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Old 11-08-2014, 06:17 PM
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rmuhlack (Richard)
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Narrowband with a OSC camera - Ha/Hbeta/OIII at same time...?

At times of full moon like this, I have often wondered about the merits of using a UHC or multi-narrowband filter with a DSLR.

If I were to use a filter such as the Thousand Oaks LP4 (see here), would this filter not allow me to collect Haplha on the red channel and Hbeta on the green/blue channel of my DSLR...? Meaning that I can use the full bayer matrix to collect meaningful data to boost a conventional OSC image?

Alternatively one could use a OIII filter (eg the Lumicon OIII shown here) to collect OIII across both green and blue channels of a DSLR.

Am I missing something fundamental here or 1) would this be feasible, and 2) could it be done effectively under moonlight?
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Old 11-08-2014, 06:22 PM
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multiweb (Marc)
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You can certainly collect oiii in the blue and green channels of your dslr, not with the moon up though. I doubt your camera would capture ha though, unless it's modded. What's the bandpass of the 1000 oaks?
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Old 11-08-2014, 06:28 PM
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Also be aware that Hb is quite a bit lower intensity than Ha (ratio of 1:2.92 when emitted and then Hb will suffer more from scattering.)
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Old 11-08-2014, 06:31 PM
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Thanks. Having no experience to date with narrowband I wasn't sure what wavelengths could be captured when the moon is up. Both my DSLRs have a full spectrum mod, normally I use each of them with an Astronomik L clip in filter. If I remove the clip in filter I have sensitivity into the NIR if I want it. I mention the 1000 oaks as the response curve linked above seems to suggest that it will pass both Hbeta and Halpha with >95% and a reasonably narrow bandpass for both (narrower than a typical UHC filter).

Last edited by rmuhlack; 11-08-2014 at 07:36 PM.
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Old 11-08-2014, 07:50 PM
LewisM
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Yup and yup.

I use an H-a filter - 15nm - on the front of my Atik 4000 OSC, and then extract the red channel for the H-a. Nothing hard about it

Not bothered with OIII or SII, as I don't have them in 2". Sticking to Colour+Ha.

I also use an IDAS LP filter to good effect with it.
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Old 11-08-2014, 08:25 PM
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Definitely feasible, this shot was done with a 60Da and a 12nm Ha and OIII filter.
http://www.astrobin.com/full/98888/0/

The Ha was shot near new moon and the OIII was shot near full moon, Astrobin has averaged these and therefore reports almost 50% moon phase.
The 12nm OIII was definitely affected by the moon, not ideal but still way better than unfiltered.

The noise in the green channel OIII was definitely lower than the red and blue channels for the same ISO and exposure, benefit of the two greens in the bayer being combined. Half the noise? maybe.
I then combined the green and blue channels for further noise reduction, then mapped them back to green and blue.
Whether this makes, say, 9 of OSC OIII subs equivalent to 27 of the Ha with regard to noise, I don't know(I might try measuring one day) but it seems to work well enough
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Old 11-08-2014, 08:44 PM
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I was aware that narrowband clip in filters were available for DSLR.

Where this is leading to is the idea that I could use a filter like the 1000 oaks LP4 filter (transmission graph linked to in my original post) which has a relatively narrow bandpass for both Hbeta and Halpha to capture H data across each of the RGB channels of my DSLR at the same time, rather than just collecting Ha on the red channel only. Then use pixel math to combine the data from each channel using the LP4 filter to give a combined mono H channel, and then assuming a fixed ratio for Hbeta:Halpha (as per Rick's post) I could then subsequently blend back into the LRGB channels of a conventional OSC image from the DSLR. Thus achieving a HLRBG image with "H" collected using the entire bayer matrix rather than just 1 in 4 pixels.
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Old 11-08-2014, 08:47 PM
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If there was another filter that combined both OIII and SII in the same filter that would again allow those wavelengths to be collected simulateously on blue/green and red bayer pixels respectively
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Old 11-08-2014, 09:02 PM
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Sorry, yeah know you were aware of the clip in filters, I was addressing the comments below, just got slack and didn't include the quote (on phone with 3 month old in my arms )
Quote:
Alternatively one could use a OIII filter (eg the Lumicon OIII shown here) to collect OIII across both green and blue channels of a DSLR
...
2) could it be done effectively under moonlight?
I can't see any probs with your idea.
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Old 11-08-2014, 09:03 PM
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no dramas

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Old 12-08-2014, 01:15 PM
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that thousand oaks lp4 filter looks like it could be a goer there Richard - although they are meant for visual so you perhaps may end up with halos or other aberrations?
How about the LP2 - it looks a bit narrower?

Russell
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