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Are different exposure lengths needed for Orion Nebula?

Acquisition HDR Deep Sky M42
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Paul Macias avatar
Paul Macias avatar
Hey guys, 

First post by me! 

So, most of my imaging is narrowband since I'm in Bortle 7, but I occasionally do LRGB. However, one target I haven't done is the Orion Nebula. For most targets, I will generally shoot 5 minute exposures for RGB and for L, but at 0 gain. Most of the time it's ok, as I don't tend to clip much at all, but I've read that some people combine stacks of Orion Nebula shot at different exposure lengths, as the very bright parts might get completely blown out at higher lengths. 

Do people still do this? Or would simply lowering exposure length as to not clip much be fine?

If it matters here's my equipment
533mm
Sky Rover 80mm f/6 triplet
Astronomik deep sky RGB and L3

Thanks in advance!
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Tony Gondola avatar
Tony Gondola avatar

People still do it. M42 has an enormous range of brightness.

Paul Macias avatar
Paul Macias avatar
I'll do a little research on the combination process then. Not something I've done before but might as well learn something new. Thank you Tony!
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Jan Erik Vallestad avatar
Jan Erik Vallestad avatar

For what it’s worth I never bother doing different exposure lengths as long as nothing is saturated/blown in the histogram. If the histogram is fine then it’s all in the processing of the image and you simply don’t need to do a whole range of other datasets. Orion is a tricky one for me however, as it’s quite low on the horizon, but the one time I shot it from B3 I used 60s exposures at gain 100 (imx533MM) with my Evostar 80ED and did not saturate the core at all.

📷image.png

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akshay87kumar avatar
akshay87kumar avatar

I have attempted to bring out the core details of Orion Nebula without getting into multiple exposures. I did 120s exposures, ensuring that the histogram is not clipping off. Although I did have to take a lot of care while post processing - masked stretches, HDR Multiscale Transformation, etc to bring out the dust while not blowing out the core.

Link: Orion Nebula | Dr. Gopal Krishna Kunwar Observatory

📷 OrionNebula-1-scaled.jpgOrionNebula-1-scaled.jpg

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Mikołaj Wadowski avatar
Mikołaj Wadowski avatar

Whether you’ll need to take shorter exposures for the core depends on your setup, settings, and tolerance to clipping. For me, 120s RGB subs at gain 100 gain (~16ke- well) at f4 I was clipping the central star cluster and the very brightest emission features. For me it wasn’t a huge deal - I’m personally not a fan of over-HDRed M42, so I wouldn’t try to preserve these features either way. But if I want to show the trapezium region in my image, I’d need to do ~20s or shorter to avoid clipping.

With some crude estimates, you can expose for ~7x longer than I did before clipping (f6 and 50ke- vs f4 and 16ke-), so 300s might be fine for RGB but I’m guessing it won’t be for L. Best way to check is to take 300s in each channel, view the linear frame, and decide whether you tolerate how much of the image is clipped or not.

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Paul Macias avatar
Paul Macias avatar
Thanks guys. I think I'll just have to just do some testing. I suspect 5 min RGB will be ok. I typically do L at 0 gain, 5 min, but mostly out of convenience and less frames to deal with, but I will have to see what works best. If I'm doing 100 gain, it's typically 2minutes. 

My preference is to not have to shoot different exposure lengths, but I might just do it anyway for learning purposes.
Tony Gondola avatar
Tony Gondola avatar

If I were to do it I would have a constant gain and just change the exposure time.

V avatar
V avatar

I simply set gain to 0 and shoot 1 minute exposures (hundreds of them is optimal for drizzle 2x). Star sizes are good, core is perfect, and the fainter dust can be resolved by taking more data.

AstroChristie avatar
AstroChristie avatar
Paul Macias:
Thanks guys. I think I'll just have to just do some testing. I suspect 5 min RGB will be ok. I typically do L at 0 gain, 5 min, but mostly out of convenience and less frames to deal with, but I will have to see what works best. If I'm doing 100 gain, it's typically 2minutes. 

My preference is to not have to shoot different exposure lengths, but I might just do it anyway for learning purposes.

You will surely blow out the core. I recently used 5 second exposures with RGB filters and still had blown out stars: M42 
Test out your exposures and measure the star brightness in your subs
Wim van Berlo avatar
Wim van Berlo avatar

Paul Macias · Aug 20, 2025, 07:53 PM

Hey guys, 

First post by me! 

