A system comprising a high temperature furnace and an apparatus for thermal imaging of an interior space of the high temperature furnace through an opening in a wall of the furnace, the apparatus being installed on the high temperature furnace and comprising: a camera comprising an optical sensor; a rigid borescope comprising an elongated housing having a viewing end and a sensor end, and a multi-element relay lens assembly within the elongated housing having at least two optical pieces for directing a real image viewed by the rigid borescope to the camera, the viewing end comprising a lens, the sensor end of the rigid borescope operatively connected to the camera; an outer housing having a sensor end portion and a viewing end portion and defining an interior space within the outer housing, the interior space containing at least a portion of the rigid borescope, the outer housing and the at least a portion of the rigid borescope defining a passage therebetween, the sensor end portion of the outer housing corresponding to the sensor end of the rigid borescope, the viewing end portion of the outer housing corresponding to the viewing end of the borescope, the viewing end portion defining a hole, the hole cooperatively arranged to permit the real image to pass to the viewing end of the rigid borescope, the outer housing having a port operatively disposed between the sensor end portion and the viewing end portion,, wherein the camera is operatively disposed outside the passage and outside the port; and a partitioning member operatively disposed to obstruct fluid communication between the passage and the camera; characterized in that the hole is in fluid communication with the passage; the port is in fluid communication with an unenclosed space proximate to and outside the furnace and proximate to and outside the outer housing, the furnace configured to be operated, at least in the vicinity of the viewing end portion of the outer housing, at a pressure that is less than the pressure of ambient air in the unenclosed space; and the port is in fluid communication with the passage and designed to permit the aspiration of ambient air from the unenclosed space into the port, through the passage, and through the hole into the interior space of the furnace. a camera comprising an optical sensor; a rigid borescope comprising an elongated housing having a viewing end and a sensor end, and a multi-element relay lens assembly within the elongated housing having at least two optical pieces for directing a real image viewed by the rigid borescope to the camera, the viewing end comprising a lens, the sensor end of the rigid borescope operatively connected to the camera; an outer housing having a sensor end portion and a viewing end portion and defining an interior space within the outer housing, the interior space containing at least a portion of the rigid borescope, the outer housing and the at least a portion of the rigid borescope defining a passage therebetween, the sensor end portion of the outer housing corresponding to the sensor end of the rigid borescope, the viewing end portion of the outer housing corresponding to the viewing end of the borescope, the viewing end portion defining a hole, the hole cooperatively arranged to permit the real image to pass to the viewing end of the rigid borescope, the outer housing having a port operatively disposed between the sensor end portion and the viewing end portion,, wherein the camera is operatively disposed outside the passage and outside the port; and a partitioning member operatively disposed to obstruct fluid communication between the passage and the camera; characterized in that the hole is in fluid communication with the passage; the port is in fluid communication with an unenclosed space proximate to and outside the furnace and proximate to and outside the outer housing, the furnace configured to be operated, at least in the vicinity of the viewing end portion of the outer housing, at a pressure that is less than the pressure of ambient air in the unenclosed space; and the port is in fluid communication with the passage and designed to permit the aspiration of ambient air from the unenclosed space into the port, through the passage, and through the hole into the interior space of the furnace.