H-alpha: This filter is suitable for photographing hydrogen nebulae from areas with light pollution, but also from places with dark skies. The contrasts between objects that glow in H-alpha light and the sky background are greatly enhanced.
SII: This filter is suitable for photographing SII regions from light-polluted areas, but also from locations with dark skies. The contrasts between objects that glow in the light of the sulphur line at 672 nm and the sky background are greatly enhanced.
By combining a low half-width with high transmission of almost 100% in the SII line range, the filter delivers maximum signal in the desired wavelength while blocking all other radiation, ensuring an extremely dark sky background.
The half-width of 6 nm is optimally matched to the use of CCD and CMOS sensors with particularly low dark current. The 6 nm filters are the first choice when observing from a location with extreme light pollution or generally whenever faint objects are to be recorded in very starry areas of the sky.
The 6nm OIII filter increases the contrast between galactic nebulae and the sky background to an extreme degree. By combining a narrow half-width with high transmission in the OIII line, the filter delivers maximum signal in the desired wavelength while blocking all other radiation, resulting in an extremely dark sky background.
Which half-width is the right one? When shooting under dark skies, images taken with a DSLR and even with many cooled CCD cameras are limited by the dark current of the camera and not by the background brightness of the sky. In this case, further suppression of the sky background by a smaller half-width will not reveal any more details in the object! Compared to the 6 nm filters, the 12 nm filters have the advantage that they usually make it easier to find tracking stars with cameras that have a built-in tracking sensor!
If you have a camera with particularly low dark current and good cooling, the 6 nm filters offer all their advantages: Even stronger suppression of sky brightening allows even longer exposure times and thus even deeper images! Due to the low half-width, the stars become tiny, and faint stars disappear almost completely. Especially in Milky Way regions with many stars, the 6 nm filters allow even faint objects to be displayed with high contrast without getting lost in the swarm of stars.
In short, we recommend using 12 nm filters for DSLRs and all cameras with limited dark current.
The 6 nm filters are the right choice in locations with strong light pollution, for cameras with extremely low dark current, and when faint objects require maximum contrast in the image!
