The primary goal of cool down four was to test the 250µm detector array in the flight cryostat for the first time. In order to install the array, many components needed to be added and configured within the cryostat. Chief among these was the
cold optics assembly which was delivered by ASU at the beginning of March 2016. The optics were mounted to the cold plate without incidence along with the backing portion of the magnetic shielding. A flat polished aluminum mirror was also mounted at the location of the 250µm dichroic filter surface as the dichroic filters had not been delivered. The mirror blank was offset from the dichroic mount surface by three 0.375 in precision aluminum blocks to ensure the surface was at approximately the same location as the dichroic surface to ensure the incoming beams were correctly focused on the array.
We require a FPA alignment procedure as the FPA structure was not included in metrology tests done on the cold optics prior to delivery. With BLASTPol a laser beam was projected from the cold optics window through the re-imaging mirrors and back again after being reflected by a flat mirror mounted in front of the feedhorns. The FPA could then be aligned correctly with the optics by adjusting its pointing until the reflected beam overlapped with the output beam at the window. We attempted to repeat this procedure with BLAST-TNG by having ASU hand polish the center of M3 and M5 to reflect the incident laser beam in addition to having them manufacture an M4 specifically for alignment tests with no central hole in the mirror. Unfortunately, even with polishing, the mirror surfaces did not have an optical quality finish resulting in diffraction effects that significantly broadened the laser beam and made the test intractable.
Creating far field beam maps with the cold optics can serve as an alternate method to check the alignment of the FPA. The beam map shows an image of the Lyot stop with the Gaussian response from the feedhorns projected on the annular shape Lyot stop. The FPA is aligned correctly if the projected Gaussian peak is centered on the image of the Lyot stop in the beam map, as was observed by BLASTPol in Figure 3.14. A symmetric beam map would confirm that the detectors are well aligned and are looking at the center of the Lyot stop. A non symmetric image would point to
G10 Support Rod 1 K Plate 270 mK Plate 500 μm Array G10 Support Rod 350 μm Array 250 μm Array Thermal Extensions (500 μm) Thermal Extensions (250 μm)
Figure 7.5: Schematic of the back side of the optics box illustrating the parts of the refrigerator and thermal distribution system. Each array has rigid copper rods extending from the back of the FPA and the 1 K ring through the magnetic shielding. These rods are then couple to the rigid distribution system from the refrigerators with copper braid sections to decouple the structures such that they are not over constrained. The copper rods extending from the 1 K plate are reinforced with G10 supports while the rods extending from the 270 mK system are supported by Kevlar trusses.
the detector plane not being perpendicular to the beam axis which would require adjusting the FPA alignment by an amount determined by the offset of the Gaussian response from the Lyot stop image. Repeatedly adjusting the alignment with this method is prohibitive as it requires cool down cycles of the cryostat between each adjustment prompting us to design a simpler alignment procedure to make future array installations easier.
7.6.1
Thermal Distribution System
A system of heat straps connect the 1 K and 270 mK refrigerators to the 1 K and 270 mK FPA stages as shown in 7.5. The distribution structure provides a stable attachment point for sections of copper braid that couple the structure to copper rods extending from each of the FPAs through the magnetic shielding. We used 0.275 inch diameter Oxygen Free High Conductivity (OFHC) copper braid that was welded to the support section and bolted to the FPA stage extensions. The copper braids were included for two reasons, to ensure the system is not over-constrained and to isolate and dampen any vibrations from the rigid distribution structure. Initially, only the section extending to the 250µm array was constructed to test the design and manufacturing process. The installation required some adjustment to make all the parts connect easily but was effective overall.
7.6.2
Array Mounting
Installation of an array requires the thermometry for the array to be attached, the thermal system to be bolted on, the magnetic shielding around the array to be mounted, and the coaxial cables for the array readout to be put in place. The coaxial cables carrying the input frequency comb route from a feedthrough on the cold plate to the 1 K FPA intercept ring where the cables are heat sunk before routing to the array. The coaxial cables leaving the arrays that carry the detector signals are made from super-conducting cables to reduce signal attenuation. The superconducting cables go from the arrays to low noise Silicon Germanium LNA are manufactured at ASU. From the LNA the signal is fed back out of the cryostat to the readout electronics. The installed 250 µm array is shown in Figure 7.6.
250 μm Array Thermal Coupling for 1 K and 300 mK FPA Stages 300 mK Fridge Cold Head
Pump Pot and 1 K Stage Pump Pot Feedline Valve 350 μm Array Magnetic Shield Cover
Figure 7.6: An image of the back of the optics box with the first installation of the 250 µmarray. It can be viewed through its access hatch in the magnetic shielding. One can also see the 1 K pumped pot and valve along with the 270 mK refrigerator and how they connect to the array via the thermal distribution system.