From the start, the Astronomik OIIIfilter has been very specifically designed for the visual observation of gaseous and planetary nebulae. The extremely narrow pass band of the two OIIIlines brings a substantial contrast gain to these lines even under best observation conditions. On faint super nova remnants and faint planetary nebulae the Astronomik OIIIfilter will often make the difference as to whether the object can bee seen or not!
The optimal aperture ratio for the use of the filter is 1:3.7 to 1:15 with apertures of more than 6" (150mm). Transmission losses and chromatic distortions, which arise with other filters, occur with Astronomik OIIIfilters only when extremely bright aperture ratios of 1:3 and more come into play. The Astronomik OIIIfilters thus can expand the view of dispersed objects, generally speaking, to the whole visual field of view of the eyepiece and are not limited to only the center of the eyepiece.
Since enough light must be available to make use of the OIIIfilter it is best to use this filter with apertures of more than 6" (150mm). Smaller instruments do not gather enough light for meaningful and satisfying astronomical work. However, please note: Many experienced Deep Sky observers, with apertures of more than 10" (250mm), prefer using the OIIIfilter instead of the more versatile UHC filter. Due to the high optical quality of the Astronomik OIIIfilter substrate you will see the same needle-sharp stars as you would from your regular telescope.