Sii filters: The narrow-band filter allows light from ionised sulphur atoms at 671.7 and 673.1 nanometres to pass through. These are the lines in which many planetary nebulae, emission nebulae and supernova remnants shine.
Astrophotography: Monochromatic cameras are best for this. Ideally, you should combine shots with sii, oii and h-alpha filters. This will give you an image that shows the delicate nebula structures clearly and with high contrast.
The narrow passband of this filter, with a bandwidth of only 6.5 nanometres, works even under difficult city conditions to bring out faint planetary nebulae, emission nebulae and supernova remnants, or make them visible in the first place.
