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Danman53
Vendor (DGM Optics)
Reged: 11/17/11
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Re: Filtered - Unfiltered
[Re: RichD]
#5581205 - 12/20/12 06:09 PM
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Dan, i'm talking telescopes mainly. I use a UHC and OIII with 5" and a 12" as I find the best effect with filters for me occurs with at least 100mm of aperture or more. I don't have very dark skies here so a pair of nebula filters on my 16x70s or 10x50 don't work too well (I tried briefly once).
As far as the 10 x 50 mm binos, I didn't care for the NPB (UHC type) view either, although from what others are saying, I should give it another try with some more challenging objects.
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Danman53
Vendor (DGM Optics)
Reged: 11/17/11
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Re: Filtered - Unfiltered
[Re: GlennLeDrew]
#5581350 - 12/20/12 07:53 PM
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Dan,
In the case of emission nebulae, for all intents and purposes the *only* light we see is that emitted at the specific wavelengths passed by the filters we use. From a visual perspective (especially when dark adapted) we can eliminate the deep red H-alpha; our eye is so insensitive to it that it can never contribute more than a couple of percent to the visible light (except for a couple of very unique objects.) To our eyes, the vast bulk of nebular light is O-III and H-beta emission. Depending on the relative intensity of each emission component, we either employ an O-III, H-beta or 'UHC'-like filter, depending on whether we wish to pass one, the other or both.
In any case, because the emission lines are effectively of sub-nanometer width, the narrowest we can humanly make the passband the better, for then we increase contrast the most. And no desired emission light is lost.
Glenn, you could actually demonstrate that theory. This is an 03 modeled with a thin film design program. It has a bandwidth of about 15 nm. At 0 AOI the oxygen lines transmit over 90% TX. If you tilt the filter about 20 degrees the average 03 will shift short about 8nm, nearly completely suppressing 505 and knocking 501 to around 30% TX. So you would suppose an oxygen rich object would dim, or even disappear, if there is no significant energy outside the emission lines.
Edited by Danman53 (12/20/12 07:59 PM)
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GlennLeDrew
Postmaster
Reged: 06/18/08
Loc: Ottawa, Ontario, Canada
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Re: Filtered - Unfiltered
[Re: Danman53]
#5581549 - 12/20/12 10:14 PM
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Quite correct, Dan. Light entering an interference filter too far from perpendicular becomes 'de-tuned.' However, for even a fast scope of f/4, marginal rays enter a little over 7 degrees from perpendicular. A visual nebula filter can well handle this.
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Danman53
Vendor (DGM Optics)
Reged: 11/17/11
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Re: Filtered - Unfiltered
[Re: GlennLeDrew]
#5581642 - 12/20/12 11:08 PM
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Quite correct, Dan. Light entering an interference filter too far from perpendicular becomes 'de-tuned.' However, for even a fast scope of f/4, marginal rays enter a little over 7 degrees from perpendicular. A visual nebula filter can well handle this.
Glenn, I was actually talking about tilting the filter about 20 degrees to shift the the bandpass shorter. Just as a kind of a "what if" to test the emission line hypothesis. But you`re correct about cone angle. It is non-problem because visual filters are so insensitive to AOI, unless you get over about 15 degrees, or an f/2 system ... yikes!
Edited by Danman53 (12/21/12 01:51 PM)
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