Taking the Twinkle Out of the Stars: an Adaptive Wavefront Tilt Correction Servo and Preliminary Seeing Study for SUSI
Abstract
The Sydney University Stellar Interferometer (SUSI) at Narrabri N.S.W., Australia, operates at optical wavelengths and has a maximum baseline of 640m. In order to form fringes with this instrument the distortion introduced into the wavefront by the atmosphere must be minimized. In SUSI this is done by restricting the aperture to r_0 or less, thereby sampling a basically flat but tilted wavefront. A pair of tip/tilt servos are then used to adjust for angle of arrival. The thesis contains a review of atmospheric turbulence theory including a new approximation for the temporal power spectra for any Zernike coefficient due to Kolmogorov turbulence. The temporal power spectra is calculated using the Taylor hypothesis of frozen turbulence. Unlike previous work of this kind the analysis is performed in the Fourier domain resulting in a solution that does not require numerical integration. A description of the tip/tilt servo follows along with a review of digital servo theory and an analysis of the servo performance. The final section of the thesis deals with atmospheric turbulence measurements made using the system. The tilt correction servo consists of `pyramid' quadrant detectors and piezoelectrically controlled tip/tilt mirrors. With a sample frequency of 1000 Hz the system measures image position and recenters the stellar image. Based on measurements of many stellar objects over several nights the system was found to hold the two beams of the interferometer parallel with a standard deviation of 0.164 + 0.025 arcseconds. With an aperture size of 0.06 meters this implies less than a 2% loss in the visibility measurements made by SUSI. The system has been used on stars as faint as magnitude 6.5 and is predicted to have a limiting magnitude of 7.5 and possibly as high as 8.5. By logging the mirror positions required to center the image a direct measurement of the wavefront phase gradient is obtained. Using this data it is possible to investigate atmospheric turbulence theory and, in particular, the wavefront tilt temporal power spectra. It is shown that at low frequencies the wavefront tilt power spectrum follows the 2/3 power law predicted by the theoretical analysis. At high frequencies the power law becomes 11/3, while the theory predicts 17/3. Since only a small part of the energy of the spectrum is contained in the high frequency domain this is of little practical importance. The difference is due to the fact that the servo measures average phase gradient which is not identical to the Zernike polynomials representing tilt. The system therefore not only corrects for the tilt introduced by the atmosphere but will supply a good estimate of seeing conditions using the same optical path as that used by the visibility measurement system itself. A preliminary investigation of seeing at the Narrabri site was undertaken, resulting in a median r_0 value of 7.1_1.6^+2.1 cm at a wavelength of 500nm. Based on these measurements the seeing at the site has been shown to have a median of 1.3 seconds of arc with seeing better than 1.5 seconds of arc 75% of the time. These values are consistent with similar seeing studies at other observatory sites. (SECTION: Dissertation Summaries)
 Publication:

Publications of the Astronomical Society of the Pacific
 Pub Date:
 August 1994
 DOI:
 10.1086/133459
 Bibcode:
 1994PASP..106..915T