1. A systematic analysis of caustic methods for galaxy clsuter masses
Gifford et al ApJ
Jeeseon Song

2. Caustics (optics)
In optics, a caustic or caustic network is the envelope of light rays reflected or refracted by a curved surface or object, or the projection of that envelope of rays on another surface.(wikipedia)

3. Caustic (cluster mass estimation)
Direct measure of potential Φ(r)

4. Problem? No 3D information in real data
Observational uncertainty in radius and velocity

5. How to define surface