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My experience with a diagonal (and a Nikon) is that the camera is too heavy and the diagonal gradually rotates. You need a 80 mm extension tube but probably not the barlow for nebula. I am still experimenting but am held up by cloud. The big problem with all those tubes and joints is the weight of a DSLR camera on them, CCD and web cameras are a whole lot lighter. But there is nothing you can do about it from what I can see the ED80 is designed to allow for a diagonal (which adds 80mm to the folcal path) so you need the extension tubes if you don't use one.
Roger
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