Quote:
Originally Posted by Amaranthus
Short answer is, the wider the band, the more non-target (wavelength) photons get through. This means the image is brighter, and the subs can be shorter, but the effects of background skyglow (moon, urban LP etc.) are also more prominent. The narrower you go, the longer subs you'll need, but the less light pollution and other extraneous contamination you'll suffer. Trade-offs...
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Thanks Barry. That's how I was thinking too. Yes, the expensive trade-offs...
Quote:
Originally Posted by RickS
As Barry said, the narrower filters will reject more of the sky glow. This will give you better contrast and won't really need much longer exposure time - the light that you're rejecting isn't signal that you want anyway.
With Ha, 5nm is a good compromise because it will give you NII as well as Ha and that will help with a lot of objects (e.g. the Helix Nebula is significantly fainter with a 3nm Ha filter.)
You didn't mention whether you were considering other narrowband filters as well as Ha? Assuming you are: Moonlight will affect OIII more than Ha, so if you want to image with the moon up a 3nm OIII filter is a good investment. If you're getting SII as well then a 5nm is adequate.
The other filter parameter that is important is transmittance. This tells you how much of the light you're trying to capture gets through the filter. A filter with 90% transmittance will give you quicker results than one with 70%.
Oh, and narrow filters won't work so well with very fast optical systems but unless you're near f/3 or faster it's not a problem.
Sorry, don't know of anyone that carries Astrodon filters locally. I've always ordered mine direct from the US.
Cheers,
Rick.
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Hi Rick, yep I am intending to get the OIII and SII also, but I still haven't purchased my LRGB set so I'm going for that, plus the Ha filter first. I'm not satisfied with the Horsehead nebula I did last summer, so I want to go again with the Ha added when Orion is in season.
Also, I'm using a newt at f4 and a refractor at f7.5 so the 3nm shouldn't be an issue I don't think.
Quote:
Originally Posted by SamD
Putting some numbers to what Rick and Barry say, I get about 7e-/min skyglow at zenith with my 7nm Baader Ha (15km from Brisbane, no moon, with 314L+, and at f/7.5). This skyglow number is similar to what Craig Stark reports in his 314L+ pdf review at a similar f ratio.
If I doubled up from a 7nm to a 14nm Ha filter and follow the SNR calculations through, I would expect to need double the integration time to get the same SNR, or half the integration time if I went down to 3nm (assuming you don't chop off any signal like NII).
Even with NB I find that I'm better off to image when moonless (ie lower integration times). Less of a problem of course for the brighter NB stuff, but for the deeper stuff and a reasonably dark suburban sky I find no moon is the way to go.
Another factor of course is that the narrower filters are pretty expensive !
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Hi Sam,
Thanks for that info. I think a set of narrowband filters would help with getting a bit more use out of my kit, for the money spent, a few moonless nights a month makes it a hard sell at times.
It appears that maybe a 7nm or similar maybe a good compromise for the Ha filter?