Some open issues related to the mechanical design

• For the 7 pixel there are 4 G-10 standoffs per pixel. 61 pixels will need a different arrangement.

• A ¼ turn “quick-locking” mount is being designed for the feed horns. A prototype is in the shop.

• A refrigerator dewar extension tube will be used to allow access to the down converters.

• An extension tube will be used to house the bias circuitry. This will allow access to the pcb without opening the entire cryostat.

• A “dead” pixel will require removal of the entire receiver for maintenance. Each pixel is NOT in a cartridge.

• The output waveguide is being studied. These 122 waveguides require thermal isolation and thermal flexing.

Thermal Expansion

Estimated thermal coefficient of expansion considering aluminum components with TCE of 22 x 10^-6 m/m K and length L = 0.5 m, delta T = 285 gives 3.1 mm. With order centimeter wavelengths this change is significant. Compensation for this compression during cooling must be accomplished by either short rigid stainless steel waveguide, SS coaxial cable bent to flex, a waveguide gap similar to the thermal transition, or a flexible waveguide. Paramount to the method chosen is baseline stability considerations. Flexible waveguide is known to have stability problems while SS coaxial cable presents stability problems to a lesser extent. For improved stability, development and testing thermal transition techniques, or other unknown options is required.