Common imaging train

WO Flat 73A + ASI533MC Pro + filter drawer

The Flat 73A is the fixed 1.0× flattener for the Zenithstar 73 and asks for more than the usual 55 mm behind its M48 thread, which is why the stock ZWO extenders do not land on it. 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 longer spacing means a bigger gap than any single ZWO extender — the tool finds the fewest rings.