About Central ObstructionGenerally, there are two main problems caused by central obstruction, and only one of them is important when using a telescope with high magnifications (i. e. for planets). We will discuss that one later, but first, lets see the less important one, which limits the scope usability with low power and wide field of view in the sky! | |
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The problem with central obsruction at low magnificationsAs a result of central obstruction, the exit pupil of the telescope is not a fully illuminated circle anymore, instead, it has a "ring" shape. This is usually not a serious problem until the diameter of the "ring" is larger than the pupil of our eye (though, during the daylight, when our pupil size is small, this can still become a problem again). But as we decrease the magnification, the exit pupil grows, and finally becomes larger than our pupil. This is the point when obstructed and unobstructed systems start to behave differently: in an unobstructed system, we can easily continue to dectease the magnification and thus increase the exit pupil size of the scope, we lose some light, but the image remains practically perfect. But if the system is obstructed, then parts of the field of view will become dark, as the central obstruction in the exit pupil might cover our pupil, so, in some directions, only a little or zero light will make its way through the exit pupil of the scope and the pupil of our eye.This way, central obstruction limits the scope's usability at low magnifications. | |
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