A system for performing an assay, the system comprising: a chassis, comprising: a reaction vessel chamber comprising: a first portion of a kinematic spherical joint adjacent to a bottom of the reaction vessel chamber; and a first portion of a kinematic cylindrical joint adjacent to a top of the reaction vessel chamber; and a light passage intersecting the reaction vessel chamber; a reaction vessel comprising: a fin that, when the reaction vessel is placed in the reaction vessel chamber, forms a second portion of the kinematic cylindrical joint, and a nose that, when the reaction vessel is placed in the reaction vessel chamber, forms a second portion of the kinematic spherical joint; wherein the reaction vessel is positioned within the reaction vessel chamber by the kinematic cylindrical joint and the kinematic spherical joint; a cap that, when in a closed configuration, forms a dark chamber by preventing light emitted by external light sources from entering the reaction vessel chamber and from entering the light passage, and, when in an open configuration, provides access to the reaction vessel chamber; and a light detector optically coupled to the light passage, the light detector comprising: a sensing element for receiving light from the light passage. a chassis, comprising: a reaction vessel chamber comprising: a first portion of a kinematic spherical joint adjacent to a bottom of the reaction vessel chamber; and a first portion of a kinematic cylindrical joint adjacent to a top of the reaction vessel chamber; and a light passage intersecting the reaction vessel chamber; a reaction vessel chamber comprising: a first portion of a kinematic spherical joint adjacent to a bottom of the reaction vessel chamber; and a first portion of a kinematic cylindrical joint adjacent to a top of the reaction vessel chamber; and a first portion of a kinematic spherical joint adjacent to a bottom of the reaction vessel chamber; and a first portion of a kinematic cylindrical joint adjacent to a top of the reaction vessel chamber; and a light passage intersecting the reaction vessel chamber; a reaction vessel comprising: a fin that, when the reaction vessel is placed in the reaction vessel chamber, forms a second portion of the kinematic cylindrical joint, and a nose that, when the reaction vessel is placed in the reaction vessel chamber, forms a second portion of the kinematic spherical joint; wherein the reaction vessel is positioned within the reaction vessel chamber by the kinematic cylindrical joint and the kinematic spherical joint; a fin that, when the reaction vessel is placed in the reaction vessel chamber, forms a second portion of the kinematic cylindrical joint, and a nose that, when the reaction vessel is placed in the reaction vessel chamber, forms a second portion of the kinematic spherical joint; wherein the reaction vessel is positioned within the reaction vessel chamber by the kinematic cylindrical joint and the kinematic spherical joint; a cap that, when in a closed configuration, forms a dark chamber by preventing light emitted by external light sources from entering the reaction vessel chamber and from entering the light passage, and, when in an open configuration, provides access to the reaction vessel chamber; and a light detector optically coupled to the light passage, the light detector comprising: a sensing element for receiving light from the light passage.