A near-infrared filter system and study of solar umbral dots at 1.6 µm

Author

Jingshan Wang, New Jersey Institute of Technology

Document Type

Dissertation

Date of Award

Spring 5-31-2001

Degree Name

Doctor of Philosophy in Applied Physics - (Ph.D.)

Department

Federated Physics Department

First Advisor

Haimin Wang

Second Advisor

N. M. Ravindra

Third Advisor

Philip R. Goode

Fourth Advisor

Dale E. Gary

Fifth Advisor

Ken K. Chin

Sixth Advisor

Zhen Wu

Abstract

The observations of solar magnetic fields are very important because they influence events on the sun. Almost all of magnetographs in the world work in the visible wavelength and they are powerful instruments for measuring strong magnetic fields (above 1000 Gauss) on the sun. To measure the weak solar magnetic fields (below 1000 Gauss), a new filter system (magnetograph) working in near-IR wavelength has been developed in this work. This system includes an interference prefilter (FWHM - 40Å), a liquid crystal magnetic analyzer, a near-IR bireffingent filter (FWHM - 2.5Å), a near-IR Fabry-Perot filter (FWHM - 0.12Å), a near-IR InGaAs camera and an image processing computer. The most important part in this system is the near-IR bireffingent filter. A new method of wavelength tuning is developed with combination of calcite and liquid crystal variable retarder. The liquid crystal variable retarder is the wavelength tunable device in every element of the filter. This is the first original design for a tunable Lyot bireffingent filter in near-IR wavelengths. The optical design and test results of this near-IR bireffingent filter are presented in this dissertation.

The bireffingent index of calcite in the near-IR range, μ = n_e - n₀ is a critical parameter in the design of the birefringent filter. The values of μ in the literature have been found to be unreliable in the near-IR range. A new method is developed to accurately measure the bireffingent indices from 1.0 to 1.65 μm using sunlight as the light source, a spectrograph as spectrum analyzer and the InGaAs camera as the detector. The temperature coefficient of μat 1.56 μm is also measured. The value of pat 1.56 gm was used in the design of the near-IR birefringent filter and the filter specifications are found to be satisfactory.

The optical components including the four pairs of calcite and the half waveplates have been tested. The results show that the optical components satisfy the specifications of the filter system. The basic theory of a Fabry-Perot filter is presented and the test procedures of the nearIR Fabry-Perot filter, ET70FS-104, are also considered.

In this dissertation, the first observations and studies of umbral dots at 1.56 μm are made. The image data are obtained with a 50Å wide interference filter centered at 1.56μm with the nearinfrared InGaAs camera as the detector. The image data include two sequences of images of a complicated sunspot and a simple sunspot. The dynamic motion properties of umbral dots are studied from the movies of the two sunspots. From the data analysis, the contrast distribution, size distribution and lifetime of umbral dots are presented.

Recommended Citation

Wang, Jingshan, "A near-infrared filter system and study of solar umbral dots at 1.6 µm" (2001). Dissertations. 489.
https://digitalcommons.njit.edu/dissertations/489