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Veil Nebula fail - where did I go wrong?


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I'm just getting back in DSO AP after a year and a bit away and would really appreciate any input or suggestions what might have gone wrong here - thanks in advance 🙏🙏🙏

On the night of 16-17 Aug I imaged NGC6992 Veil Nebula (from Berkshire in the UK, about 51°N) and did an OK job for a relative newcomer (post here)

On the night of 18-19 Aug I tried again but it was an abject failure 😭😭😭 The images that came out of it just looked horrendous and noisy which was a surprise as I really don't think I changed much in the way of settings or setup. No problem to chalk it up to experience but I want to understand what went wrong to learn and avoid mistakes in the future.

Below are the two images after stacking and basic tweaking & stretching in Siril - I've rotated them so the nebula is in the same orientation in both. The first one is OK - after a bit of Starnet action and tweaking in GIMP I was able to make a half decent image. The second one is just an ugly horrible mess and I don't really know why :(

Looking back, in the first session the initial test images I took for framing, focus and exposure looked good - with a little histogram stretch I could already see some feint nebulosity.

But in the second session the same test images already looked brighter, overexposed somehow, like a misty fog had been laid over them. I can't be certain but I don't believe the atmospheric conditions, seeing, cloud, moon, light pollution were dramatically different between sessions - both seemed good conditions.

In both cases I took about 75 x 120s lights between about 0030-0330. In each session I took new bias, darks and flat frames (10-20 of each, similar number in both cases).

What I changed between imaging sessions:

  • I reduced ISO on the camera (modified Canon 450D) from 1600 to 400 to try and reduce noise in the image. N.B. my previous best imaging was M51 in May 2023 and I note that I used ISO 400 in that case, so I don't believe this setting is fundamentally bad, but maybe wasn't ideal for NGC6992 or conditions on the day maybe???
  • I rotated the camera 90° just for framing, then refocused to make sure the image was still sharp
  • Switched on dithering on the ASIAIR mini - I'm new to autoguiding but everything seemed normal to me.
  • On the first go the ASIAIR told me polar alignment was so-so, but on the second attempt I actually got it much closer to zero error (<5' error) and got a smiley face from the app

Other gear:
SW150PDS + modified Canon 450D + HEQ5
Altair 60mm guidescope + ZWO ASI120MC-S guidecam
ASIAIR mini

ngc6992_veil_finish_2024_08_16-17.thumb.jpg.36b40abf8dbc336d881fce9d461497cc.jpg

ngc6992_veil_finish_2024_08_18-19.thumb.jpg.d1947c550430d694a2da7394e32b9f92.jpg

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Second image is a vomit bucket. Calibration frames all wrong, flats not worked, darks look like they are wrong, and banding. I would check what is the best ISO for the Canon. Nikons are invariant but Canons are not, so higher ISO might be required. 

Also, if you had cloud or very poor seeing then this has killed the subs. Have you flicked through the subs? 

Did you check the histograms? Were you clipping the left hand side?

Edited by 900SL
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34 minutes ago, 900SL said:

Second image is a vomit bucket

Hahaha 🤣 LMAO at this! 

35 minutes ago, 900SL said:

Calibration frames all wrong, flats not worked, darks look like they are wrong, and banding.

Interesting. When I looked at the stretched master dark & bias images I noticed some bands but I don't really know what I'm looking at. Amazed you can deduce that from the image.

36 minutes ago, 900SL said:

I would check what is the best ISO for the Canon. Nikons are invariant but Canons are not, so higher ISO might be required. 

Yup, noted, thanks. I also have a D5500 so was aware of this. Curious though that I've used ISO400 on the Canon before and achieved very good results.

38 minutes ago, 900SL said:

Also, if you had cloud or very poor seeing then this has killed the subs. Have you flicked through the subs? 

A bit, yes. The first set all look good, hunts of nebulosity even in a single sub. The second set look awful! 

39 minutes ago, 900SL said:

Did you check the histograms? Were you clipping the left hand side?

Interesting. Yes, I compared some and didn't think either was terrible but will look again w.r.t clipping. 

Thanks for your suggestions, much appreciated 👍 

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51 minutes ago, imakebeer said:

Ah, yes, I was just reading through your thread. 

May I ask roughly where you are? Within a million miles or so of Reading, Berkshire, SE of England?

No, look under his avatar…👍🏻

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1 hour ago, imakebeer said:

Ah, yes, I was just reading through your thread. 

May I ask roughly where you are? Within a million miles or so of Reading, Berkshire, SE of England?

Nw England so not that far

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14 minutes ago, Elp said:

Also the moon is close to 100pc phase. Just because it's low down or you can't see it, it's still reflecting sunlight into space.

I think my "I can't see it so it can't see me" tactic is not as well thought out as I'd hoped :(

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My money would be on the moonlit wildfire smoke. I could just about see arcturus on 19th and nothing less bright than that (I'm in Dorset, UK) the sky looked like reddish-grey milk before sunset, but didn't obviously look cloudy as it became dark, apart from the lack of stars.

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3 hours ago, imakebeer said:

Canon 450D

A few pointers:

450d, so no dark frames of any description. They will introduce noise along with other artefacts. ISO 400 is correct. 

Do take flat frames, but calibrate simply by subtracting the offset (1024 for the 450) from them and the light frames. 

Wait until the moon has gone.

Try with a uhc filter.

Cheers, CS and HTH.

Edited by alacant
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The smoke from the fires being transported across the Atlantic in the Jetstream was definitely  present at the time your poor capture. I was planetary imaging at the same time and was negatively affected to the degree that I abandoned the session. I am in Oxford so not far from you.

Edited by bosun21
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Thanks for all the feedback. I am relieved to hear that there is a good chance that the sky was broken rather than I had done something stupid! 

FWIW we did see the orange moon low in the sky the other night (17th/18th maybe, can't remember for sure?) so there's a big clue. 

I didn't have direct line of sight to the moon on either of those nights (low in the sky and blocked by our house) but I checked on Stellarium - it was lower and set earlier on 16/17, but went higher and set later on 18/19 so could have been a contributing factor... apart from glue from the moon I guess moonlight would also reflect and scatter off any dust or smoke in the atmosphere, right???

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Yes I had a sepia coloured moon on the night in question as well. I didn't initially click on that this was the reason I had to excessively increase my exposure times on Saturn to maintain the histogram level.

Edited by bosun21
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2 hours ago, alacant said:

A few pointers:

450d, so no dark frames of any description. They will introduce noise along with other artefacts. ISO 400 is correct. 

Do take flat frames, but calibrate simply by subtracting the offset (1024 for the 450) from them and the light frames. 

Wait until the moon has gone.

Try with a uhc filter.

Cheers, CS and HTH.

Ooh, this is interesting! Could you expand a bit, what is it about the 450D that causes issues with dark (and bias too?) frames? Is it all/many Canons, all DSLRs or just this one?

The basic script I use in Siril won't run if you don't have at least 2 each flat, dark & bias frames so I'll need to figure out that hurdle 🤔 

I'm not quite sure what you mean about subtracting an offset of 1024 - I wonder if this can be managed on Siril (sorry, still very much learning the ropes!)

I'll have a look into UHC filters and what they do. 

Thanks for the suggestions 👍🙏

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They do look so different, but if you look closely both images have colour banding, though it is much more obvious in the second one. It may well be true that the viewing was poorer for the second image and if you stretched it a lot more then that may well have exacerbated the defects.

However, for my money I think the main problem lies elsewhere. If the stacking had worked properly you (a) shouldn't be seeing the dust doughnuts, and (b) the banding shouldn't be so obvious. You could try re-stacking from the beginning, being careful that you are using the right files for each of lights, darks, flats and bias frames. Check a few sample subs from each to see that they are indeed what you expect and haven't got them muddled. Do a quick auto-stretch on each of the stacked results to see if there is any improvement. Otherwise, it might be worth trying stacking in another program like Deep Sky Stacker. Note that darks need to be taken at the same temperature as the lights to be effective, and this can be a problem with non-temperature controlled DSLRs. I used to take them at the end of the imaging run so that the sensor had reached some kind of equilibrium temperature. Also, you are imaging at a time when ambient night-time temperatures have not been low, so this might have exacerbated the issue.

Good luck.

Ian

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4 minutes ago, The Admiral said:

They do look so different, but if you look closely both images have colour banding, though it is much more obvious in the second one. It may well be true that the viewing was poorer for the second image and if you stretched it a lot more then that may well have exacerbated the defects.

However, for my money I think the main problem lies elsewhere. If the stacking had worked properly you (a) shouldn't be seeing the dust doughnuts, and (b) the banding shouldn't be so obvious. You could try re-stacking from the beginning, being careful that you are using the right files for each of lights, darks, flats and bias frames. Check a few sample subs from each to see that they are indeed what you expect and haven't got them muddled. Do a quick auto-stretch on each of the stacked results to see if there is any improvement. Otherwise, it might be worth trying stacking in another program like Deep Sky Stacker. Note that darks need to be taken at the same temperature as the lights to be effective, and this can be a problem with non-temperature controlled DSLRs. I used to take them at the end of the imaging run so that the sensor had reached some kind of equilibrium temperature. Also, you are imaging at a time when ambient night-time temperatures have not been low, so this might have exacerbated the issue.

Good luck.

Ian

Thanks for the tips, I will check on those points 👍

I definitely used the right dark, bias & flat frames - I've moved to a new computer so there weren't any others to choose from in the first case, and I already checked this in the second image.

Since you mention taking dark frames at the end - that's a good point about reaching an equilibrium temperature...

But I had been pondering this point... Is there any way to automate this? My guess is those with dedicated astrocams & filter wheel have a black opaque filter for this???

But for me with a DSLR I have to go out and put the lens cap on the telescope - so since I'm lazy and by 1am I want to go to bed I've been doing darks & biases at the start of the session 😳

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9 hours ago, imakebeer said:

Ooh, this is interesting! Could you expand a bit, what is it about the 450D that causes issues with dark (and bias too?) frames? Is it all/many Canons, all DSLRs or just this one?

The basic script I use in Siril won't run if you don't have at least 2 each flat, dark & bias frames so I'll need to figure out that hurdle 🤔 

I'm not quite sure what you mean about subtracting an offset of 1024 - I wonder if this can be managed on Siril (sorry, still very much learning the ropes!)

I'll have a look into UHC filters and what they do. 

Thanks for the suggestions 👍🙏

You can find alternative scripts for siril that allow stacking without dark, bias or flat frames

 

Also, learning how to manually stack in Siril will give you much more understanding and flexibility

image.thumb.png.11e726f9620f0a37b24d1fb1308ae24c.png

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11 hours ago, 900SL said:

learning how to manually stack in Siril

+1. IOW, you are in control, rather than the computer. In fact there isn't a script to do what you need.

20 hours ago, imakebeer said:

Could you expand a bit

The old 12mp sensors can produce good results, but you'll need a bit of help from software to get you there. The -very simple- manual process for the reduction of noise, mottle and banding is described here. To which you may want to add:
p1.png.0fedc9af96a0a8d386166ee2153aa02c.png

You will also need to address the flat frame division, which hasn't worked. Again, it can't be stressed too strongly to understand what is happening during the calibration process. Simply pointing a script at a folder and hoping, perhaps isn't the way forward.

Anyway, great re-entry and a few easy bits of stuff to get you over the line.

Cheers, CS and HTH.

Cheers and HTH

 

Edited by alacant
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Many thanks @alacant, I appreciate your input 👍 I will look into those topics.

