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Imaging strategy for Flaming Star Nebula: filter rotation and SHO ratios

Stefan Berg Nighttime Imaging 'N' Astronomy (N.I.N.A. / NINA) Deep Sky IC405 IC Objects Nebula
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Jan Hierscher avatar
Jan Hierscher avatar

Hello everyone,

I am currently planning my next project: the Flaming Star Nebula (IC 405).
My setup consists of an Askar SQA106, ToupTek SkyEye62AM, UMi20s, and a ToupTek G3M678M as a guiding cam via OAG. Since I want to go very deep on this object, I will definitely invest several nights.

I have two fundamental questions regarding the strategy:

1. Filter Rotation vs. Completing Filters Individually:
When imaging over multiple nights, what is the best workflow in N.I.N.A.? Does it make sense to "finish" the filters one by one (e.g., one night only Ha, the next night only OIII), or should I rotate through the filters each night?

2. Number of Subs per Filter & SHO Ratio:
Are there any guidelines for the ideal ratio of sub to each other?
For LRGB, it seems logical to collect roughly the same number of subs for R, G, and B (perhaps more for Luminance). But what about SHO? Since IC 405 is an emission nebula, I'm wondering if Ha, OIII, and SII should be exposed in a 1:1:1 ratio, or if OIII and SII need significantly more weight because the signal is often much weaker there?

I look forward to hearing about your experiences and tips!

Best regards,
Jan

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andrea tasselli avatar
andrea tasselli avatar
For SHO I'd recommend 1:2:4 (Ha-OIII-SII) ratios. I doubt that that nebula is just emission lines though…
Quinn Groessl avatar
Quinn Groessl avatar

1) It depends. I prefer rotating filters because clear nights aren’t guaranteed and I’d rather have some data than no data. Some caveats are that if it’s a clear night with no moon, I’m probably going to stick all my time in to Oiii if I need that filter. Even if I’m “done” with that filter, if those are the conditions I’ll add more time to it. On some nights I’ll go with Oiii exclusively while the moon is far enough below the horizon.

As for workflows in NINA. When shooting LRGB I’ll have it take something like 3 or 6 exposures per filter and repeat until dawn. That way if clouds or something comes up in the night they won’t be all in one filters shots hopefully.

2) I usually do 3 L to 1 RGB. It’s one of those things that people debate the merits of. Occasionally I’ll do all the same LRGB, but I usually don’t. For SHO it depends. Usually I try and get more Oiii than anything, but it really depends on the object and how faint the signal is.

For example, if you’re trying to get M31 with Ha and the Oiii arc, you could probably get away with 20 hours of Ha, but you’ll need 100 hours in Oiii to really get the arc. On the flip side, something like NGC6995 you could do equal amount of Ha and Oiii because it’s just so bright.

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Arun H avatar
Arun H avatar
Jan Hierscher:
Since IC 405 is an emission nebula, I'm wondering if Ha, OIII, and SII should be exposed in a 1:1:1 ratio, or if OIII and SII need significantly more weight because the signal is often much weaker there?


IC 405 is most definitely not just an emission nebula. One of its most beautiful characteristics is the bluish reflection nebulosity caused by starlight from AE Aurigae. You can catch a hint of this using an OIII filter, but RGB imaging captures in in great depth and beauty.
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Tony Gondola avatar
Tony Gondola avatar

My only comment would be on the narrowband side. When working RGB we take great pains to keep the exposure times the same so that the “natural” color balance is maintained. Narrowband can be looked at the same way. If you keep the exposure time the same between the bands you’ll maintain the natural relationships between HA, Oiii and Sii. Once you start favoring one over an other you’re changing that balance. On a lot of emission targets that interplay of tones in NB can be very subtle. It really depends on what your after in your final rendering.

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Arun H avatar
Arun H avatar
Tony Gondola:
When working RGB we take great pains to keep the exposure times the same so that the “natural” color balance is maintained. Narrowband can be looked at the same way. If you keep the exposure time the same between the bands you’ll maintain the natural relationships between HA, Oiii and Sii. Once you start favoring one over an other you’re changing that balance.


I don't believe this is accurate.

There is no inherent necessity to keep the exposure times the same between RGB. Were this to be the case, OSC cameras would  not be very useful in any type of imaging. Moreover - even if one does keep the filter imaging times the same in a mono setup, the QE curve if the sensor would modulate the signal captured. In reality, the color rendered is determined by the application of white balance and background subtraction which determines the final ratio between the channels in an RGB setup.  It is necessary to capture enough signal in all channels to get a good SNR but that does not presuppose that a certain imaging time ratio is needed.

As for narrow band - if the purpose of NB imaging was to preserve the actual ratio between filters, even if such a thing was possible, everything would be green. The very purpose of narrowband is to accentuate the structural aspects of a nebula or object using the different emissions as a guide. Since the emissions of OIII and SII are a lot weaker, it usually pays to spend a lot more time on those channels than on Ha to improve the SNR, and, particularly for OIII, to image during times of less moon interference. These aspects are what should determine an imaging strategy for a NB image.
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Tony Gondola avatar
Tony Gondola avatar

I agree, adjustments for color are made in post for RGB and NB. I’ve don’t see anyone actually doing unbalanced exposure ratios in RGB. In NB, you can certainly make the case both ways.

Brian Puhl avatar
Brian Puhl avatar

I've gone decently deep on this target, you can find my image if you like in my gallery.

Spend a decent amount of time on LRGB. The center (blue) of the region is mostly broadband. There's also a very neat reflection nebula near the top of the tail. LRGB will also allow you to grab some of the Hb.

