Common imaging train

Sky-Watcher Evostar 72ED + 0.85× reducer + ASI533MC Pro + filter drawer

The 0.85× reducer/flattener for the Evostar 72ED asks for 55 mm; Sky-Watcher does not print its rear thread, so the first joint is flagged for you to confirm. One-shot colour usually runs a single dual-band filter in a drop-in drawer instead of a wheel. The M42 drawer threads straight onto the camera and leaves a gap that one of the extenders in the camera box closes — once the filter glass is accounted for. The unknown rear thread is flagged; the arithmetic itself is the familiar drawer-plus-extender answer.