Recently I've been trying to spot a couple of nebulae, IC59 and IC1318. The skies at the site are modestly dark (4.5 on the Bortle scale), I'm using an 8" scope, and I've slowly scanned around the areas endlessly with a low magnification/wide angle eyepiece but I haven't been able to find either of them.

Is a filter likely to significantly help in this situation? Would a nebulae filter be more helpful than a light pollution filter?

  • $\begingroup$ By definition a filter filters out some light so if your just using your eye then by using a filter you will just be making it harder for yourself. If your using a camera though it would make more sense to use a filter as you can just increase the exposure time to counteract the effect of the filter. For nebulae a H-Alpha filter would be the best one to use. $\endgroup$ – Dean Feb 27 '16 at 16:39
  • $\begingroup$ @Dean You should consider posting this as an answer. Also from hat I've heard the increase in contrast caused by a filter can cause items otherwise hidden in sky glow to, "pop", a bit making objects easier to spot. Obviously I'm not sure about that info. $\endgroup$ – Ceribia Feb 27 '16 at 18:11
  • $\begingroup$ I wanted to make sure I knew what you were talking about, if it was for human eye or camera imaging. The filters do help see "hidden" objects, but again thats with a CCD camera and longer exposures, increasing the photon count in a particular wavelength, so making such features as the Hydrogen in nebulae stand out more. $\endgroup$ – Dean Feb 27 '16 at 20:50

With an 8" scope, a filter will very likely give you better results than observing without a filter. Although a filter does block light, the crucial aspect is that a filter increases contrast (by blocking light pollution and extraneous wavelengths of light more than the nebula), thereby allowing you to spot low contrast diffuse nebulae (like IC59 and IC1318) much more easily. This is in fact more critical for visual observation than for photography because it is possible to increase contract in post-production with photography. You will find that visual astronomers go to great lengths to increase contrast--baffling, flocking, premium mirrors, etc.

Light pollution filters are broadband, meaning they allow all light except light emitted by streetlights. From darker skies (like bortle 4.5), this will not give dramatic results. I would recommend using an OIII filter to start with. The OIII filter is a narrowband filter, meaning it cuts all light except at a very narrow wavelength range from ionized oxygen. Very often OIII filters make the difference between being able to see an object and not being able to see it, even from very dark skies. As an excellent test of the OIII filter, check out the Veil Nebula too; from your skies, this object would be rather difficult without an OIII but rather stunning with an OIII filter.

  • $\begingroup$ I think that the objects in question are predominantly H II regions,which means that most of their light is red. Consequently an OIII filter will not help - quite the opposite - but I agree an OIII filter is excellent with the Veil nebula. $\endgroup$ – Dr Chuck Sep 15 '16 at 21:35

I found this article, which suggested a UHC filter could cover more bases if you can only buy one:


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    $\begingroup$ This is what we consider a 'link-only answer'. They are not a good fit for our format, so please include vital information from your source in your answer. $\endgroup$ – SE - stop firing the good guys Sep 15 '16 at 10:18

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