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Basically it sounds like the primary and secondary are set to far apart , if the focus is happening at. 1/2 km you need to bring then closer together. , by a good 50mm or so judging the fairly long fl of the 16 inch GSO,s .
How much in focus do you have left at 1/2km ? , also check the focuser draw tube for a removable extension tube first before moving anything as they sometimes come with one pre-attached that just needs removing for visual.
Also what type of 1 1/4 - 2 inch adapter did the scope come with. ? The longer type would allow the 2 inch and longer 1 1/4 ,s to focus but not the shorter 1 1/4 eyepieces , try just holding the shorter eyepieces in the 2 inch hole (carefully) without the adaptor and see if they focus to infinity .
Photos of the focuser would help .
Brian.
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