Carol Chan

The Cosmology Large Angular Scale Surveyor (CLASS) is a complex ground-based observatory featuring an array of four precision telescopes situated in the Atacama Desert in Chile. CLASS is specifically designed to measure the faint polarization signals in the earliest light of the universe, the Cosmic Microwave Background (CMB). At the heart of each CLASS telescope is a sophisticated cryogenic receiver, which cools the ultra-sensitive superconducting detectors to nearly absolute zero temperature (~50mK). In this project, I built and characterized the cryogenic receiver for the fourth CLASS telescope, and led team efforts to deploy the telescope in the Atacama Desert in June 2025.

Key Responsibilities & Tasks:

• Managed the production lifecycle of custom parts (CNC, laser cut) for the CLASS cryogenic receiver by liaising directly with international manufacturers, reducing production cost by over 30%
• Conducted metrology measurements using a portable Coordinate Measuring Machine (CMM) (FaroArm) to validate optical component alignment
• Coordinated and led efforts for the deployment, assembly, and successful commissioning of the CLASS 90GHz telescope in the in the high-altitude site at the Atacama Desert in Chile
• Managed operation and troubleshooting of an ultra-low temperature (below 100mK) dilution-refrigerator-based cryogenic receiver of the CLASS telescope
• Fabricated custom parts using manual machining (milling and drilling), achieving over 50% cost saving

A rendered model of the cross section of the CLASS cryogenic receiver, which is built off a custom-made Bluefors cryostat. The different stages of the receiver are defined by their nominal temperature, namely 300K, 60K, 4K and 1K. Sky signal comes from the left, passes through a series of filters and lenses, eventually arrives the focal plane where the detectors are continuously cooled to 50mK by the dilution refrigerator.

The reflective half-wave plate (HWP) is a polarization modulator designed for the CLASS telescope that modulates incoming polarization signal, thus separating the faint polarization of the Cosmic Microwave Background from the much stronger instrumental noise. The HWP consists of a wire grid and a reflective surface, which is rotated to modulate the polarization signal. I took part in the design, characterization of the HWP, and successfully deployed it into the CLASS W1 telescope in the Atacama Desert in June 2024.

Key Responsibilities & Tasks:

• Designed and modeled with SolidWorks
  • (1) a mounting structure to hold the HWP inside the CLASS telecope
  • (2) a stretcher frame for tensioning and mounting the wire grid of the HWP
• Performed structral analysis of the HWP mounting structure and the stretcher frame using SolidWorks finite element analysis (FEA) simulations to keep the deformation of the stretcher bar to be less than 2% of the designed tensison profile
• Managed the production lifecycle of custom parts (CNC, weldment) for the HWP by liaising directly with international manufacturers, reducing production cost by over 30%
• Coducted metrology measurement of the HWP using portable coordinate measuring machine (CMM) (FaroArm), confocal distance sensor and microscope to minimize mirror tilt with respect to the rotation axis and ensure the uniformity of wire-mirror separation
• Fabricated custom parts using 3D printing and manual machining (milling and drilling), achieving over 50% cost saving

3D model of the HWP

Left: 3D model of the HWP stretcher frame. Right: In lab assembly of the stretcher frame

FEA simulation results for the deformation of the stretcher frame

Me measuring the grid-mirror distance of the HWP before installing it into the telescope!

See relevant publication: Design and characterization of a 60-cm reflective half-wave plate for the CLASS 90 GHz band telescope (Shi, R., et al. 2024)

Infrared filtering is crucial in telescopes employing cryogenic detectors for observations in the far-infrared through microwave. The aerogel filters developed by NASA are made by embedding diamond in polyimide aerogel substrate to scatter infrared radiation. I performed receiver-level testing of these novel aerogel filters with the CLASS cryogenic receiver, and validated its performance with thermal finite element analysis using COMSOL Multiphysics.

Key Responsibilities & Tasks:

• Designed plastic mounting ring for holding the aerogel filters in the CLASS cryogenic receiver
• Integrated the aerogel filters and temperature readout electronics into the CLASS cryogenic receiver
• Performed thermal finite element analysis (FEA) using COMSOL Multphysics to optimize the configuration of filter and validate thermal performance of the filters
• Presented the results in the poster session of the 2024 AAS meeting

Aerogel filter (yellow) mounted in the CLASS cryogenic receiver

Result from COMSOL thermal simulation of a stack of 5 foam filters and 5 aerogel filters inside the cryogenic receiver

Me presenting my work in AAS 2024 winter meeting :)

Check out my AAS digital poster!

Active cooling is essential for photon detectors to minimize thermal noise. I designed a cooling system, for a Silicon Photomultiplier-based astronomical single-photon detector to cool it down to -51 °C. The cooling system consists of a thermoelectric cooler, which actively extracts heat from the detector attached to it , a CPU cooler, which carries heat away from the hot side of the thermoelectric cooloer , and a 3D-printed enclosure, which reduces heat transfer by convection by creating a air-tight chamber around the detector.

Key Responsibilities & Tasks:

• Integrated a CPU cooler, thermoelectric cooler and 3D-printed enclosure to achieve active cooling of the detector
• Implemented temperature tracking and logging using Arduino
• Designed an interface for detector readout visualization using LabVIEW

Set up of the cooling system, which includes a CPU cooler, a thermoelectric cooler, and a 3D-printed enclosure

CAD model of the cooling system enclosure, which is used to optimize detector cooling by reducing heat transfer by convection

I designed and programmed an instant photo printer that prints B&W photos from photos taken by a camera using a STM32 microcontroller.

Key Responsibilities & Tasks:

Programmed STM32 using C to perform the following functions:

• Stored BMP images taken by the camera into an SD card
• Converted 24-bit colored image from the camera to a binary B/W value
• Designed an algorithm to adjust the contrast of the image based on the user input value
• Transmitted the binary data to the thermal printer for printing via UART

Designed and built a four-bar linkage quadruped robot for the Hong Kong Rocobon competition.

Key Responsibilities & Tasks:

• Designed and built a four-bar linkage quadruped robot (400+ parts) using SolidWorks
• Fabricated custom parts using 3D printing and manual machining (milling and drilling)
• Liaised with external manufacturers to facilitate the production of components for the robot

3D model of the quadruped robot designed for Hong Kong Robocon 2019

Demonstration of robot finishing the competition path