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Some people asked me to explain my workflow and the method to prevent stars while bringing up the faint and dark structures. I would like to list up the steps here with an example but it will not work as a tutorial. But perhaps it helps somebody to play around a bit and practice until my tutorial is ready (whenever that will be ). And perhaps somebody can bring in some idea for improvement.

I would like to start this thread with a before/after image of M78 as a teaser and will try to give the textual explanations later this weekend.

I shot M78 on two evenings with the K-5 this week with dozens of shots. But the sky was very bright (even more than normal here) from the citylights and haze and I decided to use only the best 30 (ISO 400, 360s). This is what came out of the DeepSkyStacker and how it looked like after I did a first levels adjustment in PS:



And after a crop (that I regret because I saw too late whats in the box) calibration, star removal, stretching and bringing back in the stars this is the final result:

M78 by pete_xl, auf Flickr
It shows very much more detail , dark clouds and even (what I did not expect under these conditions) a piece of Barnards Loop.

Beautiful stuff Pete! Really looking forward to seeing the process.

Subscribed. Nice work.

Hi Pete, you've got me convinced that I should put more effort into my post-processing. Outstanding result for M78. I find it is a too often neglected object in Orion. Looking forward to a more detailed description of your workflow. I have enough data on my hard drive to keep me busy for a year

Well done
HJ

So here it starts.....

Some preliminary remarks before the tough stuff beginns.Though I get better and am content with my development I know that there still is and always will be much to learn for me. Concerning astrophotography I would say I am a kind of a red dwarf compared to the bright giants out there. But they inspired me and brought me foreward to where I am today. The best kick forward I got from Scott Rosen‘ s and Josh Smiths’s tutorials. In my current workflow I adapted a lot of their principles and mixed it up with own ideas so that it suits my needs and leads to results I never had before. Please be aware that it only scatches the surface and may contain errors.

I use the DeepSkyStacker and Photoshop. Within the process I use the HVLG (freeware) and the GradientXterminator plugin.Though there are good „manual“ alternatives to handle gradients it is much faster and somehow more reliant to use the plugin if used properly. The same with the Astronomy Tools actions that I use for selecting stars and noise reduction at important steps of my workflow. I tried several other methods for selecting the stars to be eliminated but in the end I came back to the "Select Brighter Stars" action in the toolset.

While writing down the steps of my workflow I realized that it must look terrible and complicated for anybody else. But it isn’t. I a video it would be no problem to show that its an easy job- mainly because ist a recurring principle that is easy once you have got it. And though there are a lot of little steps the major work to clean the background can be done in a few minutes with a bit of practice - I promise!

The description is in shortform I will post the whole thing in several steps beginning with the basic processing and the steps to clean up the background.

Some but not all general advices:

• Shoot in RAW.
• Flat frames are a MUST – without them you reach the stretching limits too early.
• Shoot and use offset frames. They don’t cost you anything.
• Use dark frames. In frosty winter I sometimes do without and shoot more lights instead

2 Extract brighter stars

1. Copy layer „Start“. Make it the active layer. PS Name it "Start copy"
2. Select brighter stars.Careful not to get DSO into the selection.
3. Expand selection by 7-12 pixels (value depending on general star size and „quality“).
4. Feather selection (steps c. and d.are functions in menu SELECT).
5. Add layer mask with selection active.
6. Click on mask with alt-leftclick
7. With levels (ctrl-L) maximize contrast in mask (maximize stars to white, background to black).
8. Apply gaussian blur of 3 Pixels on the mask.
9. Leave mask (click on layer symbol).
10. Choose„FILTER -> Dust & Scratches“. Use value ruler, watch what happens to the stars. Apply filter value 8-12. Depends on image, you have to experiment.
11. Select all layers. Copy merged (shift-ctrl-alt-E) or via EDIT menu.
12. Name the layer copy „Stars I“. Delete layer „Start copy“ (the one with the mask).
13. Copy layer „Start“ (ctrl-J). Put it on top. Make it the active layer. Set layer mode to „darken“. Merge down with layer „Stars I“.
14. Freq. Separation #1: copy layer „Start“ (ctrl-J). Put it on top. Make it the active layer. Name layer„Res. Stars I“. Click „IMAGE -> Apply Image“. Reference layer = „Stars I“. Select mode = „Subtract“. Scale = 2, offset =128. Click Ok. Set layer mode to „linear light“.
15. Create a new layer group. Copy Group once. Name one group „Steps“ and the other „Residuals“.
16. Copy Layer „Stars I“(ctrl-j). Shift layer „Stars I“ into group „Steps“. Then shift layer„Start“ into the same group. Deflate and deactivate group.
17. Shift layer „Res. Stars I“into group „Residuals“. Deflate and deactivate group.
18. You now have the big stars out (or/and heavily blurred) and the image looks a bitshabby like this:
    
