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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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Mikolaj Wadowski avatar
Mikolaj 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.