Common imaging train

Askar FRA400C + ASI2600MC Pro + filter drawer

The FRA400C is the compact f/5.6 Petzval: 55 mm behind its M48×0.75 thread, 44 mm image circle. 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. A fixed 55 mm optic: the drawer and the boxed extender land on it, and the filter glass is the only adjustment.