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The F/ number (referred to as the F/Stop) is the aperture, which is a measure of how 'open' your lens is. Because you're using the telescope instead of a lens the camera can't control or measure it so it's displayed as F/00 or whatever Nikon decided to call it (yes, this is the same regardless of what camera you use). Basically your only options for controlling the exposure is the shutter timing and ISO.
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