The H-alpha 6 nm CCD Filter, size 36 mm,
Mounted in Low Profile Non-Threaded Cell by Astronomik:
The Astronomik 6 nm 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 6 nm is optimized for a maximum contrast between the (H-alpha emitting) object and the background. The contrast gain is much higher than with our 12 nm 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 (6 nm 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 12 nm version. The gain in contrast is much higher due to the smaller FWHM. The 6 nm 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 6 nm 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 6 nm H-alpha together with the 12 nm 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 6 nm 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 12 nm 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 36 mm 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 31 mm and 36 mm filters in a low profile (3 mm) non-threaded cell. O-rings, washers, or spacers are no longer required to hold the filter in place, and because the cell is only 3 mm thick, it works well in all appropriately sized filter wheels. Mounted 31 mm and 36 mm Astronomik filters will also play nice with unmounted versions in the same carousel.