    
    

Your layer heap should contain the layers (top to bottom):

• Stars I copy
• Res. Stars I (in group „Residuals“, group deflated andinactive)
• Stars I (in group „Steps“, group deflated and inactive)
• Start (in group „Steps“)
• Background (original image)

3. Extract dim stars

1. Copy layer „Stars I copy“ (ctrl-j). Rename it to „Stars II“ and make it active.
2. Open a curves adjustmentlayer above. Apply heavy contrast until dim stars stand out against the background. Select all layers. Copy merged (shift-ctrl-alt-E). Rename to „temp“. Make it active. Delete adjustment layer.
3. Select dim stars and expand/feather a tiny bit. Be careful that DSO is not selected or deselect it again manually. Delete layer „temp“. Make layer Stars II“ active.The selection is still active (tiny marching ants).
4. Hit delete and use FILL function with content aware setting in the EDIT menu (shift-F5). Inactivate selection (ctrl-d). The tiny stars are gone (hopefully). If there are too manystars it may not work with delete and fill. In that case use dust & scratches like above unter point 2.
5. Freq. Separation #2: Shift layer „Stars I copy“ to top. Make it the active layer. Rename it to „Res. Stars II“. „Stars II“ is the layer below. Click „IMAGE -> Apply Image“. Reference layer = „Stars II“. Select mode =„Subtract“. Scale = 2, offset = 128. Click Ok. Set layer mode of „Res. Stars II“ to „linear light“.
6. Shift layer „Res. Stars II“ into the group „Residuals“.
7. Copy layer „Stars II“. Shift layer „Stars II“ into the group „Steps“. Work on with layer „Stars II copy“.
   
   Your layer heap should contain the layers (top to bottom):

• Stars II copy
• Res. Stars II (in group „Residuals“, groupdeflated, inactive)
• Res. Stars I (in group „Residuals“)
• Stars II (in group „Steps“, group deflated,inactive)
• Stars I (in group „Steps)
• Start (in group „Steps)
• Background

EDIT: One comment to step d above (extraction of the tiny stars): I learned that using the delete-key and the fill function sometimes produces little black holes in the image that disturb the further processing. In this case use the dust & scratches filter instead with a low radius on the star selection or (better) on a mask generated from the selection as in step 2.

4. Extract blotches (Halos)

1. Copy layer „Stars II copy“ (ctrl-j). Rename the copy to „Blotches“. Make it the active layer.
2. Take the clone stamp tool with quite a bit of feathering and stamp all the blotches away. You can put a curves adjustment layer with a high contrast curve on top to also see less prominent blotches to be eliminated. Always stamp in the same brightness and color level. Look for bright structures to be preserved. Be very carefull in the region of the DSO. If in doubt, leave this region untouched. This step needs a little experience that is acquired by practice.
3. Shift layer „Stars II copy“ to the top of the layer heap. Set layer mode to „darken“. Merge with the underlying layer „Blotches“. With this you gain back the dark details lost in the previous stamping.
4. Freq. Separation #3: Shift layer „Stars II copy“ to top. Make it the active layer. Name the layer „Res. Blotches“. Layer „Blotches“ is the layer below. Click „IMAGE -> Apply Image“. Reference layer= „Blotches“. Select mode = „Subtract“.Scale = 2, offset = 128. Click Ok. Set layer mode of layer „Res. Blotches“ to „linear light“.
5. Shift layer „Res. Blotches“ into the group „Residuals“.
6. Copy layer „Blotches“. Shift layer „Blotches“into the group „Steps“. Work on with layer „Blotches copy“ on the top. Rename it to „Clean“.
   
