This is a picture of the main cage assembly. The three rods are 3/8" Invar-36 which is a virtually zero expansion metal used to maintain the critical

film to mirror distance. the upper ring is the support for the spider/ focuser. The lower disk is the rear cap of the tube and holds the borofloat /

mirror and has a access cap for the twin thermocouple wires to monitor the temperature of the primary mirror and the tube.

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