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PostPosted: Mon Sep 19, 2011 2:17 pm 
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Hi folks,

In my thread "Big Sky Astrophotography" I mentioned use of the PhotoShop "Dust and Scratches" filter to remove the stars from an image. The procedure is surprisingly easy and quick provided you click the OK button for each iteration of the filter!
  • Open the ”Dust & Scratches” filter
  • Set radius to 12
  • Set Threshold to 50-120, by looking enlarged image, so that the center of the brighter stars disappears but image stays ”crispy”.
  • Lower radius two points to 10 and set Threshold to 30-80. Follow image quality and details.
  • Repeat several iterations with lower values, the more rounds the better.
  • In the final step radius should be 1 and the Threshold between 3-10.
  • If there are leftovers from brighter stars, use the clone tool to clean them up.

    All Stars are now removed.
  • There are usually some other missing details as well.
  • Place the original H-a image with stars to top of the Starless one.
  • Turn blending mode to ”Darken”
  • Blink layers to see the effect
  • Merge layers
  • If some lighter shades are missing, fix them by using the History Brush
  • After fixes, blink the image again with the original one, to be sure, that there is no missing details other than Stars!

    Extract from this PDF file.
There are updates and other tutorials on the Astro Anarchy website and I urge a visit if the above procedure has whetted your appetite.

As you can see, the basic steps are very straightforward and quick to do. Although reference is made to Hα in the clean-up section I've found the technique works just fine on regular full colour images.

As for why one should bother, if you've ever tried to extract the faintest detail from areas of nebulosity you'll almost certainly have found that the degree of manipulation required can do horrible things to the stars. Conversely, if you try to reduce the size of the stars or their impact through sheer number then you may rapidly find your hard won nebulosity either disappearing or exhibiting halo artefacts where star sizes have been reduced.

Once you have a "no stars" version of your original image it's a simple matter to subtract that from the original (PhotoShop layers) to obtain a "just stars" version and you can then process the stars and nebulosity separately achieving the optimum result from both. How successful you are still relies on skill and judgement and I'm at the bottom of the learning curve with both but, borrowing from images in the aforementioned thread, and with apologies to those who have seen them before, here is an example of the best I managed to achieve with normal processing followed by an example of the two strand approach after just a couple of days of practice. Both are from the same data though the final crop is slightly different.
    Image
    Image
I thought it worth creating a separate thread to highlight the technique because the only investment needed, if PhotoShop is already in your arsenal, is a few minutes of time to gain familiarity and, IMHO, it allows significantly better results without having to invest in extra hardware. The images above were captured with a regular DSLR (a 5D2 as it happens), albeit one that was piggybacked on an equatorial mount to track the stars.

Bottom line: if you spend time and money on astrophotography then you owe it to yourself to try and get the most out of that investment!

Bob.

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Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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PostPosted: Tue Sep 20, 2011 2:45 pm 
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Hi folks,

I thought it might be helpful to graphically illustrate how star removal works. My starting point was a small 100% crop from the original image above as it entered PhotoShop for the first time. I selected an area around the "Gulf of Mexico" and, as you can see, it has not been rotated to the usual orientation. Here is an animation of how star removal progresses:
    Image
The PhotoShop "Dust & Scratches" filter was applied successively with the settings 12/120, 10/100, 9/90, 8/80, 7/70, 6/60, 5/50, 4/40, 3/30, 2/20, 1/10, 4/8 and 2/3. I did a sanity check of the Preview at each pass but didn't feel the need to deviate from the 1:10 ratio until the last two passes which I felt were needed to get rid of the last stellar remnants. The final two steps shown in the animation were application of noise removal and some "de-purpling" using the Patch tool. I could certainly have spent more time on that last step but for today's purposes it was good enough.

Here is the initial frame and the one with the stars removed:
    Image Image
Subtracting one from the other (PhotoShop layers) resulted in a "Stars Only" version which I tweaked slightly to get the star brightnesses about the same as they were in the original:
    Image
It's not my purpose here to describe subsequent processing which was, in any event, quick and dirty but here are enhanced versions of the "just stars" and "no stars" images:
    Image Image
I took the opportunity to adjust the black levels of the "no stars" version and so hide some of the artefacts left over after use of the patch tool. I also tweaked the white balance of the "just stars" image slightly to remove a slight cyan tint. All processing was done entirely in PhotoShop.

I think it would be helpful to see the before and after versions as another animation as well as a still image of the end result:
    Image Image
Star removal has allowed me to retrieve more detail but there are costs. If you look in areas where fainter stars are in the nebulosity it seems pretty clear that in the final image they no longer appear as stars but instead as nebulosity! Whether that's good or bad is a matter of taste I suppose. If one believes that the objective is to produce a scientifically exact representation of the scene then it's a bad result, though how one defines "exact representation" when so many of the images we see are composites of RGB and narrowband data is open to question. On the other hand if one believes that the objective is primarily to produce a pretty result which doesn't deviate too far from what's actually up there then maybe the compromise is acceptable.

