Common imaging train

Askar FRA400 + 0.7× reducer + ASI2600MC Pro + filter drawer

Askar's f/3.9 0.7× reducer for the FRA400/500 turns a Petzval back into a fixed-spacing optic: 55 mm behind its M48×0.75 thread. 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. f/3.9 is where half a millimetre shows in the corners — tighten the tolerance in the tool and shim accordingly.