Old 15-07-2011, 07:01 PM
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pmrid (Peter)
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Choosing filters

I'm having a look at the NB filters I'm using and considering some changes/improvements. Doing the usual reading has not made the decision easier - rather the opposite.
I'm reading, for example, that as between Astrodon and SBIG, there has been a fair amount of strong lanugage including the suggestion that the SBIG line of filters are only suitable for fast scopes whereas Astrodon are better on slower. But I have no idea why. Also, it seems that some recommend using IR/UV cut filters in addition to the usual NB range. Also, it seems that bandwidth (the +/- on either side of the centre-line) is a significant factor.
I wonder if anyone could demystify the choice of filters for me. I am using what would probably be regarded as a 'slow' system with ratios of 6.3, 6.7 and 11.
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Old 15-07-2011, 07:12 PM
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multiweb (Marc)
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From what I gather the narrower the filters the more precise your tricolor imaging will be. a Ha 3nm will give you a better result than a 7 or 13nm when you recombine your channels with the adjacent Sii for example, no channel bleed. So the narrower the better, but the dearer as well.

The other thing to consider is reflections. Some have better coating than others.

Finally I don't know about fast versus slow scopes but I know that the light angle of incidence on the filter does matter. Filters are designed to have light hitting the glass square. If the angle is too big then they don't work the same way. This can be a problem with an hyperstar for example where the light cone is very steep and I hear that in those circumstances a wide bandpass filter (Ha 13nm for example) would be recommended vs. a 3nm.

That's as far as I know about it. Sure others will drop in with better explanations.
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Old 15-07-2011, 07:16 PM
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Bassnut (Fred)
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Very narrow band filters (<5nm) are marginal for fast (<f4) optics due to the steep light cone (frequency shift). I have used 3nm on f6.7 and its fine. Narrow filter BW from any manuf wont be a problem at f6.3 and up.
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Old 15-07-2011, 09:50 PM
jase (Jason)
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I've contributed to a few threads on this topic in the past such as this one and others - a quick search will reveal them.

...but don't listen to me, what would I know
Get the facts from none other than Dr. Don Goldman. His AIC2007 presentation is the richest source of information on the topic.

What You Always Wanted to Know About Narrowband Filters but Were Afraid to Ask - 1.6mb PDF - Enjoy!

FWIW, in speaking with Don some time back he suggested going 3nm for SII and OIII to provide maximum signal. SII can be a real pig to dig out of the background ADU and a narrow bandpass of OIII has no sign of impact at full moon. This of course requires longer sub exposures but the results speak for themselves.

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