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Old 12-06-2019, 03:07 PM
casstony
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H alpha with DSLR: Prawn, Lobster, Liberty

Since modifying my d5600 I've got a Baader 7nm Ha filter and tried it in the last couple of nights while the moon is around.

The Prawn/IC4628 turned out well I think. Not sure what I'm doing with these but I guess I'll get some OIII images and try making a bi-colour image?
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Old 12-06-2019, 09:02 PM
Paulyman (Paul)
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Wow they look great, some great details in there. How are you processing to get the colour? I isolate the red channel and discard the blue and green so it comes out monochrome.
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Old 12-06-2019, 09:50 PM
Sunfish (Ray)
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They look interesting in yellow. A lot of detail there. Seems the mod is delivering a lot of fun.
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Old 12-06-2019, 11:11 PM
casstony
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I haven't discarded anything Paul, just aligned the RGB and bumped saturation to give the yellow colour for aesthetic reasons. Bear in mind I don't know what I'm doing with narrowband, just starting to play.

In the long run if I stay with a dslr/mirrorless the Triad Ultra filter looks interesting, after it drops in price a bit.
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Old 12-06-2019, 11:13 PM
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Originally Posted by Sunfish View Post
They look interesting in yellow. A lot of detail there. Seems the mod is delivering a lot of fun.
Yeah it's nice to be able to image while the moon is about, plus the extra signal for normal imaging.
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Old 13-06-2019, 06:22 AM
Paulyman (Paul)
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The green and blue channel may have some data in them but are largely just contributing noise. If you grab just the red channel that is where the majority of your good data is and it will have the best snr, of course you would lose some resolution due to the Bayer array. But if you dither and collect enough data (more than about 16 frames) then you can stack using the Bayer drizzle algorithm which will use the dithered subs to basically remove the loss of resolution from the Bayer array. Isolating the red channel at this point you haven’t lost as much resolution thanks to the drizzle.

I’m still learning but using the above has definitely improved my Ha data.

Last edited by Paulyman; 13-06-2019 at 06:23 AM. Reason: autocorrect issues.
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Old 13-06-2019, 06:25 AM
Paulyman (Paul)
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I do really like the colour though forgot to mention that.
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Old 13-06-2019, 09:04 AM
casstony
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Originally Posted by Paulyman View Post
The green and blue channel may have some data in them but are largely just contributing noise. If you grab just the red channel that is where the majority of your good data is and it will have the best snr, of course you would lose some resolution due to the Bayer array. But if you dither and collect enough data (more than about 16 frames) then you can stack using the Bayer drizzle algorithm which will use the dithered subs to basically remove the loss of resolution from the Bayer array. Isolating the red channel at this point you havenít lost as much resolution thanks to the drizzle.

Iím still learning but using the above has definitely improved my Ha data.
Thanks for the hints Paul. I already dither but have a long way to go with processing.
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Old 13-06-2019, 09:06 AM
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Very nice. I agree with Paul. Pull out and work on the red channel, monochrome. The BG channels will have a load of noise.
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Old 13-06-2019, 07:26 PM
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I extracted the red channel of the Prawn image and it does look better than the previous image. Lots to learn
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Old 13-06-2019, 07:49 PM
Paulyman (Paul)
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Oh wow, yeah that looks fantastic. How many hours is that and how long were the subs?
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Old 13-06-2019, 08:29 PM
casstony
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Prawn is 1 hour 20 minutes, 16x5min subs, ISO 400. (the original image looks much better than the compressed one above too). I'll do the other two images sometime too, keen to see how Liberty turns out.

I also figured out how to combine two channels which is handy for an image of Sh 308 I took with an OIII filter. The red channel had heaps of noise and no signal so it was discarded. The image isn't worth posting as I only have 50 minutes worth of subs.
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Old 13-06-2019, 08:39 PM
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Originally Posted by casstony View Post
I extracted the red channel of the Prawn image and it does look better than the previous image. Lots to learn
Tony, just to check, were the images de-Bayered before you extracted the red channel? If yes this will add noise from the other three channels without signals.
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Old 13-06-2019, 09:13 PM
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Luka, I stacked the image in DSS and then extracted the red channel in Pixinsight. Should I do something differently?
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Old 14-06-2019, 02:01 AM
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Tony, maybe I am over-complicating things but there are two ways you can "extract" the red channel from the images.

