[h0ughy (David)]

Can't quite get my head around this backfocus distance for the camera the manual says that the adapter is 17.5mm and the camera sensor is 6.5+4 =28
So do I need a spacer to get 29 or doesn't it need it?

From the celestron manual

Backfocus– For all flat-field astrographs, including the RASA 8, it is important that the camera’s sensor is positioned at the proper distance from where it mounts to the telescope. Otherwise, performance at the edge of the sensor, especially larger sensors, may suffer. For the RASA 8, this location is 29 mm from the top surface of the lens group cell. A camera’s backfocus, also known as its flange focal distance, is the distance between the camera sensor and where the camera body mechanically connects to a camera lens or telescope. Therefore, cameras with sensors deeply recessed within their body having more than 29 mm of backfocus cannot be used with the RASA 8 as the sensor cannot be placed at the proper location. DSLR cameras cannot be used with the RASA 8 due to their large obstruction and long backfocus. Installing a Camera If using a camera that mounts with M42 threads, the supplied M42 camera adapter can be used. This will likely require utilizing optional M42 extension rings to place the camera sensor at the proper location. When using the M42 camera adapter, the required backfocus distance is 25 mm. So, for example, if the backfocus of a camera is 17 mm, then 8 mm of M42 extension ring(s) between the camera and M42 camera adapter will be needed. For best results with larger sensors, get the backfocus within 1 mm of the required distance. The supplied C-thread camera adapter can be used to attach any camera which mounts with a standard C-thread. If the camera’s sensor spacing meets the C-thread standard (i.e. 17.5 mm backfocus), no additional adapters are required.