[ngcles]

Hi John,

The filter you are contemplating will give, at best, a mild contrast-boost on the Horsehead Nebula (IC 434) in suburban areas. It is designed to block the emission from certain street lights such as the Mercury vapour and low-pressure Sodium lights. Other than that it has very wide pass-bands . It will be of very limited use visually on nebulae -- a quite mild contrast boost in suburban areas where there is quite a bit of this sort of lighting. It is primarily an imaging filter, rather than visual use.

As Jethro has already said, you need either a (1) "UHC" type filter -- a narrowband filter that allows H-Beta and OIII (doubly ionised Oxygen) light through. This will give a good contrast gain on nearly all emission nebulae.

Even better for this particular object and just a few others (2) a Hydrogen-Beta filter, that has a very, very narrow pass-band centred over 486nm giving a somewhat higher contrast boost on nebulae like the Horsehead where there is little emission other than via the several Hydrogen lines.

H-Beta filters, because they block so much light are really only usable on good-sized telescopes. As to what the definition of "good-sized" is varies depending on who you speak to. Personally I don't think it has any great benefit with telescopes smaller than 10" aperture, but opinion varies. Smaller than 10", I'd personally be inclined to go with the UHC type that has a wider band-pass.

Most H-Beta filters are designed to be used at a magnification providing an approximately 5mm exit-pupil. That's not to say they won't work at other magnifications/exit pupils, but that's where they work best.

There are seveal leading manufacturers, I'd be inclined to stick with those with a proven track-record or "your mileage may vary ..."


Best,

Les D