Ouroboros

Wow! That is a fantastic image. You really have pulled out the detail in the dust.  Nice to see the bubble so well. 

Personally, and this isn’t a criticism because it’s all a matter of personal taste, I don’t like the colour pallet. But then, I instinctively react against false colour astrophotography unless it’s for justifiable scientific reasons. I know that’s against the current trend, but there you are.  That’s my view.

Given the obviously good quality of your data, have you considered processing it more realistically?  Might be interesting. The crescent nebula looks like you’ve got lots of OIII there. Keeping Ha as red and OIII as green/blue might look very striking. 

Fantastic image. I am so impressed with what some of you ‘real’ astrophotographers can do.

Thanks for posting your workflow. So useful for those of us still padding around in the foothills of astro processing. 

What does integer resampling do for you? 

Interesting. I was wondering whether doing some initial stretch to the whole image followed by star separation might have provided a better approach.

I had to look up DDP as I have not come across it. It is available in Pixinsight, but seems to be considered obsolete compared with other methods. That’s not to say it is in the applications you’re using of course, and anyway seems to work for you. 

Very nice colours and nice image generally. You must have been up half the night processing. It is a fascinating set of objects  though isn’t it?

Coincidentally I had another go at widefield imaging the whole Cygnus Loop last night.   I packed in about 11pm after getting three hours of data. I want to get a few more hours of data before processing.

I had my first go at dual band imaging this region a few days ago. How did you find separating processing the stars?   I found stretching the stars to give a satisfactory star field difficult. Did you separate the stars at the linear or nonlinear stage? 

Very nice pic.  And great to capture that in such a short time.  

Oh, I know the Cornish weather very well.  Warm, wet, westerly winds as they taught us in school geography.  Not 'arf!.  Some of the best astronomy weather I've had down there is when the weather has been coming from the east.  

Surprisingly we had clear(ish) skies here yesterday evening.  I was beginning to give up hope of ever testing my new Askar Colour Magic 6nm dual-band filter with my recently acquired Askar FRA300 I posted on in October. 

Inevitably perhaps the first target had to be the Cygnus Loop.  I should really have framed it differently.  I've chopped off some of the object at the bottom of the picture. But I didn't know whether I was going to get 20 minutes of clear sky or several hours. So I just went for it and in the end managed a modest 71 x 120s  reasonable subs out of 85 or so.  

I found my first process of dual-band interesting, and need to study how to do this rather better.  Stars in particular were a bit of a challenge.  Using  an unfiltered star field would probably be better I imagine.   

What a fascinating object and an excellent image of it.  It looks three dimensional. 

22 hours ago, wimvb said:

The stf is much too aggressive for a permanent stretch. It’s function is to show as much detail in a linear image as possible. It does this by bringing in the black point and white point and moving the mid point to achieve maximum contrast. If stf is applied permanently, you suddenly need to fix things that didn't need fixing before.

It is too aggressive.  It took me a long time to realise this fact.  Unfortunately a lot of introductory videos use the STF/Histogram as the standard method to stretch to non-linear.  So you sort of assume that's the correct way. They should really carry a health warning.  For a while now I've been using GHS as my go to stretcher. 

Yes, better still.  I should have run NoiseXT slightly harder maybe. 

OK. So this is my most recent effort after reviewing some of your comments.  I think I’m still struggling but at least the blotchiness in my initial attempt has largely disappeared, and I’ve managed to pull up some of the background not unlike your various examples.  I could go on fiddling with it for example by attenuating the noise a tad more. Incidentally I didn’t start applying noise reduction until towards the end.  Anyway, thanks for all your help everyone. 

Thanks, Bryan. I will look those out. I like AB’s vids. 

@Nikolai De Silva Excellent.  Amazing what they did with photographic plates isn’t it?  I bet someone had to guide those long exposures by eye for the whole duration of the exposure too.  Also amazing that those images were the best of what could be achieved then with big very expensive equipment ….. and that people can now produce better images in their own back gardens.

The coloured plate of spectra is amazing.  I checked my own copy of encyclopaedia Britannica published in 1953 and it does not have such good images.  

