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Dark Nebula

ZWO ASI2600MC Pro Processing Deep Sky NGC7023 Dark Nebula
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Dominic Wijker avatar
Dominic Wijker avatar
Hi!

I work on my first dark nebula project, the Iris Nebula. I use my TS-Optics GSO Photon 8" F5 Newtonian Telescope with a coma corrector, ZWO ASI2600MC pro camera and the Optolong L-Quad filter. 

I got around 7h of data with 300s subs, in Bortle 6-7.

I did some processing, but it seems i don't have any of the dark nebulosity in my data. I overstretched the data to see if there is anything at all and I cant see any except around the center where it reflects the beautiful blue light of the central star.

Is it my equipment, acquisition or my simple processing? I use Photoshop and have limited experience.

Any advice will help!
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Brian Puhl avatar
Brian Puhl avatar
dark nebula from bortle 6-7 is going to be extremely tough.   It's going to be even thougher using an OSC camera.     On top of that filtering your light isn't doing you any good either.   I was reading that this filter for some reason claims to be marketed for this purpose, but I'd disagree.   Filters like these are designed for emission nebula, not so much dark nebula.  

If you're going to try, stick with a standard UV/IR filter, and be prepared to dump ALOT of time into this project.   You've got alot going against you.   I've seen respectable results from higher light polluted areas like yours, but they've required 30-40 or more hours, and most of them are using mono.

If you have the time, just a few hours in a darker site outside the city will produce MUCH better results.
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Dan Kearl avatar
Dan Kearl avatar
https://cdn.astrobin.com/solutions/images/97817/2024/7eb9df67-d209-4c99-a41e-009719b828ce-1715365339.jpg
Biran Puhl is correct that it will take 40 hours.
I imaged this in my Bortle 8 yard with a 533mc and a Radian Triad filter.
Not the Best image but it is doable with enough time.
This is 40 hours with 600 sec. exposures.
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Dominic Wijker avatar
Dominic Wijker avatar
Brian Puhl:
dark nebula from bortle 6-7 is going to be extremely tough.   It's going to be even thougher using an OSC camera.     On top of that filtering your light isn't doing you any good either.   I was reading that this filter for some reason claims to be marketed for this purpose, but I'd disagree.   Filters like these are designed for emission nebula, not so much dark nebula.  

If you're going to try, stick with a standard UV/IR filter, and be prepared to dump ALOT of time into this project.   You've got alot going against you.   I've seen respectable results from higher light polluted areas like yours, but they've required 30-40 or more hours, and most of them are using mono.

If you have the time, just a few hours in a darker site outside the city will produce MUCH better results.

Good to know! Appreciate your advice!
Dominic Wijker avatar
Dominic Wijker avatar
Dan Kearl:
https://cdn.astrobin.com/solutions/images/97817/2024/7eb9df67-d209-4c99-a41e-009719b828ce-1715365339.jpg
Biran Puhl is correct that it will take 40 hours.
I imaged this in my Bortle 8 yard with a 533mc and a Radian Triad filter.
Not the Best image but it is doable with enough time.
This is 40 hours with 600 sec. exposures.

Seeing this picture makes me want to spend another 30++ hours! Amazing!
andrea tasselli avatar
andrea tasselli avatar
This talk of 40+ hours is nonsense. Even more so for a bright nebula like the Iris (bit early in the year, me thinks, but never mind). I shoot in B7 skies (on average) and I just use a L-PRO. 
This is with around 12h @ f/4, 1.35"/px (OSC, of course):

NGC7023 RGB (OSC) - The Iris Nebula - AstroBin
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James Peirce avatar
James Peirce avatar
You ought not to anything near 40 hours of data or the like to get significant nebulosity here (speaking obviously about the dust; the reflection nebula is bright and not hard to resolve). It will be much harder, and with much weaker signal-vs-noise than vs dark skies, but the most important parts of what you’re able to resolve from your exposures are going to come down to assorted best practices.

For example:
  • Moonless nights (no need to salt the wound).
  • Well-executed calibration; especially flats
  • Aim for high in altitude (light pollution degrades dramatically as you shoot through more and more atmosphere)
  • Carefully avoid light contamination (local light sources, from light leaks, etc.) as these will quickly add complexity to light gradients, which make it much harder to bring out faint detail).
  • Avoid under-exposure (get the most out of data over time).
  • Do collect a healthy amount of data.


And post-processing choices are important:
  • Use a sort of image normalization (e.g. Local Normalization in PixInsight) if available; simplifying light gradients helps considerably when bringing up faint detail.
  • Crop out any portions of the stacked image which are extremely problematic.
  • Use the right tool or tools for background extraction. Blanket sampling is bad on a target like this as the field is littered with nebulosity. Sampling which takes points needs to be placed well. If using Pix, the new Multiscale Gradient Correction is perfect for this (as long as the Iris is in the MARS data; not sure).


