Doing narrowband images in helium, neutral oxygen or even argon bands requires some narrowband filters that aren't (yet?) made by astro-filters makers. The problem may be solved by machine vision / scientific lab filters. More narrower and better they get the more they will cost - even more than astronomical filters. Semi-narrow filters may be found cheaply (Omega Optical on Ebay for He-I and He-II). 10 nm narrow filters are also available through Edmund Optics - and I've chosen 4 EO filters for experimental narrowband imaging.
I've ordered 4 EO filters through gerdneumann.net which also mounted them in Astronomik filter cells (double cells as filters are thick) with narrow collars as the filters are only 25 mm in diameter.TechSpec 470nm filter that can be used for this band: TechSpec 589nm filter, but no detailed specs, so this is a interpolation from a filter centred on other band: TechSpec 632nm filter for this band. This filter except of main O-I band should catch weaker emission at 636,3 nm and S-III (not S-II that we use) 631,2 nm if it exists in given nebula. TechSpec 710 nm, but it has only ~55% transmission. There is no detailed specs, so I had to interpolate from a 730 nm filter specs:
Exotic narrowband seems to work best for planetary nebulae or similar supernovae nebulae (Crab Nebula). For H II region nebulae the emission is to much diffused.
It also prefers bigger apertures providing more photons and bit better resolution than average small APO scope.
[O I] may give most interesting image (if the emission exists), while helium will be more "boring" diffused blob near the central star. [Ar III] seems to be to weak to produce usable and nice images (except maybe M1 and similar objects... to be tested).
- M27 - H-alpha and [O I]
- M16 - H-alpha and [O I]
- M42 - [O III], [Ar III], green continuum
- M42 - [S II], [He I]
- M42 - [O I] and wide-band CLS-CCD
- NGC 7009 - [He II] and other
- M27 - [He II], [Ar III] and others
- M57 - [He II], [Ar III] and others
Quick [O I] test on M27
M27 in He II band