The H-alpha 6nm CCD Filter, size 36mm,
Mounted in Low Profile Non-Threaded Cell by Astronomik:
The Astronomik 6nm H-alpha filter is an extremely narrow emission-line filter for CCD photography. The filter lets the H-alpha light of emission nebulae pass and blocks nearly the whole remainder of the spectrum where the CCD is sensitive. The Full Width at Half-Maximum (FWHM) of 6nm is optimized for a maximum contrast between the (H-alpha emitting) object and the background. The contrast gain is much higher than with our 12nm emission line filters but you won't see as much stars in the resulting images. The design of the transmission curve makes it possible to use the filters with instruments from 1:3.75 to 1:15.
The Astronomik H-alpha-CCD (6nm version) increases the contrast between objects, in this case between the H-alpha emission line and the skyglow background in the same way as the 12nm version. The gain in contrast is much higher due to the smaller FWHM. The 6nm H-alpha filter is the right choice for extremely dim objects, even in the Milky Way, and for observations from heavily light-polluted places. The Astronomik 6nm H-alpha-CCD completely suppresses the emission lines of artificial lighting (mercury (Hg) and sodium (Na)) and skyglow. Transmission losses and chromatic distortions, which arise with other filters, only occur with Astronomik filters when extremely bright aperture ratios come into play.
Using the Astronomik 6nm H-alpha together with the 12nm H-alpha gives you the ability to have a maximum contrast in the H-II objects and to keep the stars in the final image. The Astronomik 6nm H-alpha filter may NOT be used for solar observation. If you plan to do false-color imaging with other emission line filters, you should get the matching filters with 12nm FWHM. Images taken with filters with differing FWHM are very difficult to combine into the final image.
Now Mounted in Low Profile Non-Threaded Cell: Sometimes unmounted filters can be hard to handle. The smaller diameters, such as this 36mm H-alpha filter, can be especially difficult to pick up and insert or remove from the filter wheel, and that can lead to possible damage as well as unwanted reflections from incorrectly installed filters. Astronomik has solved this problem by mounting their 31mm and 36mm filters in a low profile (3mm) non-threaded cell. O-rings, washers, or spacers are no longer required to hold the filter in place, and because the cell is only 3mm thick, it works well in all appropriately sized filter wheels. Mounted 31mm and 36mm Astronomik filters will also play nice with unmounted versions in the same carousel.