So, most of my imaging is narrowband since I'm in Bortle 7, but I occasionally do LRGB. However, one target I haven't done is the Orion Nebula. For most targets, I will generally shoot 5 minute exposures for RGB and for L, but at 0 gain. Most of the time it's ok, as I don't tend to clip much at all, but I've read that some people combine stacks of Orion Nebula shot at different exposure lengths, as the very bright parts might get completely blown out at higher lengths. 

Do people still do this? Or would simply lowering exposure length as to not clip much be fine?

If it matters here's my equipment
533mm
Sky Rover 80mm f/6 triplet
Astronomik deep sky RGB and L3

Thanks in advance!

In my opinion; no, multiple exposures are not needed. Modern cameras have enough dynamic range for this object. Here’s my version with 72 × 30 s exposures from Bortle 4. I only used RGB and no L. Two versions of the same data with slightly different processing.

https://www.astrobin.com/700v3r/C/?nc=&nce=

https://stargazerslounge.com/topic/411071-m4243-once-again/

Cs,

Wim

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Paul Macias avatar
Paul Macias avatar

Thanks guys. I will probably just do a shorter exposure than I normally do for this when it comes time to do so. I will just have to find the right length.

Ferran Bosch (S.A.C.) avatar
Ferran Bosch (S.A.C.) avatar

It will depend on the quality you are aiming for. If you don't have very high expectations, you can do a very long series with relatively short exposure times. At F6, I think 180" is a good compromise, but you will need to take a lot of shots to get a good sample of the background and the weak and dimly lit elements. On the other hand, if you want to take a very high-quality photo, I recommend using HDR, for example, taking 5“, 15”, and 30“ shots, then moving on to 180” shots, and finally 300" shots. It's a hassle, but the difference in the result is substantial. In this hobby, every detail counts; nothing is trivial.


BR

Ferran.

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Read noise Astrophotography avatar
Read noise Astrophotography avatar

Short answer: Yes, people still do HDR for M42. If you only shorten exposures enough to save the Trapezium, you’ll starve the faint outer dust. Best results multi-exposure HDR.

Why

Orion’s dynamic range is brutal: the Trapezium core saturates fast, while the outer dust needs long subs. Your 533MM helps (big well at Gain 0, clean short subs), but you’ll still clip the core at 300 s unless you layer in shorter data.

Exposure plan (Bortle 7, f/6, 533MM)

  • Gain: 0 (keeps full-well high; 533 read noise is low enough that short subs still calibrate well).

  • Luminance (L3):

    • Core: 1–2 s subs (100–200 total) + 10 s subs (60–100)

    • Mid-tone: 60 s subs (60–100)

    • Faint outer dust: 180–300 s subs (80–150)

  • RGB (Deep-Sky RGB):

    • Core: 5–10 s (40–60 per channel)

    • Field: 120–180 s (60–100 per channel)

If time is tight: shoot L at 10 s + 60 s + 300 s and RGB at 10 s + 120 s. That already covers the range well.

How to combine (PixInsight, but same idea elsewhere)

  1. Calibrate & integrate each exposure length/channel separately (don’t mix lengths in one stack).

  2. HDR on L: HDRComposition with your 1–2 s → 10 s → 60 s → 300 s L stacks (shortest first). Check Auto Exposures; leave Reject black pixels off unless you truly clipped blacks.

  3. HDR on RGB: Two options:

    • Do HDRComposition per channel (R, G, B), then ChannelCombination.

    • Or build a normal RGB from the long subs, then run HDRComposition again feeding in the short-RGB masters.
      Either way preserves color in the core.

  4. Process L (DBE/ABE → mild decon if needed → noise reduction → gentle contrast).

  5. LRGB combine (LRGBCombination), then finish color tweaks.

  6. If you see halos/joins around the core, blend with a RangeMask to soften the HDR transition and keep stars natural.

If you skip HDR

You can choose a single exposure that barely avoids clipping the core (e.g., 30–60 s at Gain 0), shoot a ton of subs, and stretch carefully. You’ll keep the core but you won’t reach the faint outer dust like with 300 s. That’s the trade-off.

Practical checks

  • On a linear sub, the brightest pixels in the core should not hit the camera max. In PI, inspect Statistics: you want no white-point clipping on your longest subs; let them clip only the very inner Trapezium if you’re doing HDR.

  • The 533MM has no amp glow; short subs calibrate cleanly perfect for Orion HDR.

Bottom line: For Orion, multi-exposure HDR wins. Shoot short subs to save the Trapezium, long subs to dig out the dust, then merge. Your 533MM at Gain 0 is ideal for this approach

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