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Hi James,
In theory you will gain an extra 30mm of back focus by going to F5 but that is an expensive fix. The secondary would be OK.
Another slightly less expensive fix is a low profile focuser that is an inch shorter in height to the one you have, just make sure the mounting holes line up without having to use an adapter.
Before spending heaps of money why not try getting some longer collimation bolts and springs.
Alternatively take the OTA tube to a machine shop and get them to cut off an inch off the bottom and re-drill the mounting holes for the primary cell.
Another thing to be aware of is, you don't want the focuser racked all the way in as the focuser tube will cast a shadow on the primary.
Regards
Bill
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