   Your layer heap should contain the layers (top to bottom):

• Clean
• Res. Blotches (in group „Residuals“, groupdeflated, inactive)
• Res. Stars II (in group „Residuals“)
• Res. Stars I (in group „Residuals“)
• Blotches (in group „Steps“, group deflated,inactive)
• Stars II (in group „Steps“)
• Stars I (in group „Steps“)
• Start (in group „Steps“)
• background
  
  Your image should look more or less like this:

For control purposes and for your good feeling you can copy the group „Residuals“ (ctrl-j) and shift it to the top. With all grouped layers active on top and in „linear light“ layer mode you should see no difference to the „Start“ layer if the workflow was Ok up to here. You have the backgrond isolated and the big stars, tiny stars and halos on separate layers. Even if your result is not perfect it should enable a much better processing of the background than with all the extracted stuff in. You can leave this group always on top and switch it on and of after the subsequent steps to get an impression how the complete picture looks like. But leave the content of the group untouched and do not mix it in accidentially when copy-merging layers in later steps (happens frequently to me thats why I leave it away).

Depending on the image quality you can now do another frequency separation for the noise in the same way as decribed above. Be careful not to destroy too much detail here.


This is the end for today (puh!). Hope it is understandable....

Next steps (hopefully comming soon):

5. Process the background to the maximum
6. Work out the details
7. Combine background and residuals
8. Final adjustments

Woha Pete!

Even though I don't have PS with all of its advanced functions, this is great inspiration for anyone working with layers in astrphotography.

A great, many thanks.


Steen G. B.

Can't wait to try it, Thanks for sharing!

WOW!!!! Absolutely wonderful! I've been thinking that I could try something a bit deeper while I'm out in the desert freezing my rear off. One of the barriers is that I really have never tried DSS - and this is a great tutorial to start with. I am wondering how well Affinity Photo could be substituted for PS - since it is oriented to working with layers?

The results from you all are spectacular!

I will be trying this; well done Pete, you deserve a gold star!

Thank you - good luck trying, hope it works for you!

Some more words to the Levels tool because ist proper use is essential in many steps of the workflow.

In a virgin astrophoto you normally have in the Levels histogram from left to right:

- a gap or no gap at the left end
- sometimes a short horizontal line of data where the data beginn on the left side that usually contains only scrap data
- a sharp peak that contains the background data and the dark DSO with a short ascending left slope (darkest data, i. e. dark nebulas) and a steep falling right slope
- a long stretched flat right part reaching to the very right end of the histogram (stars)

When I say "stretch with the Levels tool" I mean the following:

- make a working copy of the active layer (ctrl-J, cmd-J on the Mac)
- do a ctrl-L on the layer copy to bring up the tool.
- use only the upper left and mid slider, don't touch the right one or the lower ones
- if there is a gap at the left side of your histogram shift the left slider to the beginning of the data -> OK -> ctrl-L
- then iteratively shift the mid slider a bit to the left -> OK -> ctrl-L and then the left slider to the beginning of the ascending slope.
- do that in several consecutive steps, in my step 1c above just a little bit to not bring foreward the star halos too much.
- merge the working layer with the underlying one - ready

Doing like described you stretch the sharp peak (the background data) and eliminate the crap data. The bright stars remain unchanged.
If you are done you always end the stretching by setting the black point back to RGB 30,30,30 by clicking the black eyedropper of the Levels tool (see 1.g above) at the point you have considered as the darkest in your image.

Cheers
Pete


P.S. do not try my workflow with too many stars in the beginning. With the Milky Way in it you will be frustrated and fail. Better start with images that do not have many stars and that are in a good focus. The Andromeda Galaxy is a perfect object to start. Also the Orion nebula or the Horse and Flame Nebula region.

I've spent a couple of hours trying to at least learn the basics of this technique and first I started with a picture of the Milky way -> bad result. Could not get it to work.

Then I read your last sentence in post #14, paraphrasing, "Do not start with the Milky way". Right, always read the entire instruction first. I know that.

So I start again, this time with the Andromeda galaxy. I jumped to part 2 (post #7) as I didn't quite understand the use of the eyedropper. Still couldn't get it to work at first.
But I eventually found the problem, step "g" was a bit confusing and I guess there is a typo there, "(stars dark black, background brightwhite)." That meant I had to invert the mask, which I did, which I shouldn't have done. When I skipped the inverting of the mask I finally made it through post #7.

So now I can turn the stars on/off, cool.

Thanks.