M57 - The Ring Nebula

[kirkster501]

Ten hours of data LRGBHa - from summer 2018 but only just got round to processing it.  Very hard to bring the faint outer shell out without blowing out the stars.

Lots of very small galaxies in the background.

[kirkster501]

Hmm, I am not so sure about this picture in hindsight.  I've got the ring OK but the background looks a but plasticky in retrospect.

I will have a play around a bit more.

[Magnum]

Maybe tone down the brightest part of the ring it’s lost it’s colour.

star colour is good 

[kirkster501]

Here's another version.  A new technique I learned.

[ollypenrice]

The data's clearly good and will support lots of processing routines and attempts. It would be a great data set to use for experiments in processing or on a processing course.

I'd begin by looking at the colour gradient roughly right-left but a little different in distribution in the two renditions. In Photoshop it would take seconds to measure the background sky values in R and G and B at various points. I don't know how that's done in PI but a colour-neutral background sky is my first building block. I never take a second step till until I have that. In the first image a screen grab shows blue too low on the left hand side and way too low on the right. This can be hard to judge by eye so I always measure.

Olly

 

 

[kirkster501]

How do you equalise a subtle gradient (especially in one colour) from left to right Olly? What tool or method would you use pls?

[ollypenrice]

12 hours ago, kirkster501 said:

How do you equalise a subtle gradient (especially in one colour) from left to right Olly? What tool or method would you use pls?

DBE. Simple as that. Unfortunately it has to be applied to the linear image so it involves going back to square one. After an edge crop it's the first thing I do in post processing. I follow Rogelio Bernal Andreo and Harry Page in applying a small number of carefully placed background markers, scrupulously avoiding anything but pure background sky. Give the linear data a very aggressive screen stretch (called STF in PI) so as not to put any markers on fuzzies, the glow around stars, or outlying nebulosity - particularly when processing galaxies which get bigger as you stretch. The advantage of fewer markers is that they let the software estimate broad changes in gradient rather than more local ones. Don't use DBE to correct star flare, for instance. When the gradient map appears look at it before applying it to ensure that it really is a broad gradient map has no local details in it whatever. In your image I'd try one marker in each corner and one half way along each side at the edges. Maybe two more, one each side of the ring but half way to the edge in each case. You don't want it to mess with that faint outer glow or to read that little spiral. I can't promise that this will be right but it would be my own first guess processing this data.

Screen stretch the result image after the DBE map has been applied using subtraction. If it still doesn't have a neutral background increase the 'Tolerance' value and try again. Once you get to a stage where there's maybe just a bit of green gradient left, stop. Switch to SCNR green at a low value on the slider and increase it till you get rid of the green gradient. You want to apply it as lightly as you can. Remember that you don't need to get a perfectly neutral background if your screen stretch is far more extreme than your real stretch will be.

Final head-banging stretches for the faintest detail I do in Ps using layers. I make a copy layer and work on the bottom, top invisible. In Curves, pin the background sky where it is (Cursor on the sky, Ctrl click) and fix it just below that with a point,  then lift the curve just above the background marker. This will stretch everything above the background including stars but you then go to the top layer and erase it over the stretched items you want to see in the final image. (Galaxies, nebulosity etc.) But the top layer will not have the stretched stars in it. This is a method unsuitable for images with widespread nebulosity but it's good for galaxies, PNs and images with lots of starfield.

Olly

[kirkster501]

Thanks Olly, yep, I did a DBE on the LRGB and got rid of it (I think).  Here is another version.  I brightened the background sky a bit and knocked back the colour saturation a bit too. 

 I need to work on the stars which have blown out - the Ring is also now blown out a bit  -  and many stars are suspiciously the same colour.  This is like a game of whack-a-mole; you address one issue then another pops up elsewhere!

I am convinced this data set is a good one and I am focusing on my processing skills in PI and Photoshop.

 

[kirkster501]

Yeah, the linear image....  I don't mind going back Olly.  Have to learn this stuff or else I won't get anywhere.

[ollypenrice]

33 minutes ago, kirkster501 said:

Thanks Olly, yep, I did a DBE on the LRGB and got rid of it (I think).  Here is another version.  I brightened the background sky a bit and knocked back the colour saturation a bit too. 

 I need to work on the stars which have blown out - the Ring is also now blown out a bit  -  and many stars are suspiciously the same colour.  This is like a game of whack-a-mole; you address one issue then another pops up elsewhere!

I am convinced this data set is a good one and I am focusing on my processing skills in PI and Photoshop.

 

Much better background. The gradient is almost gone but you could hit it still harder with an aggressive DBE I think. 

I know you want the outer glow of the Ring but not the brighter stars. You could try a custom stretch n Curves. Give the bottom a bigger lift than the top. Lift the bottom but bring it to a straight line from quite low down. The thing is that I can see nothing in the image which needs such a hard stretch in the upper brightnesses. Only the outer nebula and the little galaxy really need to be stretched hard. In PI the idea would be to mask the rest. No idea how to do that, though. In Ps you can just use layers as outlined above.

Olly

 

Edited October 18, 2019 by ollypenrice
clarification

[wimvb]

46 minutes ago, kirkster501 said:

Thanks Olly, yep, I did a DBE on the LRGB and got rid of it (I think).  Here is another version.  I brightened the background sky a bit and knocked back the colour saturation a bit too. 

 I need to work on the stars which have blown out - the Ring is also now blown out a bit  -  and many stars are suspiciously the same colour.  This is like a game of whack-a-mole; you address one issue then another pops up elsewhere!

I am convinced this data set is a good one and I am focusing on my processing skills in PI and Photoshop.

 

Much improved with better stars and a flat background. To lift the outer ring while keeping the core, you can try starting with masked stretch in PI. Repair the star cores of bright stars before you stretch.

Use HDR processes or local histogram adjustments (lhe) to tame the core. In general, hdr will change local contrast by a general darkening of an area, while lhe will brighten the image locally. 

[kirkster501]

How to repair the stars Wim?

[wimvb]

9 hours ago, kirkster501 said:

How to repair the stars Wim?

There is a script for that in Scripts - utilities: HSVrepair. Just run with standard settings, but also with the "unrepaired V" box checked. Then create a new image from the H, S, and unrepaired V channels. Do this on the unstretched image. 

[kirkster501]

Thanks Wim, I will check into that.  I am struggling with this problem of blown out stars in may of my images.

[ollypenrice]

10 hours ago, kirkster501 said:

Thanks Wim, I will check into that.  I am struggling with this problem of blown out stars in may of my images.

A curve which straightens early will protect your stars (but perhaps at the expense of contrast in brighter nebulosity.)

Olly

 

[gorann]

I gave it a quick and, some may say dirty, treatment with curves, layers and the brush tool in PS. First I stretched the part of the image containing the outer shell, then I used the brush tool to only apply the stretch to the nebula (and the little galaxy). After that I also used a curve to bring the brightest parts of the whole image down (including the core of the nebula). All this would probably work better on the original image than on the 8 bit jpg that I downloaded.

[geoflewis]

I’m following this thread with great interest as I too have experienced great difficulty blending Ha with LRGB to reveal the outer halo of M57. Unfortunately my image processing laptop died, so I’ve just ordered a new one from Dell, but the shipping timeline is a few weeks. Currently I’m still trying to capture Ha data on my Obs laptop, which is a low spec machine, not suitable for heavy image processing. Hopefully I’ll be able to get back to image processing by mid November.

Cheers, Geof

[Rodd]

I think it is darned good.  I would bring the brightness of the ring way down.  Bightness steals dynamic range and color.  I bet if you bring the brightness down you will start to see more details and structure.  

Rodd