Spitzer's telescope was a lightweight reflector of Ritchey-Chrétien design, with a mirror measuring 85 centimeters in diameter. It weighed less than 50 kg (110 pounds), and was designed to operate at an extremely low temperature.
All of its parts, except for the mirror supports, were made of beryllium, which is extremely light but very strong. Beryllium works especially well in the construction of infrared telescopes, because it has a low heat capacity at very low temperatures, meaning that it is very easy to cool down quickly. It is important for as much of the telescope as possible to be built from a single type of material, because different materials expand and contract by different amounts as they heat up and cool down. Building the telescope out of different materials could cause it to flex out of shape as it changes temperature, putting extra stress on the joints and causing the telescope to go out of focus.
The telescope was attached to the top of the cryostat, which kept the scientific instruments very cold.