Backtracking a bit first, the camera sensor has a Bayer matrix in front of it. See here for the explanation of the Bayer matrix. Basically in any block of 2x2 pixels on the camera sensor one pixel will pass "mainly red" light, one will pass "mainly blue" light and two will pass "mainly green" light. Note that "mainly red", "mainly green" and "mainly blue" have wide transmission ranges and partially overlap (see the two examples of the transmission curves a few paragraphs below).
Anyway, the RAW images from the camera contain information about intensities for each pixel. To get to the colour information they need to be de-Bayered first, i.e. the information from the neighbouring pixels is used to calculate the colour using a mathematical algorithm.

Now, if you are imaging H alpha (656nm) most of the signal will end up in the red sensor pixels. The green and blue will mainly only have noise (again emphasis on "mainly", have a look at the transmission curves in the last paragraph below).

So you can process your images in two ways:
1. If you stack images in DSS it will de-Bayer them for you. Basically all channels (red, 2xgreen and blue) will get mixed before you use PixInsight to get the red channel. Green and blue channels may not have any useful H-alpha signal but they will add noise to the final image.

2. The other way would be to extract the red channel from the RAW files before doing any stacking. You would basically discard 3/4 of the pixels (green and blue) and your image resolution will be halved horizontally and vertically. But the image will be sharp; the larger resolution of the image from method 1 is not quite real, it was artificially increased by de-Bayering calculations with green/blue pixels which had not much signal.

What is the better way? It depends on the Bayer matrix in front of your sensor (and your camera noise). For example, have a look at the transmission curves for different sensors here and here, in particular look around 650nm.
From the first link (sensor 1) red has about 90% transmission while blue and green have sub-5% transmissions. Basically red pixels will see almost 20x stronger H-alpha signal then blue/green.
From the second link (sensor 2) red has about 40% transmission while blue/green are about 3% / 8%. While the numbers are closer the red will still have 5-10x stronger H-alpha signal.

So... I would be tempted to discard the blue/green channels. While the sensor noise in the modern cameras is low, using blue/green pixels will increase it 4 times while not contributing much to the real signal. The fainter features will be swamped in the extra noise and hence lost.
(Keep in mind that we are dealing with very small signals here)

Hope this all makes sense.

Maybe pick your best RAW image and separate the RGB channels. See if you can see any H-alpha signal in the blue/green channels.
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Old 14-06-2019, 06:25 AM
Paulyman (Paul)
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This is where the magic of Bayer drizzle comes in! You dithered, so with enough subs the idea is that the camera moved around enough that each of the 4 bits of the array (RGGB) visited the location of the others at some point. The algorithm uses that to create the RGB channels, it doesn’t need to interpolate and introduce noise from the other channels because it has data from the dithered subs. So stacking and extracting the red channel at the end is okay in this case. At least that’s how I understand it.

I use PI but I know there is a Bayer drizzle option in DSS as well.
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Old 14-06-2019, 09:50 AM
casstony
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Thanks for the info Luka and Paul.
I found the bayer drizzle option in DSS so I'll try that for a start.

I'll be moving to PI over time but I'm still using a mishmash of DSS, PS and PI at the moment.
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Old 14-06-2019, 06:02 PM
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Even though there aren't enough subs (16x5) for the bayer drizzle to work well, I tried it and a lot more features became apparent. Are all those extra lines across the image real or artifacts?
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Old 14-06-2019, 08:51 PM
Paulyman (Paul)
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There is definitely more faint detail than the previous image and it looks awesome. If you zoom in the Bayer pattern is showing up when you zoom in, or maybe it’s just the remnants of it. Either the settings aren’t quite right or there isn’t quite enough data so some artifacts remain. I use PI so cant help with the DSS settings unfortunately. The images I’ve processed using the method have 4 hours of 10 minute subs, so 24 frames. Maybe it is as simple as a few more subs for the algorithm to have enough to work with to avoid artifacts?
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Old 14-06-2019, 09:21 PM
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I suspect it is just a shortage of subs that lets the grid pattern show up - probably need more like 50 rather than 16.

So the curvy lines on the left side of the frame and the sharper lines across the image are real nebulosity?
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