First Veil (West). Processing issues

[Shiinsuh]

Hi all,
I've had my first go at NGC 6960 a few days ago. I have been trying to process it but i keep coming across two issues. First is that the nebulosity comes out green rather than blue, adjusting it to blue makes everything else off colour (Reds are fine with no colour correction). Second is I'm getting a lot of noise, particularly in red. I'm unsure if this is due to there not being enough subs or the calibrations frames are wrong some how, or I'm just missing something during processing. 
I'm after a bit of advice on both issues, It appears I have a lot of data, i just don't know how to get the best out of it. I'm unsure if the colour issue is due to my CLS filter washes everything out with blue. 
(info at bottom of post)


This is the best I have been able to get out of my data. I'm happy enough with it, but id like to get rid of a lot of the noise and to get the nebular blue rather than green.

Subs: 31 x 120s, 7 x 360s
Darks: 18 x 120s, 6 x 360s
Flats: 26 x 1/640s

Camera: Canon 600d(T3i) unmodded with Astronmik CLS CCD clip filter
Scope: Skywatcher 130PDS with Baader MPCC mkII
Mount: Celestron CG5-GT, Guided

Stacked in DSS 3.3.4, Processed in Photoshop CS6.

[steppenwolf]

Your data seems pretty good, focus is fine etc. CLS filters tend to give a blue cast so perhaps the answer lies in correcting this.

Can you post up a stacked version before post processing so that we can have a look at that?

[Shiinsuh]

3 minutes ago, steppenwolf said:

Your data seems pretty good, focus is fine etc. CLS filters tend to give a blue cast so perhaps the answer lies in correcting this.

Can you post up a stacked version before post processing so that we can have a look at that?

I have suspected that the green colour be due to the CLS filter, but I'm unsure. Stacked / unprocessed tif attached.

161116 NGC6960 DSS 02 (na).tif

[steppenwolf]

I'm downloading your data now.

 

[steppenwolf]

I've looked at your data and it doesn't have a blue cast so it is not your processing that has slewed the colours. I have also looked at some of my earlier images of this object captured with a one shot colour camera and the same regions are green so I think your data is accurate from the point of view of the equipment that you are using.

[Shiinsuh]

22 minutes ago, steppenwolf said:

I've looked at your data and it doesn't have a blue cast so it is not your processing that has slewed the colours. I have also looked at some of my earlier images of this object captured with a one shot colour camera and the same regions are green so I think your data is accurate from the point of view of the equipment that you are using.

That's a shame then, It would have been nice to have the blue parts of the nebula naturally blue rather than having to colour shift them. But also it's good to know that it's not due to my processing or the filter doing it. Thank you.

Do you have any idea about the red blotchy noise? I don't know if its due to lack of subs or if the darks are bad (I know its difficult to use darks with a DSLR due to temp variation), or some other issue causing it.

[steppenwolf]

Normally, I'd say that you need more subs but in fairness, you have a fair number. However, the longer subs (likely to be the more thermally noisy) are short on quantity and I suspect that this is where the problem lies. On that subject, I aim for at least 20 darks and again you are down on that with the longer exposure darks.

DSLR Darks are a compromise, that's true but another argument for more dark frames to get a better average.

[Shiinsuh]

I'm certainly going to try get more long exposures along with some darks. I'm not happy with only having 7, 6 minute ones. But the weather isn't being friendly at the moment, I just hope it clears because the Veil starts dropping west too early to image it. Hopefully that will clear up the noise.
 

[steppenwolf]

Good luck with the additional data capture - you have made a good start.

[Shiinsuh]

Thank you very much! I'll be sure to post results if / when I get the extra data.

[alacant]

Hi. Try losing the dark frames and take bias instead. On my 700d, this cleared a lot of the noise. If you can dither too, even better. HTH.

[ollypenrice]

I'd also try using bias as darks plus a large 12 pixel-ish dither.

After palying with your data I came to the same conclusion as Steve: it is determined to be green! The OIII line lies on the blue-green border so this is not a disaster but I agree that bluer would be better. You do have twice as many green-filtered pixels as blue, which won't be helping. This is as blue as I could get it and it has knocked the data about going even this far. I'd let it be, myself.

I thought your background was pretty decent for the time and for a DSLR. I've seen far worse colour mottle than this and you data does preserve an important feature of the target: the sky outside of the bow shock (ie the right hand side here) is redder than that of the swept part within the Cygnus Loop.

 

Olly

 

[Astrosurf]

My veil was also green with my DSLR.