I was interested and intrigued to see @bdlbug how you returned to generalised hyperbolic  stretch again at steps 18/19. I don’t expect you to recall why you did that here but is that something you do regularly?  I’ve come across GHS as a means to stretch in a more controlled way from linear to nonlinear than is achieved by using the STF + histogram transform method. But I haven’t seen it used in the later stages as you have here. 

23 minutes ago, windjammer said:

The only issue I have with your data is the star haloes - there are a lot of them! and they largely defeated me.

I see what you mean. I’ve only recently started using StarXTerminator. Perhaps a bit slap dash of me, but I have so far naively assumed, unless the residues left behind are really bad, that since I’m going to put the two images back together they’ll be hidden again*.  Previously I used Starnet2 for a short while.  I thought StarXT did a slightly better job on balance. I didn’t use filters with this image. It’s straight OSC. The haloes could be caused by faint mist I suppose. Perhaps I should reblink the original subs, weed out if necessary and reprocess.  

* I can see this doesn’t necessarily work because the starless image is probably going to get stretched more than the stars image.  

Hey!  Lots of responses. Thanks all.  

On 31/10/2023 at 18:11, wimvb said:

That's your main culprit. TGV denoise mainly removes small scale noise but can leave blotches, which is probably what we see here.

Yes, my experiments with TGVDN last night seems to confirm this.

9 hours ago, windjammer said:

One could tweak this around forever - a fascinating field of view.  And very good base data.

Yes. In fact I’ve played around with this image so much I’m having difficulty seeing it anymore.

Thanks for the breakdown of your workflow. I very much like your second attempt particularly, along with @Fegato’s, for the way you’ve brought up the background nebulosity. 
 

3 hours ago, bdlbug said:

So I have enjoyed working with your data, yes its 3hrs and yes its got noise, but in UK getting 3 hrs broadband is a result....

Indeed it is. My recollection is that I got three hours of good data and then the mist rolled in off the Atlantic.  Thanks for taking the trouble  to provide your workflow. I shall study that.   You’re right - there are any number of ways to skin a cat process an image.  It’s one reason I take so long to process an image. Three steps forward, two steps back.  Your comment about giving the data a bit of noise reduction at the linear stage. I think I have that stuck in my head as recommended procedure when I first started learning PI.  Warren Keller even has a section on it.  Kill the noise before you stretch it seemed to be the idea ….. but not to the extent that you kill the important details. Anyway, maybe that rule of thumb has been superseded.

I also like the way you’ve developed the background. You’ve lost the purple colour and made it look more misty. Nice. You’ve also brought up the Cave Neb nicely too. 

1 hour ago, wimvb said:

SCNR is the best way to remove a green cast. But I seldom use it at full strength, because it can leave a purple cast where the sky is supposed to be neutral. Try it at 0.4 - 0.6 strength.

Yes. I often use a mask with it too to target the background for example.  Those residual green casts can be quite subtle can't they?  It's not until you remove them does it become obvious that they're there. 

Thanks, @Fegato.  Your background looks much better than in my effort, which encourages me to have another go.I think we all agree there is insufficient data. But then I’m thinking of this as a learning exercise.

I am amazed how quickly you produced a result. I work very slowly, not least because I don’t quite know where I’m going so I spend ages on each process trying something, going back, trying something else.  But how else can you come to understand what is after all a very complicated processing tool which can be applied any number of possible ways to achieve different results?Watching videos only takes one so far. 

7 hours ago, windjammer said:

mmmm, well, you could try posting your master lights here (just calibrated, coscor and aligned) and see what the ravening, opinionated, barbarian hordes here make of them.  Seeing as how its raining and we don't have anything else to do : )

OK Let's see if this works ...

This is 90 x 120s pre-processed data.  All I've done is a slight dynamic crop of edge effects. Enjoy!  

NGC281_dc.xisf

1 hour ago, ollypenrice said:

Can't agree on NR. It already looks like 'vaseline on the lens' and that's the root of the problem. But I do agree that it's a good image.

How about resampling it down to a lower resolution , accepting a smaller rendition, and getting a cleaner image?

