Re: (IAAC) Markarian 205 - 8inch F/10

Possibly one for the "it's never that simple" file...

>One exception could be the use of filters for observing nebulae.
>I have little experience with filters on scopes other than the
>30" but have to believe that, even at low powers, a large scope
>will outperform a small scope when using filters such as the
>H-Beta or O-III.  Have the more experienced observers on the
>list found this to be the case?

In my own limited experience, Dave, I suppose I'd have to ask what you mean by "outperform". If the question is whether filtered images of emission features are clearly improved under the same skies by larger aperture, I'd have to say yes. But then I also have to say *MY* experience is that even WITHOUT any filtering, images of extended objects (emission or continuum) are better in larger apertures than smaller, with few exceptions.

I think it's pretty clear that image brightness (which is related to aperture, but also to sky conditions and particular features of each object) has a major impact on quality of view through a narrow-band filter. In fact, generally the only NEGATIVE effect of using narrow-band filters (aside from false color distractions) is image darkening! So that in order for a filter to be worth using (e.g., on a planetary vs. a reflection nebula), any increase in contrast vs. surrounding sky must overcome the general reduction in image brightness caused by the filter. In the end, any improvement may even become a function of an individual observer's spectral response curve!

Clear skies!

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