As I mentioned in the opening post I'm right at the bottom of the learning curve with this technique. My purpose here isn't to provide a masterclass, as I'm no master, but rather to show that star removal is actually not as difficult as it sounds and can offer benefits if applied judiciously.

Bob.

P.S. On the remote chance that anyone would find it useful the initial crop is available as a PSD file here. As a PSD, rather than a JPEG, it has the advantage of being 16 bit and in Adobe RGB so if you want to experiment and post your own results in this thread then feel free.

Edit: Corrected the white balance of the final "just stars" and dependent images.

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Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


Last edited by Bob Andersson on Wed Sep 21, 2011 12:34 pm, edited 1 time in total.

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PostPosted: Tue Sep 20, 2011 9:07 pm 
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Very interesting work Bob, thanks for the details!


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PostPosted: Wed Sep 21, 2011 12:36 pm 
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Hi Gordon,

Thanks! I'll consider the effort more than repaid if a few readers find that they can use the technique to good effect in their own work.

Bob.

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Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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PostPosted: Sat Sep 24, 2011 5:51 pm 
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Bob, this is great and thanks for sharing.

I have struggled trying to get to grips with this technique and I think your thread has unlocked the key for me to it. Roll on the clear skies so I can give it a whirl. Havent had the scope out since March :shock: :shock: :shock:

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PostPosted: Sat Sep 24, 2011 11:41 pm 
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Hi Lee,

My pleasure. Clear skies? :? What are they? They either seem to appear contrary to the weather forecast or when there's a full moon around here! :evil:

Bob.

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Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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PostPosted: Sun Sep 25, 2011 9:42 am 
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Hi Bob, your not wrong! The last few clear nights have always coincided with a full moon, argggh! Still when I eventually get my CCD and Narrowband filters this won't be much of an issue.

Hopefully, looking at the weather forecast we might get a clear night next week I just hope it doesn't coincide with the night I'm out with mates celebrating a birthday (not mine BTW).

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PostPosted: Fri Oct 21, 2011 10:10 pm 
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wow thanks


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PostPosted: Fri Dec 30, 2011 1:05 pm 
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Hi folks,

I thought I'd update this thread and share some of my recent experiences with the "Star Removal" technique. It's not all plain sailing!

First the good news. Having now got the kit to do some narrowband imaging of the H-alpha emission line (Wiki article) I'm finding it incredibly useful to be able to extract the stars from those Hα images, process the nebulosity as seems fit and then add in stars from separately processed RGB exposures. The RGB exposures, which are quick to do, let me reproduce the star colours and, because I have removed the nebulosity, I can process those stars to get as little or as much colour as I want and also reduce the star sizes as appropriate without ending up with artefacts such as "ringing" which I would see if the stars were still embedded in nebulae. And, of course, I can process and colourise the nebulosity without having to be concerned about the star colours.

Pictures are better than words so I thought I'd use a crop from my recent North America Nebula image.
    Image
Star removal from the Hα exposures was straightforward although if one looks very closely one can also see that a few dark spots (Bok globules, I presume) have also disappeared. They could be manually restored if required. I thought it might also be useful to compare the same 100% crops from the Hα image and the RGB image. The RGB image was prepared from 3 x 200 second exposures through each of red, green and blue filters while the Hα image was prepared from 5 x 1000 second exposures. More stars in the RGB image but the benefit of the Hα filter is obvious to see when one is after Hα nebulosity.
    Image
Just for fun here is an animation of the "no stars" Hα, the "just stars" RGB and the final colourised Hα plus stars 100% crops after all post-processing was completed.
    Image
It's not all a bed of roses with this star removal technique, though. With my Flaming Star Nebula widefield image (here) in addition to 8 x 1000 second Hα and 5 x 200 second RGB exposures I also took 3 x 1,000 second exposures through the blue filter to see if I could pick up the reflection nebulosity. Although successful (but only just) the scale of the image meant that the bloat of the brighter stars really got in the way. The next animation shows a 100% crop around the Flaming Star nebula (monochrome but taken through the blue filter as described) and then a sequence showing the results after my standard star removal, manual cleanup using PhotoShop's spot healing brush, manual removal of the remaining brightest stars using the same tool, noise reduction and, finally, use of a "curve" to clip the image and apply an S-curve.
    Image
To emphasise the problems here's an animation of just the first and last frames from the animation above.
    Image
While it's certainly true that I have extracted the nebulosity visible in the original image it's also true that I've also gained nebulosity in the final image that wasn't originally present and, of course, what is in the final image that was originally masked by those bright stars is a complete fiction. The extra nebulosity could be removed by more manual intervention but while that fictional nebulosity wouldn't be a particular problem if I added back stars of the same size later on there certainly are issues when trying to add the reflection nebulosity into an image whose main constituents are Hα plus nice tight RGB stars as described earlier. The amount of manual cleanup work required is also much greater than with star removal from Hα images, even when just working with those few areas which actually do contain reflection nebulosity.

Despite that I can't imagine not using the star removal technique for all but a very few of the images I'm likely to go after in the future but, of course, your mileage may vary.

Bob.

_________________
Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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