На сайте фокусера кажись нашел ответ. кольца предназначена для размера 2" некоторые делают небольшой углубление в трубе. из-за этого и болтается. У них есть кольца ниже чем обычное расположение, что позваляет обхватывать не срезанную часть трубы.
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4.5" travel "low position" compression ring style drawtube. This drawtube also uses 3 thumbscrews for extreme grip like the models above but the brass compression ring is located in a lower position so the brass compression ring clamps around the true 2" OD of diagonal, not at the beveled under cut area. Some Diagonals such as AP and WO brands have bevels back cut in to the 2" OD barrel. MoonLite's "Low position" drawtube solves the misalignment problem and grips the diagonal properly to maintain accurate collimation and load bearing ability. Please note- the "low position" comp ring bump out on the top of the drawtube adds .8" to the height of the focuser, not 3/8" like the regular comp ring design does.
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