For narrowband, focus on Ha and Sii primarily. There is very little Oiii to be captured here. All of the Oiii glow will be captured with broadband anyways. I took a little bit but it added absolutely nothing to my image. All your narrowband depth will come from Sii. So I would put the most time into it.

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Jan Hierscher avatar
Jan Hierscher avatar

Thanks for the many quick replies.

Is there a guideline for how many subs I need to make per filter to ensure a good SNR?

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andrea tasselli avatar
andrea tasselli avatar
No. Take as many as you can.
Daniel Cimbora avatar
Daniel Cimbora avatar

You might find this site helpful in assessing the relatively strengths of the NB channels in commonly-image objects:

I haven’t imaged the Flaming Star in RGB, but have processed my NB data a couple of different ways (mouseover to switch) to give you an idea how drastically the image changes with different mappings of colors:

CS,

Dan

Matthew Singer avatar
Matthew Singer avatar

A more general approach might be to reference these two sources, which can answer your question directly and be helpful in overall narrowband planning:

Stefan Ziegenbalg's Northern Sky Narrowband Survey (mentioned many times in the past here on AB):

https://simg.de/nebulae3/index.html

Galactic Hunter’s Narrowband Deep Sky objects and what to expect with each filter:

https://www.galactic-hunter.com/post/filter-expectations

He discusses IC 405 specifically here:

https://www.galactic-hunter.com/post/the-flaming-star-nebula

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Brian Puhl avatar
Brian Puhl avatar

Matthew Singer · Dec 16, 2025, 09:38 PM

He discusses IC 405 specifically here:

https://www.galactic-hunter.com/post/the-flaming-star-nebula

“ the Hydrogen Alpha is much more impressive than the data obtained with the two other filters. “ -quoted from that page

I can’t disagree more. Sii is so under appreciated in narrowband imaging. People just seem to get more excited about Ha because it’s much easier to image, build up signal quicker. Put the effort into Sulfur and you will be rewarded. I could have afforded more time on this one, but I had target ADHD.
📷 image.pngimage.png

andrea tasselli avatar
andrea tasselli avatar
Or shoot pure RGB and you won't loose anything.
Jan Hierscher avatar
Jan Hierscher avatar

Thank you, Daniel and Matthew, the pages really helped me a lot.

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alpheratz06 avatar
alpheratz06 avatar

Jan Hierscher · Dec 16, 2025, 03:04 PM

1. Filter Rotation vs. Completing Filters Individually:

To add my personal opinion, I prefer to keep the same imaging conditions as long as possible. In that case I would take a chance and run one color per night. This also limit the amount of swtching and flat sequences.

Jan Hierscher · Dec 16, 2025, 03:04 PM

2. Number of Subs per Filter & SHO Ratio:

I second the general sense of all the comments :

  • look for long individual exposure whenever possible; but never forget to avoid saturation and adapt the duration to the sky background luminosity

  • use HDR capture processing with high-dynamic range targets

  • adjust NB ratio with a general rule of the thumb 1 2 4 but “cheat” whenever you may have information about the spectral repartition of emission : it’s not useful to look for SII for hours if there is not SII at all !

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Matthew Singer avatar
Matthew Singer avatar

Daniel Cimbora · Dec 16, 2025, 09:32 PM

You might find this site helpful in assessing the relatively strengths of the NB channels in commonly-image objects:

Oh, sorry, Daniel, I didn’t realize you had already mentioned this site before I replied; it is indeed a very helpful site

Rob Pettengill avatar
Rob Pettengill avatar

andrea tasselli · Dec 17, 2025 at 12:50 AM

Or shoot pure RGB and you won't loose anything.

Actually RGB looses a lot, because each color bucket is very large in bandwidth. Once photons are trapped in one, you can’t differentiate the source. You catch all the photons but loose valuable information about them.

Halpha and Sii are both red. Hbeta and Oiii are both blue/green. Each of these revel different structures that is lost when we limit ourselves to the eye’s crude perception of color.

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Rob Pettengill avatar
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Jan Hierscher · Dec 16, 2025 at 06:08 PM

Is there a guideline for how many subs I need to make per filter to ensure a good SNR?

Some things to keep in mind, beyond more is better and it depends on how bright the target is…

  • It’s not the number of subs that increases signal to noise but total integration time. There is also an optimum sub exposure time. This is because noise comes from multiple sources that scale differently with sub exposure time. Shot noise from the sky and your target is independent of these. Thermal noise from the sensor is temperature dependent. Read noise from the sensor electronics is minimized by longer sub exposures (larger signal). Lots of people around who know more about this that I do can advise on determining this for your setup and environment. However, you won’t be too far off with 3 to 5 minutes subs most of the time.

  • Signal to noise ratio scales with the square root of exposure time / number of subs. This means that you will see the same 2x relative improvement going from 20 to 80 hours of integration time as you did going from 2 to 8 hours. Effectively this means that there are diminishing returns in relation to the opportunity cost of imaging another target. Going from 40 to 50 hours of exposure won’t make much difference in a dim target, but those 10 hours might yield a nice image of a brighter target.

Bottom line is that an answer to your question is complicated. Depends on your target, equipment, sky conditions, and location. Most of all it depends on you and how you want to spend your time. Personally, I’ve gone from thinking that 4 hours was a long time with my f/5 scope to being willing to spend 40 hours on one target. I’ve seen amazing images with hundreds of hours of exposure, often by a team of imagers. I should mention also that the new AI driven noise reduction software can do wonders with the aesthetics of noisy images. However, it won’t reveal details hidden in the darkness like greatly increased integration time will.

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