Filters for nebulae, galaxies & other DSOs

[25585]

Reading the forums, it seems certain filter types are best suited to certain DSOs, in particular different nebulae. 

If OIII filters are good for most nebulae, which nebulae benefit more from other types?

If UHC filters help with contrast for galaxies & fainter DSOs, are there certain suffixed types better than others. 

Hydrogen Alpha, Beta & various other filters, get mentioned. What are those for?

[John]

These articles by David Knisely are often linked to but are a pretty useful:

http://www.prairieastronomyclub.org/useful-filters-for-viewing-deep-sky-objects/

http://www.prairieastronomyclub.org/filter-performance-comparisons-for-some-common-nebulae/

Personally I've not found a filter than improves the view of galaxies although I know that some use a Baader Neodymium for this purpose.

 

 

 

 

[Ricochet]

An OII filter passes the the two OIII emission lines.
An H-beta filter passes the H-beta line
A UHC filter passes both the H-beta line and the two OIII lines.

All of these are for visual observations of nebulae. Which filter will work the best depends on the exact nebula being observed (see the links John provided) and I also think on local light pollution levels (the wider the bandpass, the more light pollution must also be passed). 

Any nebula that emits H-beta will also emit much more strongly in H-alpha but you cannot see it because the scotopic (nighttime) eye does not detect that wavelength. Astrophotographers, on the other hand, will use H-alpha instead of H-beta because the camera can see it. In comparison the photopic eye response is shifted towards the red end of the spectrum so during the day you can use H-alpha filters for visual observation of the sun (as part of a safe solar viewing setup, don't try using only an H-alpha filter).

Here is a graph I made previously to show these nebulae emission lines against the scotopic response of the eye (credit to Wikipedia for the scotopic response curve)

 

The only filter I have found to have any sort of positive effect on galaxies is an Astronomik CLS filter (and that is a very minimal improvement on a few, not all, targets). From a truly dark site I suspect it would have no effect at all. Star clusters respond best to increasing magnification (up to the point where exit pupil drops to ~1mm and Airy disk appears).