelerator_outer_edge() {
+		let space = Space {
+			tube: Tube {
+				inner_radius: 30.0,
+				outer_radius: 50.0,
+				internal_halflength: 100.0,
+				external_halflength: 300.0,
+			},
+			objs: vec![],
+		};
+		let ε = 1e-3;
+		let off = 10.0;
+		let steps = 4096;
+		for x in [space.tube.outer_radius + ε, space.tube.outer_radius - ε] {
+			let a = vec2(x, -(space.tube.external_halflength + off));
+			let b = vec2(x, space.tube.external_halflength + off);
+			let Δ = vec2(0.0, 2.0 * (space.tube.external_halflength + off));
+			let dir = Δ / (steps as f32);
+			let traced = space.trace_iter(Ray { pos: a, dir }).nth(steps).unwrap();
+			assert_abs_diff_eq!(traced.pos.x, b.x, epsilon=2.0e0);
+			assert_abs_diff_eq!(traced.pos.y, b.y, epsilon=1.0e0);
+			assert_abs_diff_eq!(traced.dir.x, dir.x, epsilon=1.0e-2);
+			assert_abs_diff_eq!(traced.dir.y, dir.y, epsilon=1.0e-2);
+		}
+	}
 }