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Old 13-04-2021, 10:05 PM
Gordy
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Why use a green filter

I am just starting my mono journey along with Pixinsight. I noticed a lot of tutorials show how to use SCNR to remove the colour green from images. One site mentions that there is no green in space so it makes sense to reduce that colour much as possible in images.



If this is the case, then why bother to take images using a green filter (LRGB) in the first place? People still do so there must be a reason - does anyone know what that is?
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Old 13-04-2021, 11:53 PM
raymo
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Well Gordy, for a start, there is plenty of green in many deep sky objects;have a look at NGC 3918, bright green. Green is oxygen. To balance the hues of your colours during processing you need RGB subs. You need to be able to
adjust your colours by making them for example a little more or less R,B, or
G. You can't do that if you don't have any G.
raymo
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Old 13-04-2021, 11:56 PM
AdamJL
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Old 14-04-2021, 08:39 AM
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multiweb (Marc)
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The oompa loompas started the green bashing campaign. There's plenty of green. Take the green off and you enter the pizza realm and it's a slippery slope of fast food imaging. Embrace the salad. Colour variety and fidelity is key of good imaging.
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Old 14-04-2021, 09:39 AM
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multiweb (Marc)
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On a more serious note this is what I mean by colour fidelity:

https://www.eso.org/public/images/eso1719a/

One would argue there are tinges of green in that field or yellow but that's what makes the field so beautiful. Variety comes from data integrity. Data talks, bullsh|t walks.
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Old 14-04-2021, 12:50 PM
JA
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Quote:
Originally Posted by Gordy View Post
I am just starting my mono journey along with Pixinsight. I noticed a lot of tutorials show how to use SCNR to remove the colour green from images. One site mentions that there is no green in space so it makes sense to reduce that colour much as possible in images.



If this is the case, then why bother to take images using a green filter (LRGB) in the first place? People still do so there must be a reason - does anyone know what that is?
You don't only need green to represent green objects, but also to help generate other colours and nuances of colours either by Bayer filter interpolation/processing in the case of a colour camera, or in creating a colour image from Red, green and blue filtered images from a monochrome sensor using software.
Leaving aside the complexities of demosaicing and interpolation in software or camera firmware, think about how, for instance ,white is generated on a colour sensor: Red+Green+Blue and some hocus pocus. This hopefully goes most of the way to an answer

Also take a look at the spectral response of the various channels from a colour camera sensor. Here are two examples: a Nikon D600 DSLR and a ZWO ASI071, most DSLRs and other colour cameras will show something similar.....
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Remebering that the Oxygen lines OIII are around 500 nanometers, take a look at the graphs and you see that the OIII lines are represented by both the Blue and Green channels, almost in equal proportion. Even with a monochrome sensor the Red, Green and Blue filters used would have comparable bandwidths to those shown above on the colour sensor array, as evidenced in the Astronomik L-RGB filter set data below...
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Best
JA

Last edited by JA; 14-04-2021 at 01:31 PM.
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Old 14-04-2021, 01:26 PM
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DiscoDuck (Paul)
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In desperation once after clouds came in after I had L, R and G images I tried to make a synthetic blue from L, R and G
(technically L is R+G+B so can do a bit of creative PixelMath and, with some cost from excessive noise, reconstruct a set of blue images).

I never achieved that misguided goal as the image never quite could be made to look right.

Sensibly, next clear sky chance I got I added some B images
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Old 14-04-2021, 10:48 PM
Gordy
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[QUOTE=JA;1516496]You don't only need green to represent green objects, but also to help generate other colours and nuances of colours either by Bayer filter interpolation/processing in the case of a colour camera, or in creating a colour image from Red, green and blue filtered images from a monochrome sensor using software.


Many thanks everyone for your responses , especially JA.
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