The IDAS NBZ-II is a new dual band nebula booster filter - the successor to the NBZ (UHS) filter. The FWHM of the NBZ-II is 8nm (+/=0.4nm) at OIII, and 10nm (+/-0.4nm) at H-alpha respectively. This has the same anti-Haloing characteristics as the NBZ did.
The transmittance curve of interference filters will naturally shift towards the shorter wave length side against oblique light incidence, and this tendency will be more noticeable as the inncidence angle becomes higher as shown in the following figure.
Some of the competitive 7nm or narrower filters will decrease in transmittance by 50% or more when used in a fast optical system (around F2), while the NBZ=II retains 80% or more of the transmittance under the same conditions.
The shift tendency will be stronger in optical systems that have a central obstruction. Taking this into consideration, we have designed the NBZ-II as you can see from the following simulations: