Lum for NB imaging

[Somnium]

i was just wondering what people use for Lum data when taking NB images. do you use Ha as the lum? or a lum filter? does it change depending on the target? i am looking at M16 at the moment and it seems like Ha lum would be ideal ...

[RickS]

You can get away with using Ha as luminance if there's no significant detail in the Oiii and Sii (or you don't mind losing it...) I mostly use a blend of Ha, Oiii and Sii.

Cheers,
Rick.

[Atmos]

I typically stack all of the lowest FWHM images together with a noise weight. In some of the bright emission nebula the SII is not as bright but has better detail than the Ha. The OIII usually being somewhere in between. Doing a noise weight does tend to preserve a lot of the lightness of the Ha and the detail from the others.

[Somnium]

is there value in doing a lum layer then ?

[glend]

I shot M16 recently and did not use Lum and thought it was fine. However, i have tried a blend, and using that for Lum but did not think it made much difference, other than increasing the output file size. I think there is a tendancy for people to produce a 'forum standard' image when using false colour narrowband - as it attracts less criticism, other than the usual 'not enough data'. Unless your entering a calendar competition, or going for an award, then express it your way. It's a bit like a beauty compeition where it seems all the girls look the same, because it has evolved to the point where the contestants must have the right height, measurements, in the right proportions, to even have a chance; but in the real world there are plenty of pretty girls. Sorry if that seems sexist but it seemed a good analogy.

[Andy01]

I use J-P Metsavainio's tone mapping method for most of my nb imaging.
The lum layer in PS is generated from the Ha, with a 15-25% screen blend of each of the S2 & O3.
More on his processing method here - https://astroanarchy.blogspot.com.au/2009/04/neaic.html?m=1

Speaking from a purely aesthetic viewpoint, I have to agree with "beauty contest" comments above as well :)

[Atmos]

I do simply because I find it a lot easier to process using an RGB image and a Lum one. The RGB I don't do much to other than noise reduction and colour calibration. I do all other processing on the Lum. Partly because it is a lot easier not having to worry about three layers (the RGB) and because your Lum image is likely to have a better SNR than the other, making it easier to process and push that bit harder.



Probably wouldn't want to actually shoot a Luminance filters on a nebula as it tends to pick up too much other stuff that you're likely to not want, it being more sensitive to stars than the nebulosity.

Using a luminance shouldn't change the file output size. If you are working in PS and you haven't merged the layers then yes, you'' have a bigger PS file but when you go TIFF, PNG, JPEG or whatever it doesn't matter.

[glend]

Colin I was referring to the layer file that has to be saved if you want to come back and edit later, etc. The more layers you put in the larger the file gets, some of my recent ones exceed 500mb. Sure you can just save a copy of the image itself, but without also saving the layers you lose the ability to do future edits without a rebuild, imho.

[Atmos]

I'm with you now :) I thought you were just referring to the final image :)

[RickS]

As Colin and Andy mentioned, there are advantages in processing lum and colour separately and combining them late in the workflow. Here's just a couple of reasons why:

some operations you'll want to perform on lum, like various types of sharpening, will have a bad effect on the colour component (enhancing the chrominance noise)
operations which modify RGB colour can also affect overall brightness and vice versa. Keeping lum separate prevents this "contamination"

There are ways to get around these issues, like working in CIE L*a*b* colour space, but I generally find it easier to just keep lum and colour separate.

Cheers,
Rick.

[Somnium]

Thanks guys, i haven't processed a NB image before, this should be interesting :P