O-III FILTER?

[BinocularSky]

IMO, the UHC should be a first filter -- it is useful on more objects than the OIII. The OIII "trick" on teh Veil was done at a very dark site, in the early-hours lull of a Messier Marathon.

[Moonshane]

doh! I was just about to order a pair and make some spectacles

[Takahashi]

I know that to be plain wrong; I have used a Lumicon OIII with a 6mm "scope" at x1 (aka the Mk-1 eyeball) -- you wanna see the veil naked eye?

So, a 2" OIII would be good for dSLR/CCD imaging on a 4.5" 'frac? I'd be a happy man if that were true. ...or is it specific ones that are more effective than others?

[MikeWilson]

Are UHC/OIII useful for imaging on an unmodified DSLR? I can't seem to find any info anywhere on this subject, other than to say that the Ha/Hb filters necessitate a modified DSLR or CCD. But what about the other bands?

[great_bear]

Seriously? Wow.

I think it's important to remember that:

aperture = magnification x exit_pupil

- and thus any specific exit_pupil has precisely the same brightness and image resolution, across all telescopes - regardless of aperture*

So when it is said that "Nebula filters are best in large scopes", it's not about the aperture so much as the magnification. i.e. what is really meant, is that it is only in big scopes that you'll get both the required exit_pupil to drive the filter and the amount of magnification required to see those small, planetary nebulae in a interesting and satisfying way.

If however you're interested only in large, extended nebulae, then sure - there's no reason why you can't use a small scope with such a filter.

*measured as a function of image, not of angle, and also assuming optics of a good standard.

[Ben Ritchie]

Are UHC/OIII useful for imaging on an unmodified DSLR? I can't seem to find any info anywhere on this subject, other than to say that the Ha/Hb filters necessitate a modified DSLR or CCD. But what about the other bands?

Replied on the other thread, but H-alpha is much better with a modified DSLR (you can do it unmodified, but the standard filter cuts hard into the signal), H-beta is in the blue/green so can be done unmodified but the signal is always weaker than H-alpha.

I reckon UHC may work out, but isn't really a substitute for a modified DSLR + H-alpha.

[MikeWilson]

Replied on the other thread, but H-alpha is much better with a modified DSLR (you can do it unmodified, but the standard filter cuts hard into the signal), H-beta is in the blue/green so can be done unmodified but the signal is always weaker than H-alpha.

I reckon UHC may work out, but isn't really a substitute for a modified DSLR + H-alpha.

Thanks for that.

What proportion of objects emit in H-alpha? Am I right that the UHC filter on a modified camera would allow through some H-alpha in addition to almost all the O-III and therefore a UHC filter would be the "best" overall (first) filter for a modified DSLR?

Or is H-alpha the way to go?

[Takahashi]

So when it is said that "Nebula filters are best in large scopes", it's not about the aperture so much as the magnification. i.e. what is really meant, is that it is only in big scopes that you'll get both the required exit_pupil to drive the filter and the amount of magnification required to see those small, planetary nebulae in a interesting and satisfying way.

*measured as a function of image, not of angle, and also assuming optics of a good standard.

Thanks, very helpful info there. I'm forever being put-off any kind of narrowband filters due to the "disclaimer"s at the bottom of them, invariably warning potential buyers (or at least giving the impression) that they're only of use in large aperture scopes.