Central Obstruction - Decreasing contrast Part 2Now returning to central obstruction, it will be easy to see why aperture has a very hard time to compensate for central obstruction. Let's examine a Maksutov with 150mm aperture and 33% central obstruction (50mm secondary obstruction)! This scope has an effective contrast equal to a 100mm perfect telescope system! But there is a serious difference: under the just mentioned (average) sky, a 100mm APO performs as an almost diffraction limited system, as the wavefront error added to the (almost zero) internal optical error of this scope is about lambda/4, so the whole system is still near to the diffraction limit. But what happens to the 150mm scope with 50mm obstruction? This scope’s aperture is 2.5 larger than the 60mm coherence diameter of atmosphere, so, it will suffer 2.5 * lambda/6 wavefront distortion, which makes it not better than a lambda/2.4 system, even if the scope features practically perfect optics. So, atmosphere adds more wavefront error to a system with larger aperture, and this way, the 150mm system with 50mm secondary mirror will have much worse wavefront error compared to a 100mm unobstucted system under the sky, regardless of the optical quality of the mirrors. The wavefront error is added by atmosphere, NOT the scope itself, and we can do nothing at this moment to avoid it (though, this situation might change in the future, when adaptive optics become popular on amateur telescopes, but this is surely NOT today's technology...) | |
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What can we do (today) to improve the situation? Naturally we can further increase the aperture of the mirror to increase contrast - and thus, we also increase again the amount of atmospheric turbulence gathered by the scope. It is true, that sooner or later the scope will outperform a 100mm refractor on planets, but this requres a significantly larger aperture compared to the smaller APO, and, as aperture grows, the internal thermal problems of the large mirror telescope start to dominate too. By applying ventillators to cool the optics, we can handle this error, but still, a mirror scope with large central obstruction usually has a very hard time to beat a smaller APO in planetary performance.
It is interesting to note, that not in every aperture range has central obstruction similarly important effect. If the scope's aperture is very large, then the huge amount of atmospheric wavefront distortions start to hide the effect of central obstruction, so, in this range of very large systems, the mirrors give similar, or even better performance than lenses. But in the size range of amateur refractors used today, central obstruction is very important. |
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| This renders all telescope systems with central obstruction out of our search for the Ultimate Telescope, so in the next part we will examine only unobstructed systems. | |
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