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 . <EOS>