I suppose in short the real trick here is to do everything you can to avoid elements which add complexity to the light gradients you are going to be dealing with normally imaging under light polluted skies, and then to remove that light gradient cleanly without compromising the subject.

And if you have the means to travel to dark skies, subjects like these are the ones worth doing it for. Getting some data from slightly darker skies can also be useful with normalization, as you can use the cleaner data to get better normalization on the more light polluted data. And if you can get to really dark skies, one night under good, dark skies will casually trump 20 nights under heavy light pollution with broadband imaging of dim subjects.
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kgav8r avatar
kgav8r avatar
Dominic Wijker:
Hi!

I work on my first dark nebula project, the Iris Nebula. I use my TS-Optics GSO Photon 8" F5 Newtonian Telescope with a coma corrector, ZWO ASI2600MC pro camera and the Optolong L-Quad filter. 

I got around 7h of data with 300s subs, in Bortle 6-7.

I did some processing, but it seems i don't have any of the dark nebulosity in my data. I overstretched the data to see if there is anything at all and I cant see any except around the center where it reflects the beautiful blue light of the central star.

Is it my equipment, acquisition or my simple processing? I use Photoshop and have limited experience.

Any advice will help!

I refuse to believe you didn't capture any dark nebulosity after 7 hours.  The best tool for stretching dark nebula that I have found is the Generalized Hyperbolic Stretch.  I see you are using photoshop.  I would strongly encourage you to move to an astro specific processing software.  How did you stack the data?  As mentioned above, calibration files will be a key to the success.  Proper background extraction will also be important to ensure you are not removing any signal.

I shot this about a year ago and my first attempt at processing was disappointing.  I gave it another go (as I usually do) and the results were much better.

Don't give up on this data, I bet there's more there than you realize.

Here is Less than 6hrs under bortle 4, but look at my original version.  Processing can really make a difference.
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Alex Nicholas avatar
Alex Nicholas avatar
Dominic Wijker:
I got around 7h of data with 300s subs, in Bortle 6-7.

I did some processing, but it seems i don't have any of the dark nebulosity in my data. I overstretched the data to see if there is anything at all and I cant see any except around the center where it reflects the beautiful blue light of the central star.


Your equipment and acquisation are most likely not the issue. The issue here is you're imaging a very dark target from very bright skies, you're going to need a LOT more than 7 hours to get it to come out. 

I'm in Bortle 4-5 and dark nebula can still be really really tricky from here, I ususally save my dark neb targets for when I travel to Bortle 1 zones a couple of times a year... 

I would double your total integration time, but potentially drop from 300s to 180 or even 120s...

Another thing I'd check is, what offset are you running your camera on, and what is the lowest ADU count in a bias frame?  if your offset is too low it can be really hard to dig those darker details out of the noise floor... I run my offset such that my lowest ADU count is about 130ADU's which is still VERY low, but it allows the me to ensure that nothing is ever rendered too dark to be able to be processed, while still affording me a very high level of dynamic range.
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Alex Nicholas avatar
Alex Nicholas avatar
andrea tasselli:
This talk of 40+ hours is nonsense. Even more so for a bright nebula like the Iris (bit early in the year, me thinks, but never mind). I shoot in B7 skies (on average) and I just use a L-PRO. 
This is with around 12h @ f/4, 1.35"/px (OSC, of course):

NGC7023 RGB (OSC) - The Iris Nebula - AstroBin

All else being equal... 12h at f/4 ~= 17h at f/5, so for starters theres a pretty high discrepency, you're shooting gain 200, we don't know what gain OP is using, but likely 100, so without changing gain, maybe push that 17h out to 24h.

And as we all know, to get a noticable improvement in SNR, you really need to double the integration time... 

Remind me... what is 2x 24? 

Yep... That's what I got, too.
andrea tasselli avatar
andrea tasselli avatar
What makes you think we have the same camera and from the same makes. Because we ain't. My 200 gain isn't his 200 gains. And never mind the different sky conditions. So it's 12 hrs and stays that way.
Huib Wouters avatar
Huib Wouters avatar
I did 6,5hours in Bortle 8 with an F7 telescope and OSC camera on the Iris nebula (https://app.astrobin.com/i/2wip98). I added another night of data, but the higher humidity of the third night didn't lead to an improved image. Post processing becomes difficult, I tried to get out the finer structures, but this increases noise and blows up the stars. Processing the nebula starless was even more difficult, so ended up discarding that image.
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