Experiments with a Novel CCD Polarimeter

Neumayer, Dirk (2000) Experiments with a Novel CCD Polarimeter. MSc(R) thesis, University of Glasgow.

Abstract

Until recently, photo electric devices have mostly been used in stellar polarimetry. They usually involve a rotatable modulator, such as either a polaroid or a half-waveplate with fixed polaroid in the beam of light. This modulator is either positioned at different angles and exposures are taken (at least 3 exposures at different positions are necessary, to compute the normalized Stokes parameters q and u) or it is rotated continuously. The second method has the advantage, that fluctuations in sky-transparency are averaged out. But a major drawback of photo electric devices is that they are essentially single pixel devices, so that only one object at a time can be observed. Another handicap are dead-time problems. If bright objects are to be observed, neutral density filters have to be used to reduce the photon flux to acceptable counting rates which is rather painful since it is the number of counted photons that determines the final accuracy. With the advent of CCD technology these detectors are now more and more applied in stellar polarimetry. CCDs, being imaging devices, allow the observation of whole sky areas or several objects at a time. The method which is usually applied involves taking several exposures with a modulator at different angles. Once again, changes in sky transparency have to be taken into account. Also the total photon count per exposure is limited, this time by the full-well capacity of the pixels and the number of pixels used. In this Thesis a new design for a CCD-Polarimeter is presented which is a hybrid between the traditional photo electric instruments and modern CCD-imaging. Analo-gous to photomultiplier devices, the modulator is rotated continuously. A co-rotating glass wedge in the light path deviates the beam and the eventual image is recorded by a CCD camera. The rotation together with the deviation converts the point-like images of the stars into rings. These rings contain, encoded as intensity modulation along their circumference, all the information necessary to determine the normalized Stokes parameters. By choosing suitable deviation angles, either a group of stars can be observed simultaneously, or a single star, slightly out of focus, can be observed with high accuracy, since a high photon count can be achieved by spreading the light into many pixels along the ring. Theoretically with the presented instrument an accuracy for p of about +/-7 x 10e-5 could be achieved with one single exposure. Variations in sky transparency do not cause any problems since they are averaged out. This design has been explored by means of a simple prototype and new data reduction techniques have been developed to reduce and analyze the obtained data. In particular, finding the circles and extracting the information from the intensity modulation along the circumference of the circle. Experiments with the prototype have been carried out, both in the laboratory using artificial 'stars' and at the Cochno observatory, this time involving real stars. Different modulators and deviators have been tested. During the tests various problems were encountered, the instrument was returned to the workshop and modifications were made. This cycle of testing and improving was quite time consuming and the main problem, the motor inducing vibrations and not rotating uniformly could not really be overcome in the current design. The obtained frames were used to check the data reduction and analyzing software. The presented algorithms fulfilled their tasks satisfactorily. The experiments with the prototype CCD-Polarimeter have shown that in principle it is a very potent technique but with some, mostly mechanical, obstacles to overcome. Based on the experiences in the described test runs, improvements for a further second prototype are proposed. It is self evident that this Thesis has been prepared using ITEX. 26 and its appropriate packages. One of the niceties of this is the possibility of inventing special fonts. It turns out that the stellar images containing the polarimetric information are so beautiful in concept and appearance that it was impossible to resist making one special character which has been incorporated in the title page.

• Item Type: Thesis (MSc(R))
• Qualification Level: Masters
• Additional Information: Adviser: David Clarke
• Keywords: Astronomy, Optics
• Date of Award: 2000
• Depositing User: Enlighten Team
• Unique ID: glathesis:2000-76164
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 16:33
• Last Modified: 19 Nov 2019 16:33
• URI: https://theses.gla.ac.uk/id/eprint/76164