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Old 02-09-2016, 12:03 AM
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luka
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Narrowband and colour sensor (DSLR)

I will have a set of narrowband filters soon and am planing of getting/building a mono camera at some stage. At the moment I am stuck with a IR modded colour DSLR and I want to play with narrowband.

I already imaged in H alpha with the DSLR. I extracted the red channel and got a 1/4 image size. It worked well. I suppose SII should be the same as it is only detected by the red channel.

My question is what to do with the OIII. It falls between the green/blue channels of the DSLR and will be detected by 2 green and 1 blue pixel. Do I just use 1 channel? Which one? Or do I separate the three channels (GGB) and then add them together into one image to get the same image size as the red channel? The latter approach will improve the signal intensity by up to a factor of 3.
What about the fact that in the final image I will be combining the 3 channels that came from different pixels/different part of the image?

I am not expecting great results but would like to get some narrowband data that I can play with.

Thanks in advance
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Old 02-09-2016, 05:49 AM
glend (Glen)
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The nice thing about narrowband, at least with mono cameras, is that you have the freedom to use whatever pallet you choose. With a OSC DSLR the Oiii filter will likely be green for you and that is a good thing because you have more green pixels than red ones in your bayer matrix. You have some choices with your camera settings. Canon will allow you to shoot monochrome, ignoring the colour data. This will, if shot through a Oiii filter give you an effective mono channel that you can colour layer the way you want, perhaps using the Hubble pallet one time and the more natural pallet another.. When shooting in monochrome on a DSLR, just select monochrome in the image type menu. When you do shoot the sub through an Oiii filter only that bandpass part of the spectrum will illuminate the pixels. If you shot the sub in colour it would be mostly green, depending on you bayer matrix, just as Ha and Sii will be mostly red. But shooting them with a mono camera setting, you won't get any more pixels exposed than a colour shoot but it will allow you to colourise the layers the way you choose in processing. When processing, you will need to watch the file type and convert the mono file to RGB (at keast in Photoshop), in order to colourise the layer in your choice of colours. Sorry if that confuses but its pretty simple really.

Last edited by glend; 02-09-2016 at 06:05 AM.
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Old 02-09-2016, 09:03 AM
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pluto (Hugh)
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You could think about it from the point of view that you're creating a mono greyscale image which represents the intensity of light at the wavelength you're allowing through the filter. In this case that's OIII ~500nm. So if that signal happens to fall across the green and blue channels then I would just combine them into a single mono image (in practice this usually means creating a normal 3 channel image where each channel is the same)
You'd do the same for your Ha and SII so you basically have 3 monochrome images which represent the light intensity at the 3 wavelengths (or wavelength ranges)
Then you can combine your Ha, SII, and OIII as you please.
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Old 02-09-2016, 10:26 PM
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Thank you for your replies.

Hugh, that is what I originally had in mind. I just was not sure what to do with the OIII channel. I suppose it makes sense to use signal from all available channels.

Glen, how is Canon actually converting the image to mono? I can imagine that it is combining all 4 colour pixels which may introduce additional noise from the unused pixels. For example, for H alpha there will be light only in the red channel. If mono conversion uses the other 3 channels (GGB) it will just add extra noise and no signal to the red. Or am I missing something?
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Old 02-09-2016, 11:15 PM
glend (Glen)
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Luka there will not be any red pixels showing up if your shooting with say the Oiii filter and the camera is set to mono. The bayer matrix is a physical pixel mask so red ones will only be red. If your shooting in mono the camera simply outputs a mono file stripped of the colour information, try shooting a daytime shot to see what i mean. When you put a narrowband filter in front, it only passes that part of the spectrum, and regardless of whether you have it set for mono or colour, you will only get data from the pixels that can pick up that spectrum area. I only suggest using a mono setting because it mimics mono camera operation and will allow you to layer colour it as per standard mono camera practice.
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Old 02-09-2016, 11:31 PM
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Got it now, thanks Glen. I will try tomorrow. I managed to crack my Baader UV-IR cut filter and get it stuck on the flattener
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Old 03-09-2016, 08:21 AM
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pluto (Hugh)
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I don't know about a 450D or 1200D but on my old 5DmkII even when the camera is set to monochrome the raw files are still colour.

I guess my workflow, in Photoshop, would be something like:

Do a raw conversion to tif or something.
Open in Photoshop.
For Ha and SII copy the red channel and paste it into both the green and blue channels.
For OIII I'd grab the green channel and copy it to a new image. Then grab the blue channel and paste it as a new layer over the green image. Then blend them together how ever you want and save that as your OIII image.
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