About narrowband:
Generally speaking, typical emission nebulae such as NGC6960 Veil, M8 Lagoon, M16 Eagle Nebula and other pure emission nebulae are suitable for shooting with narrow bands. Some targets that contain both emission and reflection nebulae are not suitable, such as M42, IC434 Horsehead Nebula, pure reflection nebula and galaxies cannot be photographed with narrow bands at all, only narrow bands can be used to improve part of the details.
Filter depends on your telescope, for 294:
- You can use 36mm filter if telescope>F1.7
- You can use 31mm filter if telescope>F3.7
- You can use 1.25" filter if telescope>F7.0
- You can use 2" filter without limitations.
