A monitoring system for a gas turbine engine, the monitoring system comprising: a viewing tube assembly having an inner end and an outer end, the inner end configured to be located adjacent to a hot gas flow path within the gas turbine engine and the outer end configured to be located adjacent to an outer casing of the gas turbine engine; an aperture wall located at the inner end of the viewing tube assembly, the aperture wall extending radially inwardly from an interior surface of the viewing tube assembly; an optical element located within the viewing tube assembly adjacent to the inner end and spaced from the aperture wall to define a cooling and purge chamber therebetween; an aperture defined in the aperture wall for passage of light from the hot gas flow path to the optical element; one or more swirl passages defined in the viewing tube assembly between the aperture wall and the optical element for passage of cooling air from a location outside the viewing tube assembly into the chamber, wherein the one or more swirl passages effect a swirling movement of air in a circumferential direction within the chamber, wherein the viewing tube assembly comprises a lens tube, and a lens housing located within the lens tube at the inner end of the viewing tube assembly and defining the one or more swirl passages, wherein the lens housing comprises a tubular member having a first end, a second end, an exterior surface, and the interior surface, wherein the optical element comprises a lens, and a diameter of the lens is at least approximately 5 times greater than a diameter of the aperture, wherein the lens tube includes an end portion that includes a flange that extends radially inwardly from an inner end surface of the lens tube and forms a support structure for engagement by an end portion of the lens housing, the flange including an opening for permitting passage of light from the hot gas flow path into the viewing tube assembly, wherein the end portion of the lens housing spaces the aperture wall from the flange a predetermined axial distance to define an annular area therebetween surrounding the aperture, wherein an axially extending air passage is defined in a gap between the lens tube and the lens housing for supplying cooling air to the one or more swirl passages, wherein, in use, cooling air is provided to the axially extending air passage from shell air passing between a radially outer casing wall and a radially inner casing wall of the gas turbine engine, wherein, in use, secondary cooling air is provided to the viewing tube assembly through openings formed in the lens tube for passage of the secondary cooling air to the annular area, the secondary cooling air passing through the openings adjacent to the end portion of the lens housing. a viewing tube assembly having an inner end and an outer end, the inner end configured to be located adjacent to a hot gas flow path within the gas turbine engine and the outer end configured to be located adjacent to an outer casing of the gas turbine engine; an aperture wall located at the inner end of the viewing tube assembly, the aperture wall extending radially inwardly from an interior surface of the viewing tube assembly; an optical element located within the viewing tube assembly adjacent to the inner end and spaced from the aperture wall to define a cooling and purge chamber therebetween; an aperture defined in the aperture wall for passage of light from the hot gas flow path to the optical element; one or more swirl passages defined in the viewing tube assembly between the aperture wall and the optical element for passage of cooling air from a location outside the viewing tube assembly into the chamber, wherein the one or more swirl passages effect a swirling movement of air in a circumferential direction within the chamber, wherein the viewing tube assembly comprises a lens tube, and a lens housing located within the lens tube at the inner end of the viewing tube assembly and defining the one or more swirl passages, wherein the lens housing comprises a tubular member having a first end, a second end, an exterior surface, and the interior surface, wherein the optical element comprises a lens, and a diameter of the lens is at least approximately 5 times greater than a diameter of the aperture, wherein the lens tube includes an end portion that includes a flange that extends radially inwardly from an inner end surface of the lens tube and forms a support structure for engagement by an end portion of the lens housing, the flange including an opening for permitting passage of light from the hot gas flow path into the viewing tube assembly, wherein the end portion of the lens housing spaces the aperture wall from the flange a predetermined axial distance to define an annular area therebetween surrounding the aperture, wherein an axially extending air passage is defined in a gap between the lens tube and the lens housing for supplying cooling air to the one or more swirl passages, wherein, in use, cooling air is provided to the axially extending air passage from shell air passing between a radially outer casing wall and a radially inner casing wall of the gas turbine engine, wherein, in use, secondary cooling air is provided to the viewing tube assembly through openings formed in the lens tube for passage of the secondary cooling air to the annular area, the secondary cooling air passing through the openings adjacent to the end portion of the lens housing.