François Amar, David Batuski, Richard Eason, Brian Frederick
A theoretical description of a simple optical train, modulated signal based spectropolarimeter is discussed. The design includes, after the telescope optical tube (in this case, a 9.25” Schmidt Cassegrain), a rotating quarter waveplate (compensator), a fixed linear polarizer (analyzer), and transmission grating of 100l/mm, with a ZWO ASI290mm astronomical camera. The practical constraints on implementing such an instrument are discussed, and the construction of the spectropolarimeter is detailed, including the necessary optics, optomechanics, and electromechanics. The rotation and recording of the rotating compensator is facilitated by a motorized connection with proportional feedback control, and the uncertainty in measuring the angle is discussed. Calibration data from measurements with linear and circular polarizations was collected and analyzed, and exhibited close to the expected theoretical performance. Full analysis of the light in terms of the 4 Stoke’s parameters was hindered by lack of knowledge of the relative angle between analyzer and compensator, leading to ambiguity in the S1 and S2 parameters; however, even with this ambiguity, degrees of polarization can be determined. Astronomical data was collected on the Moon, Arcturus, and Vega. The analyzed moonlight exhibited clear linear polarization, with a degree of polarization of 4.13%. The polarimetric analysis of Vega and Arcturus suggested potential for polarization, but more analysis is needed. Spectroscopic performance was confirmed by measuring the hydrogen Balmer lines in Vega, leading to a plate scale of 0.6434 nm px . In summary, a low cost, low complexity spectropolarimeter capable of measuring all 4 Stoke’s parameters of stellar spectra has been constructed and tested on known generated polarization signals and astronomical objects.
Marchio, Jacob, "Design and Construction of a Computer Controlled Astronomical Spectropolarimeter" (2020). Honors College. 604.