Luminosity Channel in Light Polluted Skies - Forums

Topic starter

Mar 6, 2021 · Edited: Mar 6, 2021

Hello folks,

I am planning to buy a mono camera and and a set of NB filters for deep sky (nebula) photography. I always hear that for a good image you need to make SHO NB exposures as well as L,R,G and B. Others replace the RGB with SHO, and also do the L. But I have also seen exceptional images here which were seemingly made only with the three NB channels SHO. I am confused.

Taking a luminosity image (as well as RGB) apears difficult in light polluted skies even when utilizing a CLS filter, since they dont help with LED street light. So I would rather very much hope that with the three narrowband filters SII, OIIIm and Ha I am all set to make great astro photos ?

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Ben Koltenbah

Mar 6, 2021

Götz Golla:

I would recommend that you start NB imaging with just the NB filters to get the hang of the data and processing. There's a lot of variation in how people incorporate broad L filter data and/or RGB filter data. As you've discovered, impressive images are made using any of these techniques. But you may wish to look at this as a learning process in steps, starting with just the NB data. There's lots to learn and do with that first step!

My preference is to only use the three NB filter channels mapped into RGB as SHO, although there are endless ways to mix the channels with 2 or 3 filters. I have experimented with using the broad L channel as the luminance layer, but with mixed results. Others have done this well. And others have made mixed RGB images to bring out star color. My preference is to render the stars into neutral white colors, but all these choices are a matter of taste.

Good luck!

Best Regards,
Ben

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HaSeSky

Mar 6, 2021

Hello Götz,
I mostly use SHO NB filters for my nebula images. RGB and L is not really satisfying in light polluted areas from my point of view.
With 6nm NB filters I can expose sub-frames for 600-1200 s even with heavy light pollution and full Moon.
You have a lot of processing possibilities with pure SHO frames. The famous Hubble palette for example (SHO transfered to RGB in post processing) does not need RGB and L filters. Anyway, you can get great results with a basic SHO NB filter set. RGB+L(CLS) could be a nice add on if you have good light pollution conditions.

Clear skies
Hanns

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north.stargazer

Mar 6, 2021

I suspect the information you're hearing is mostly related to people who want to bring RGB star colours into their narrowband SHO, HOO and other compositions.

The narrowband images can be beautiful, but the stars almost always look pure white... especially after correction to remove the common purple/megenta halos you'll get with something like a SHO Hubble palette.

I haven't done too much of this yet, but my plan is to simply shoot my DSOs with narrowband from my Bortle 8 home location and then shoot the same with a one-shot colour CCD or DSLR and then simply copy over the more natural stars. If your OSC and NB images are from very different sensors/scale, you can use software like RegiStar to match and align the scale and registration prior to combining.

As @HaSeSky stated, LRGB imaging from heavily LP skies is generally not great.

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Bogdan Borz

Mar 6, 2021

Hi Götz,

You do not add luminance to a narrowband image. That doesn't make sense since the structures and the details are specifically in the narrowband channels and you would practically lose it all with the L channel. SHO images are just pure SHO, sometimes you can create a synthetic luminance using various percentages of Ha and S2, O3 (Ha usually has the best SNR and details). Sometimes you can use only Ha as luminance since the other two don't have much signal.

I am in a heavy LP zone. You can do great astrophotos only with the narrowband filters. Shooting in broadband in Bortle 7/8/9 is very tough and you need very long cumulative exposures. It is possible for bright targets like galaxies and planetary nebulae. But some people living in Bortle 4 and under will produce a better image in 5h compared to yours in 25h.

We usually combine star color with the narrowband image in order to avoid unnatural colors or white saturated stars (they saturate before the nebulosity). So go for the narrowband filters the narrower the better. (and more expensive : )).

Clear skies,

Bogdan

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Chris Sullivan

Mar 7, 2021

You can stack SHO images together and create a luminance from that, or even just use Ha, for example, as your luminance (which, while not being a true luminance it can still help with structure/detail). RGB data for SHO and HOO targets is primarily used for natural star color. There will also be times when a target only has Ha in it, for example. In that case, you'll need RGB stars just to create a color image (as shooting SII or OIII, for example on the Cone/Christmas Tree, would just be a waste of imaging time since there is no SII or OIII signal present).

The one thing about SHO and HOO imaging you should be aware of is that you're only going to be able to shoot emission nebulae, planetary nebulae, Wolf-Rayet nebulae and supernova remnants. Galaxies, reflection nebulae, star clusters as well as almost all dark nebulae are broadband targets and thus require broadband imaging techniques. Imaging star clusters under light pollution is pretty easy. Imaging galaxies is not impossible either. I haven't been imaging much lately due to work obligations, but I spent this time last year tackling galaxies from my Bortle 7-8 transition zone. It definitely can be done, but it requires more patience and integration time.

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