Olly

 

well, just tried that by halving the resolution (ie reducing the image height and width by a factor of two, if that's what you meant) and it made no discernible difference. Thinking about it anyway, what you're looking at is reduced resolution because I reduced image size by a factor of about 5 before saving to jpg to post. 

.... and I don't think it is a particularly good image in that the framing could have been better ie the nebula could have been more central, but that's in the data acquisition stage.  

1 hour ago, windjammer said:

I'll go out on a limb and say, yes it is real and you haven't denoised enough.  Looking at my own image of the cave (a smaller FoV) in the area where they overlap, I have the same features as you in the murk.   I posted my pic here if you are interested - it needs to flip vertical to match your orientation.

https://stargazerslounge.com/topic/412672-cave-nebula-update-algorithm-beats-human/#comment-4403775

Simon

 

Very nice pic.   It’s interesting comparing the two. It took me a while to get my eye into spotting the similarities and differences. Interesting to see how the different techniques compare and what each bring out. I’m not sure my blotches appear in your image. But I’m going to reprocess mine and see what I get. 

1 hour ago, windjammer said:

.. but a great pic anyway !

Ta!  

2 hours ago, gorann said:

The mottled background you have could be real but NR may have made it look odd. Try processing it without NR to see what it looks like.

Yes, thanks for this suggestion. I’m going to revisit the data later when I’ve got some time to experiment with different amounts of noise reduction at different stages. Just got to experiment.  

1 hour ago, gorann said:

SCNR usually works well for getting rid of green, but not always. On some images it creates ugly blue casts on dark patches of sky. In those cases I skip SCNR and try playing with the green curve to suppress it in parts of the histogram. But I do that in PS, where I do all my stretching, but I assume something similar can be done PI.

The mottled background you have could be real but NR may have made it look odd. Try processing it without NR to see what it looks like. Like @ollypenrice I would never even think about applying any NR before I know what the image looks like (so after a fair amount of stretching) and I can se what the NR does to it (and I have the possibility to use it selectively).

OK. I can see the amount of noise with STF. 

39 minutes ago, ollypenrice said:

Edit: regarding green, try SCNR green. I run it on 9 images out of 10. DBE doesn't deal with it for me.

Yes. I use SCNR a lot. Usually later and very successfully to remove residual green after background modelisation and colour calibration.  The green I’m referring to above is the bright green colour cast observed in many OSC images immediately after preprocessing and before you do anything else. I have read it arises as a consequence of OSC sensors having twice as many green sensors as red or blue.  

Just out of interest I just applied SCNR to a just processed green image. It goes sepia, and looks yuck! I think it’s better to apply SCNR when you can see what it’s doing in a controlled way, possibly masked too to protect certain areas. 

24 minutes ago, ONIKKINEN said:

I think you might have pushed the data a little bit too hard in terms of denoising or stretch (or both). Could use more data too of course, 3 hours is really not that much. Since you have noiseXterminator i would rather use that than any other method of denoising like the TGV denoise you applied here. Try using only noiseXT with the same masks you did with TGV denoise? Use a value lower than full power, like 50% or so. Too much will turn the background into a painting which will also look blotchy since some detail that should have stayed as noise will have been evened out by noiseXT and will look unnatural.

But I'd say its not too big of a deal here, the uncropped image is not at all blotchy looking if you dont go looking for it. Image is not too bad to my eyes, would probably not have noticed any blotchiness if you had not mentioned it.

Thanks for your thoughts on this. I’ll revisit the data and have another go by trying those things.

I feel astro image processing is a constant learning experience. In a sense an image is never ‘done’.  I’ve picked up a lot in the last year or so, and feel it’s good practise to go back and redo old data.  The weather is lousy anyway so few opportunities to get new data.  

1 hour ago, Fegato said:

NoiseX - I was sure it was recommended as best on non-linear, but I can't find any advice on this on the RCAstro site now, so can't really back that up.

You might be right when they first launched it. In fact the process included a box to check for linear. That’s gone now.

I apply it early because not only does it work extremely nicely in linear but also because I thought it was recommended to do some noise reduction before stretching.