The Celestron adapter you are using is designed for telescopes having focal length greater than 650mm or "slow" telescopes, or to be more specific, you will have greater travel of the knob to achieve focus which suits this kind of barlow but your telescope is of 450mm or "fast", which is the opposite of the latter. So this will actually impact the focus and it has nothing to do with camera settings, what you can do is try to lengthen the adapter and if the problem is still not solved, buy an adapter that is designed to do the job for your telescope. Get a Universal T-adapter, which is an adapter and also a barlow lens that will suit almost every telescope. See if the telescope is collimated, although it has nothing to do with the focus adjustment it might improve viewing experience a lot, If you are not familiar with the term collimation, see a bright star you can see in your night sky and zoom it with eyepiece on until you see the secondary mirror obstruction and the spider vanes, if the dark circle is at the center it is collimated and you don't need to worry, if not adjust the screws in the secondary mirror until you see the circle at the center. And for some astrophotography tips see my answer here