The curved supports had an insignificant effect on both the PSF and the modulated transfer function (MTF) values which is indicative of a comparably high resolution. The central spike energy remained the same regardless of the number of arms. One study found that imaging using curved spiders yielded no significant discrepancies in the point spread function (PSF). Some telescopes have been constructed with no spider to eliminate diffraction from the mirror mount, or with just one supporting vane, but these can create a lack of stability, movement, and precision with the secondary mirror. Strategies for diminishing the Airy pattern include stopping down both the focal and pupil plane. The Airy pattern can sometimes be faintly seen and cause an image to be somewhat blurry. This can create diffraction rings, like the Airy pattern. Courtesy of ĭiffraction also occurs along the edge of the aperture. Instead of a spike, the light is now distributed over a less sharply defined area.Įxamples of some spider configurations and the patterns of the resulting diffraction spikes. When the structs are curved into a half-circle, the diffraction spikes are essentially invisible to an observer. The diffraction spike gets spread out over a larger area, so the diffraction lines in the image are not as sharp. It has been determined that if the vanes are curved rather than straight, the diffraction effect is reduced. A single struct creates two diffraction spikes, and two or more diffraction spikes appear as the same geometry of the support arms but rotated by 90 o. The resulting effect on the image depends on the number and curvature of the structs. These diffraction spikes are due to the arms/structs, or the spider, that supports the secondary mirror and can be referred to as diffraction spikes.ĭiffraction spikes are the Fourier transform of the support arms. The starburst effect when viewing with the eye is due to the atmospheric effects, but the spikes remain when imaged even though adaptive optics corrects for such atmospheric effects. We know that stars are (roughly) spherical, yet they sometimes appear imaged with pointed geometries. So, the mirrors are large, can be deformed, cooled, and the resulting image adjusted. The mirror(s) can be bent and cooled to compensate for whatever may be causing distortion, be it gravity, mechanical effects, temperature, wind, etc. Active optics optimizes the primary (and/or secondary) mirror shape dependent upon environmental factors. Now, both are implemented to improve image quality. Courtesy of Kayla FilipekĪbout the same time adaptive optics was beginning to be practically used, so was active optics. Here, one of the primary mirrors is shown in the center and the secondary mirror is the dark object being supported in the top of the image, left of center. The secondary mirrors of the Large Binocular Telescope (LBT) on Mt.
Among other things, namely focusing and directing the light, the secondary mirror assists with wavefront correction for better results when using adaptive optics. Adaptive optics compensates for these atmospheric effects on the wavefront that make it difficult for reflective telescopes to reach a purely diffraction limited system. In practice, larger mirror telescopes (~4 m diameter) tend to have the same image sharpness as those with small mirrors (~20-40 cm) due to the atmospheric effects. However, creating such massive, perfect mirrors is no small feat. In the case of reflecting telescopes, that means increasing the mirror size. Other measures taken to obtain brighter and theoretically better resolution images include making a larger numerical aperture. One of the measures taken to obtain better image quality is to house the telescope at a high altitude to avoid much of the local weather. Imaging Improvement Techniquesīefore diving into diffraction spikes, it must be recognized that imaging stars/planets/objects with telescopes at such distances and through layers of atmosphere can be tricky. Namely, diffraction spikes can cause images of stars to be less than fully representative of the star itself. With all these advancements one problem still remains in optical telescopes. Along the way a number of telescoping imaging techniques were developed and perfected. Over time this curiosity evolved into an entire scientific field of study. Their allure across the ages has resulted in them being imaged using telescopes for quite some time. Stars retain a level of mystique regardless of wisdom or creed despite knowing they are simply balls of gas. They have biblical, cultural, and scientific meaning.
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