Alexxx

[Shiinsuh]

On 11/21/2016 at 09:26, ollypenrice said:

I'd also try using bias as darks plus a large 12 pixel-ish dither.

After palying with your data I came to the same conclusion as Steve: it is determined to be green! The OIII line lies on the blue-green border so this is not a disaster but I agree that bluer would be better. You do have twice as many green-filtered pixels as blue, which won't be helping. This is as blue as I could get it and it has knocked the data about going even this far. I'd let it be, myself.

I thought your background was pretty decent for the time and for a DSLR. I've seen far worse colour mottle than this and you data does preserve an important feature of the target: the sky outside of the bow shock (ie the right hand side here) is redder than that of the swept part within the Cygnus Loop.

 

Olly

 

Thanks for the feedback. It's a shame that its not easy to take out the green without loosing data or making the stars unrealistically blue. Until I get a CCD I'll just have to put up with the greenness for now. At least the what you have done smooths out the blotchyness of the space background. I still have a lot to learn about processing!

I had a chance to get some more exposures last night, the cloud finally cleared. I enabled dithering (was a pain to get APT and PHD talking but got it in the end) as suggested and took around 30 x 360 second exposures. I only managed to get around 20 decent ones in the end however But it gave me a total exposure of around three and a half hours. I did some more darks but also bias, so I can play around with with that. The noise / grain has come down a lot. Still have the mottle however, but I did use the previous data as well, I'll have a look at using only the recent and see what happens.

A lot more detail is reviled now with the additional exposures with in the main part of the nebula as well as patches to the left. I'm sure there is room for much more to be brought out.
It looks like this evening will be clear so I'll go out and take some more exposures to try and further clear up the noise.


161123 NGC6960 DSS 01 (na).tif

[Astrosurf]

Cracking shot. I found with my 600D that the blotchiness is more in the red channel so did some selective blurring there.

[Shiinsuh]

25 minutes ago, Astrosurf said:

Cracking shot. I found with my 600D that the blotchiness is more in the red channel so did some selective blurring there.

Thanks!

The mottel seems to be mostly in the red here too. Must be something to do with the 600d. I'll give blurring the red channel a go (making sure to miss the nebulae ofcorse). 

[Astrosurf]

Let us know how you get on.

[Guest]

I used to use a QHY8 one shot colour camera and the images of this object were always green, and mine wasn't the only one I saw around this site and the web that looked green.

I too preferred the blue/red combination

I struggled quite a bit with this myself - here's my SGL posts on the subject (below)

Hope that is of some use :-)

David

 

 

[Shiinsuh]

Last night I managed to get another couple of hours worth of data. Its brought out a lot more detail and let me push the colours more before giving loads of distracting noise. Still have the noise however, even after trying all the types of calibration frames. But it has improved at least.
I'v tried blurring the red channel for the noise as suggested. It seems the noise isn't just in the red and makes the stars come out weird (probably need a star mask to protect them).
Here is what I have now:


Here is the stacked data to play around what can be done with it to improve it.

161128 NGC6960 DSS 01 (na).tif

[Astrosurf]

As you say, still quite noisy. What calibration frames did you do? Getting nice star colour there. I select the stars periodically during stretch iterations to protect them, and also ensure the white point doesn't go up in Curves or Levels.

Still, a very nice image!

[Dave In Vermont]

Yes it certainly is! Please do continue.

Dave

[EWHB]

These are all very clear images.

[Shiinsuh]

3 hours ago, Astrosurf said:

As you say, still quite noisy. What calibration frames did you do? Getting nice star colour there. I select the stars periodically during stretch iterations to protect them, and also ensure the white point doesn't go up in Curves or Levels.

Still, a very nice image!

I have gathered; Flats, dark flats, bias and darks. I tired with only the darks (plus flats and dark flats), darks with flats only, Only the bias (plus flats and dark flats), and all of them.
I dint really see any difference between using darks or bias.
I have also started dithering. The only thing I need to try is negating the original batch of lights (maybe they have a lot of noise in them that is still coming through). I will try that later!

I have not protected any of the stars, probably something I should do. However I take care not to clip any white or black when stretching or curves.

 

3 hours ago, Dave In Vermont said:

Yes it certainly is! Please do continue.

Dave

I will certainly be collecting more data next time the sky is clear and add it to this data set. (Tonight is going to be clear but its going to be hazy )

 

10 minutes ago, EWHB said:

These are all very clear images.

Thank you!