I've been Googling today and found a bit more information on the suggestions from you and others. I've already had a go with a different script in Siril that stacks only the light frames (OSC_Preprocessing_WithoutDBF.ssf), and I've been comparing the contents of that with the standard "OSC_Preprocessing.ssf". Shouldn't be too hard to make my own scripts if necessary. But I need to include flat frames and this mysterious 1024 next.....

I was also looking at a Siril tutorial on synthetic biases (https://siril.org/tutorials/synthetic-biases/) and if I'm understanding correctly this is telling me more or less how to do what you are suggesting w.r.t subtracting an offset of 1024 - right?

By the way - what exactly are we subtracting here? I recognise 1024 as a power of 2 which I assume isn't a conincidence. But I don't get what 1024 is a measure of - is it birghtness or something on the scale from white to black???

Also, if I've understood right I need flat frames but not biases or darks - which is fine by me as it'll save me a job 👍

Slightly off on a tangent but I've already been playing with Starnet and star masks, but I've also been having a play with star reduction which also brings something different and interesting.

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2 hours ago, imakebeer said:

if I'm understanding correctly this is telling me more or less how to do what you are suggesting

In a round about fashion. Please see the link in my previous post which is specific to your case, especially regarding the 450.

2 hours ago, imakebeer said:

I don't get what 1024 is a measure of -

It's simply the offset (sometimes called bias) which is used to prevent negative pixel values. 

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Each pixel on the camera sensor is represented by a 16 bit binary number. In theory, this could be sixteen zeros (0) for black and sixteen 1s (65536) for maximum brightness. Colour is done by putting a filter over the sensor so that there is a pattern of pixels recording red, blue and (twice as many) green brightness levels. Practical reality is a bit more complicated than that. For example, how would you know that a pixel was at zero because it received no light at all, or because it was broken, or becausse the data was lost in transmission or copying? It's quite common for camera manufacturers (and sensor manufacturers) to designate an "offset" - zero brightness is not 0, but something else (e.g. 1024). When we process an image in software, we want to get to remove that offset, so that black is really black. Simply deducting 1024 from the value of every pixel will do that.

There's other jiggery pokery going on too: our eyes are more sensitive to changes in the middle ranges of brightness than they are at the top and bottom of the range. It's very common to apply "curves" adjustments automatically to give more contrast where we will most easily perceive it. This is usually not applied to raw images in the camera (but the offset is part of the raw) but is often applied by software (e.g. it's the first thing a raw converter does). Since so many pixels in astrophotography are near the very bottom of the range, "ordinary" photo software can be unhelpful, which is why we use things like Siril.

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Posted (edited)

Thanks for the explanation @chrisecurtis , that makes sense 👍

10 hours ago, alacant said:

In a round about fashion. Please see the link in my previous post which is specific to your case, especially regarding the 450.

@alacant yeah, I did have a look at that link and tried to follow it. Honestly, I need a more explicit, more detailed guide since I've never stacked manually before* but I can Google for a more basic step by step guide. But the instructions in your link makes sense in general. And as much as we're drifting from my original question, this is really useful stuff as I'm aware Siril can do way more than the basic scripts I've been using up to now and I want to learn this stuff.

(* I recognise the "Plot" in the manual stacking section that shows the quality of each light frame in the stack - I think I had just started getting in to this shortly before I wandered away from the hobby for a year or so)

So following that link I think I did OK - I think I made the master flat, then used this and the =1024 offset to calibrate (?) my light frame sequence. But I think I might have done something wrong during registering and/or stacking - initially I ended up with star trails after stacking so I tried a different stacking algorithm. That seemed to mostly fix it but I think I'm still getting "star trailing" of the noise (?) and hot pixels (?) (see zoomed in screenshot).

EDIT: Oh and I did also try the banding reduction in Siril - it helped a bit I think but not a magic wand. More practice needed!

I shall perservere!

 

Screenshot 2024-08-22 090049.png

 

 

Edited by imakebeer
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