Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001phdt.........7s&link_type=abstract
Thesis (PhD). UNIVERSITY OF ALBERTA (CANADA), Source DAI-B 62/05, p. 2354, Nov 2001, 352 pages.
Astronomy and Astrophysics
Astronomy
Scientific paper
Many important areas of science such as geodesy, astrometry, satellite navigation, and remote sensing require accurate understanding of the amount of astronomical refraction. However, very little work has been done to actually compare the observed and modelled astronomical refraction at high zenith angles. In this study, a ray tracing model using atmospheric data from rawinsondes is for the first time compared with the measured astronomical refraction presented by the setting Sun. These measurements were obtained on December 8, 14 and 22, 1998 from the campus of the University of Alberta in Edmonton, and the Stony Plain Upper Air Station, about 25 km west of Edmonton, Alberta. Astronomical refraction values were measured through a theodolite survey (Edmonton) and photogrammetry (Stony Plain). Photogrammetric images were obtained using a Questar 3.5 inch telescopic lens and then scanned on an Agfa Studio Scan 11 si flat bed colour scanner. Before accurate measurements could be extracted from the negatives, the camera and scanner required calibration. The calibration of the scanner found systematic linear and non-linear distortions of less than 0.6%. The calibration of a consumer grade flatbed scanner has so far, not appeared in the literature. Photographs of star fields were used to determine the focal length (1445.3 +/- 3.6 mm) and the distortions of the Questar lens. A terrestrial calibration method helped verify these results. Both methods showed no measurable lens distortion. The photogrammetric calibration of a Questar 3.5 inch telescopic lens has not appeared in the literature. The theodolite measurements of astronomical refraction from Edmonton and photogrammetric measurements from Stony Plain showed good agreement with the refraction model for the December 14 and 22 sunsets. The poorest fit occurred during the December 8 sunset when a substantial horizontal temperature gradient was present. From conversations with Environment Canada employees it is also possible that the VIZ rawinsonde used on December 8 may have been from an old and possibly defective supply. A Modified U.S. Standard Atmosphere (MUSSA) profile produces a better fit between model and observed astronomical refraction than the rawinsonde profiles. The results of this study suggest that the temperature measurements from the rawinsondes may be too inaccurate to improve on a MUSSA model. Nonetheless, both models showed a significant improvement over the Pulkovo Refraction Tables.
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