Abstract
This chapter examines the characteristics of the intensity envelopes of star images formed by ground-based telescopes. These envelopes arise as the Fourier transform of the product of the atmospheric MTF and the telescope OTF. The seeing disc star images delivered by large (> 1 meter) telescopes typically measure about 1-arcsec across which, sadly, is about one or two orders of magnitude broader than would be delivered by diffraction-limited versions of the same telescopes observing in the absence of atmosphere. However, at the end of the chapter, we introduce the subject of images cores. In good seeing conditions, these features are sometimes seen in star images at visible wavelengths, but at IR wavelengths they appear routinely in almost any seeing conditions. If cores are present, the average intensity envelopes then comprise highly-resolved (diffraction-limited) core features surrounded by the much wider halo features. The intriguing properties of core and halo images are examined in detail in the next chapter.
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Notes
- 1.
Only the core energy fraction is determined by the seeing.
- 2.
For diffraction-limited telescopes, the angular width of the core depends on the imaging wavelength and the telescope diameter and is roughly given in radians by the formula, \(1.22 \cdot \lambda /D\).
- 3.
This assumes that we can continue to build ever larger telescopes. Theoretically, a diffraction-limited 100-m telescope imaging at 0.3 μm should deliver angular 0.75-mas resolution. To further improve, resolution would require an even larger instrument.
- 4.
The wavefront structure function corresponding to a Gaussian representation of \(\rho \left( \varepsilon \right)\) may be seen (Sect. 8.4) as similar to the function \(\varepsilon^{5/3}\) rolled-off at an appropriate \(L_{0}\) value.
- 5.
The visible wavelength, 0.78 μm, corresponds to the upper limit of scotopic vision.
References
CFHT Site Characteristics. (2003, January). CFHT Observatory manual, version 1.
McKechnie, T. S. (1990). Diffraction limited imaging using large ground-based telescopes. In Proceedings of SPIE, V. 1236, Symposium on Astronomical Telescopes and Instrumentation for the 21st Century (pp. 164–178). 11–17 February.
McKechnie, T. S. (1991). Light propagation through the atmosphere and the properties of images formed by large ground-based telescopes. JOSA A, 8, 346–365.
McKechnie, T. S. (1994). Another route to sharp images. Sky and Telescope Magazine, 88(2), 36–38.
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McKechnie, T.S. (2016). The Average Intensity Envelope of an Unresolved Star Image. In: General Theory of Light Propagation and Imaging Through the Atmosphere. Springer Series in Optical Sciences, vol 196. Springer, Cham. https://doi.org/10.1007/978-3-319-18209-4_9
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DOI: https://doi.org/10.1007/978-3-319-18209-4_9
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