The Tele Vue NP127is a Petzval. I.e., when you focus the OTA, once you achieve focus, you have also automatically achieved perfect back focus.
So, unless you have added flatterners and reducers to the OTA, there is no need to worry about back focus distance, as long as the rack can move the drawtube sufficiantly for your camera.
(If you add a flattener or spacer, then you need to maintain perfect backfocus distance to minimize field curvature.)
So, all you have to do is to get the threads to adapt correctly. You need not also worry about the length of the spacers as long as the camera sensor is within the range that the rack can move. You can, for example, estimate the optical path length to your eyepiece, and try to roughly replicate that distance for your camera.
After focusing, the Petzval OTA will place the camera sensor at exactly the same place as the focal point of your eyepiece. This is why beginers should start with Petzvals, but unfortuantely beginners tend to choose cheap telescopes that are usually also not Petzvals.
If you add a flattener or a reducer to the Petzval telescope, you will have to maintain perfect backfocus unless you use very small sensors as your camera.
Chen
