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 OIII filters only when extremely bright aperture ratios of 1:3 and more come into play. The Astronomik OIII filters 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.

Main use

Since enough light must be available to make use of the OIII filter 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 OIII filter instead of the more versatile UHC filter. Due to the high optical quality of the Astronomik OIII filter substrate you will see the same needle-sharp stars as you would from your regular telescope.

more information about the visual Astronomik filters

Suitability

• Visual observation (dark skies): Very good, huge contrast enhancement at O III-emission nebulas
• Visual observation (urban skies): Very good, huge contrast enhancement at O III-emission nebulas
• Film photography: It depends, very long exposure time
• CCD photography: Good, when used with an additional IR-block-filter
• DSLR photography (original): Very good, huge contrast enhancement at O III-emission nebulas
• DSLR photography (astro modified): Very good, huge contrast enhancement at O III-emission nebulas
• DSLR photography (MC modified): Good, when used with an additional IR-block-filter
• Webcam / Video (Planets): Unsuitable
• Webcam / Video (Deep Sky): Reasonable, if light pollution is a big problem and OIII Objects are being observed