For the Owl, a 'true' OIII filter like the Lumicon or Thousand Oaks would probably be best. The NPB will help as well, but perhaps with not quite as much contrast as the narrower OIII filters. As for Tele Vue's Bandmate OIII, that one is *WAY* too wide (nearly as wide as the NPB), so I would pass on getting that filter. As for "100% transmission", no commonly manufactured filter has that despite whatever passband plots you will see. The most one might expect for most production filters like these is in the high 90's (perhaps 97% or so at one wavelength). In any case, the human eye cannot easily detect a change in brightness of less than 10%, so if an OIII filter has a transmission of around 88% or more at one or both of the OIII lines (and has a Full Width at Half Maximum bandwidth of less than 15 nm but more than 8.5 nm), it should work just fine for visual use. Clear skies to you.
I think I am finally catching on!
Bandpass means alot also
I appreciate the info as you have had practical and in the field experience with several filters.
But, can you give me a quick definition of FWHM ?
Does it have something to do with the 496 nm and 501 nm OIII lines? Which have a difference of 5 nm.
I am confused about the 8.5 nm and 15 nm wavelength positions.
FWHM is the full width of the filter at half of its maximum transmission level. Most narrow-band nebula filters have FWHM values in the 23 nm to 28 nm range, while good OIII line filters will have FWHM values of between 8.5 nm and about 15 nm. Generally, the filter passband of an OIII filter will be centered at a point in the visual spectrum that is roughly halfway between the 495.9 and 500.7 nm wavelengths with a peak transmission at that point. There is some slight fall-off in transmission away from that peak transmission point, but most line filters have more than 88% transmission at the two OIII lines, so there is little to worry about there. The Baader OIII is a little on the narrow side (8.5 nm FWHM passband width), as the 495.9 nm OIII line is transmitted at only between 35% and 50%, while the other line comes through at over 90%. However, typically, the 495.9 nm OIII line is weaker than the 500.7 nm line, so the Baader filter still works fairly well. Clear skies to you.