Project Design
Systems Overview
Optical Design
Mechanical Design
Systems Overview
The instrument being designed is the optical front end. The instrument will then be integrated into a system, which will then be characterized in a manner to achieve a quality measurement. The front end will be mounted onto a telescope mount, which will point to the place in the sky undergoing investigation.
The optical front end consists of a polarization stage, a focusing optic (if needed), and a mechanical fiber attachment to allow for precise alignment to the spectrograph.
Polarizer Technology
Polarization Plate
A polarization plate is a piece of glass that is set to polarize light that passes through it. Polarization plates are not inherently dynamic. Although they can be characterized well, they must be rotated to different angles for the polarimeter to measure different angles of polarization. Different coatings can be applied to change the throughput of a polarization plate. Most are characterized to accept a certain range of polarization, but this error will vary based upon the technology used to create the plate.
Filter Wheel
A filter wheel has a set number of slots for multiple polarization plates, each one lined up to give a different angle. The wheel then switches between the different plates to give the incoming light different angles of polarization. The advantages of a filter wheel are more precise polarizations and a decent rotation speed, getting as low as 30 milliseconds to switch between adjacent filters.
Rotation Stage
A rotation stage uses a single polarized plate that is rotated to pre-determined positions to give different polarizations to the light passing through it. The response time between stages is generally the longest of all the options, many times reaching into multiple seconds instead of milliseconds. The rotation stage allows for a single optic to be used, which will increase the overall accuracy of the device. With each added element in a filter wheel, it will increase uncertainty in the measurement.
Liquid Crystal Variable Retarder
A liquid crystal variable retarder uses an applied voltage to phase shift the light passing through a transmissive element. When no voltage is applied, maximum retardance is achieved and the liquid crystal molecules align parallel to the glass. High voltages cause the liquid crystals to align perpendicular to the glass surface and a minimum retardance is achieved. The response time can be as quick as a few tenths of a second. Despite the speed advantage, LCVRs have a lower spectral range than most polarization plates. This will limit the total range of the measurement if this option is chosen.
Optical Design
Optical Design was accomplished using Radiant ZEMAX. Primary concerns were chromatic abberation (focal length dependance on wavelength) and field of view (as focal length changes with wavelength, field of view changes). The following is a model of a UV-IR silicon lens.
Mechanical Design
Mechanical design was accomplished using Dreamspark Mechanical CAD software.
Rotation Stage
Lens
Lens Tube 1
Integrating Sphere
Fiber Connector
Lens Tube 2
Custom ABS Plastic Case
Mounting Plate
