A sorting flow cytometer comprising: a flow nozzle configured to provide a fluid that includes particles, the fluid moving along a fluid flow path, wherein the flow nozzle includes an aperture that permits only one particle to pass through at a time; a light source configured to generate a light beam to illuminate the fluid; an optics system comprising: a collection optics assembly arranged and positioned adjacent the fluid flow path and aligned with a path of the light beam from the light source to collect light rays radiated from the light beam by the fluid, or by particles in the fluid, in the fluid flow path; a collimator arranged to receive the light rays from the collection optics assembly, wherein the collimator directs the light rays through a focus insensitive region in which positions of the light rays are independent of fluctuations in the position of the fluid flow path toward or away from the collection optics assembly; a filter mask positioned at the focus insensitive region to selectively filter the light rays based on radiation angles associated with the light rays; and a re-imager optically arranged between the collection optics assembly and the collimator; the sorting flow cytometer further comprising: a sensor analyser operable to receive and detect the light rays that pass through the filter mask; a computing device and control electronics, wherein the computing device and control electronics is configured to utilize information from the sensor analyzer to control sorting of the particles in the fluid among multiple receptacles. a flow nozzle configured to provide a fluid that includes particles, the fluid moving along a fluid flow path, wherein the flow nozzle includes an aperture that permits only one particle to pass through at a time; a light source configured to generate a light beam to illuminate the fluid; an optics system comprising: a collection optics assembly arranged and positioned adjacent the fluid flow path and aligned with a path of the light beam from the light source to collect light rays radiated from the light beam by the fluid, or by particles in the fluid, in the fluid flow path; a collimator arranged to receive the light rays from the collection optics assembly, wherein the collimator directs the light rays through a focus insensitive region in which positions of the light rays are independent of fluctuations in the position of the fluid flow path toward or away from the collection optics assembly; a filter mask positioned at the focus insensitive region to selectively filter the light rays based on radiation angles associated with the light rays; and a re-imager optically arranged between the collection optics assembly and the collimator; a collection optics assembly arranged and positioned adjacent the fluid flow path and aligned with a path of the light beam from the light source to collect light rays radiated from the light beam by the fluid, or by particles in the fluid, in the fluid flow path; a collimator arranged to receive the light rays from the collection optics assembly, wherein the collimator directs the light rays through a focus insensitive region in which positions of the light rays are independent of fluctuations in the position of the fluid flow path toward or away from the collection optics assembly; a filter mask positioned at the focus insensitive region to selectively filter the light rays based on radiation angles associated with the light rays; and a re-imager optically arranged between the collection optics assembly and the collimator; the sorting flow cytometer further comprising: a sensor analyser operable to receive and detect the light rays that pass through the filter mask; a computing device and control electronics, wherein the computing device and control electronics is configured to utilize information from the sensor analyzer to control sorting of the particles in the fluid among multiple receptacles. a sensor analyser operable to receive and detect the light rays that pass through the filter mask; a computing device and control electronics, wherein the computing device and control electronics is configured to utilize information from the sensor analyzer to control sorting of the particles in the fluid among multiple receptacles.