本帖最后由 cosmosEX 于 2014-3-13 19:03 编辑
The terms “stable” and “accurate” must be taken in their strictest sense here!This is because each little jerking motion and each lack of precision, while hardly noticeable to the eye of the observer, is mercilessly amplified by the magnification used with the telescope. A small jerk measured in millimeters can quickly cause the observed celestial object to disappear from the telescope’s tiny field of view. This demand for stability applies not only to the actual mount but equally to the associated tripod or stand which in most cases is the principal weak point of the system.
The requirement of having the greatest functional accuracy possible along with a low vibration setup of course entails a certain level of mechanical stability. The designer can address a critical aspect such as vibration dampening only by using materials of solid construction, and especially a reasonable weight ratio between mounting and telescope tube.
On the other hand, this requirement conflicts with the desire of the user to have a compact, light, and easily transportable unit. The mount must therefore not be too heavy since otherwise the equipment becomes unwieldy. And this is not the only area where the gap between need and necessity becomes very wide. Since the component “mount” is not very high on the list of performance criteria for the scope, the willingness to invest a significant portion of the purchase price in an effective mount is quite low, especially with first-time telescope buyers. Of course, suppliers then react accordingly by offering telescopes with highly impressive apertures and magnifications, but placed on light and cheaply produced mounts. Although these mounts may no doubt be able to hold your scope, it is often out of the question to speak in terms of stable and accurate handling. What must always be taken into account is that the best optics can never be utilized properly if the mount is not worth anything. For example, the constant backlash motion or lack of tracking can cause every observing session at higher powers to become a war of nerves, or even frustrate it entirely.
The best mount is also not of much use if it does not have the right foundation. This is generally supplied in the form of an extendable tripod made of hollow aluminum. Less commonly used as tripod material is wood, which is more effective in attenuating vibration.The steel post that used to be quite common as the support structure is rarely sold now. In the vast majority of cases, the proud owner of a purchased telescope will thus have to make do with an aluminum or steel-tube tripod. Yet it is precisely these that are frequently the weak point in the stability of the overall telescope design. One reason no doubt for this is in part the excessive trend to minimize weight that is spreading to an ever-increasing degree from the telescope itself to the mount and on to the tripod.
In fact, the opposite conclusion would make more sense for reasons of stability, since it is after all only a stable and massive support that can effectively dampen a telescope that has started to vibrate. As a result, it is not surprising when a relatively heavy telescope causes the relatively small mount underneath it to vibrate along with it, instead of being attenuated by it. And this whole system is still supposed to be supported by an overloaded tripod made of thin, light hollow sections. Another reason for this no doubt is the potential for the manufacturer to reduce costs, which can best be saved on the tripod after taking into account the expensive optics and mechanically costly mount.
————《Handbook of Practical Astronomy》
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