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Author Topic: Smoothing Deep Space Backgrounds, Partial Black Point Clipping  (Read 5970 times)

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NoelC

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Good deep sky images usually present the background at a level above pure black. Mostly this facilitates viewing the dimmest details, as computer monitors are notorious for hiding things in the shadows, but it also helps the image look more natural. After all, we're shooting through the atmosphere, and like it or not, some sky fog just seems right, visually. Of course, if you're processing Hubble images, please disregard this. :)

Ever process a ton of data, only to find that when you get your DSO looking just right, with all the stretching you've done, your deep space background is kind of noisy/blotchy/nasty?  Got dark noise in what should be smooth deep space?

Here's a way to get a really nice looking, smooth background without losing perceptible detail nor reducing the natural appearance of the image.

First, you have to make sure the background is flat (no gradients). For this you may use flat calibration, and/or my actions (e.g., Light Pollution Removal), and/or another commercial solution such as Russel Croman's Gradient Exterminator, or your own methods. You MUST ensure your image is flat.

Next, add an adjustment layer to "turn on the lights" and let you easily see the background. I do this as follows: Layer - New Adjustment Layer - Curves, and pull up on the curves significantly to brighten everything up.  You should be able to see the background easily now.

Now that we can see the background and how nasty it really is we may want to clean it up a bit. Here's how:

1. Add the Curves adjustment layer, per the above, to allow you to easily see the background.

2. Select the image layer.

3. Choose Image - Adjust Curves from the menus.

4. Grab the black point - usually at the lower-left of the graph.

5. Pull the black point up toward the white point, keeping the Input and Output values the same.

6. Stop when you *just* see the background noise get blanked out. You may have to release the mouse and enter values from the keyboad for Input and Output manually to get this just right. The Input and Output values should be identical (see the first image below).

7. Press [OK] to commit the Curves operation.

8. Choose Edit - Fade Curves from the menu, then enter 75% or so (see the second image below). Avoiding this step will leave you with an almost perfectly smooth background, but this may look unnatural.

9. Delete (or disable visibility of) the adjustment layer created in step 1.

That's it! Please look closely at the third image below for a graphic illustration of the technique in action.

-Noel



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Offline davenash

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Re: Smoothing Deep Space Backgrounds, Partial Black Point Clipping
« Reply #1 on: 00:18:15, 08 June, 2006 »
Looks great, Noel :D

Thanks for sharing.  I certainly have suffered from this, I'm sure we all have.

Now a question:  As I've said before, I tend to use other software (not photoshop).  I normally know what most things actually do, so I can replicate them.  But I'm not 100% sure on that Edit-Fade thing.

I'm guessing the effect is the same as putting an edited image as a layer over the original, and adjusting the transparency.  Am I right?

Dave
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Offline janmclare

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Re: Smoothing Deep Space Backgrounds, Partial Black Point Clipping
« Reply #2 on: 01:43:10, 08 June, 2006 »

God thats amazing. Thanks so much for that Neol.


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NoelC

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Re: Smoothing Deep Space Backgrounds, Partial Black Point Clipping
« Reply #3 on: 02:07:38, 08 June, 2006 »
I'm guessing the effect is the same as putting an edited image as a layer over the original, and adjusting the transparency.  Am I right?

Yes, exactly.  In this case, the top layer would be 75% opaque.  It's essentially applying the Curves operation 75% to the image, allowing a small amount of the uneven background to remain, to keep things looking natural.

-Noel

Offline the fordster

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Re: Smoothing Deep Space Backgrounds, Partial Black Point Clipping
« Reply #4 on: 10:13:01, 08 June, 2006 »
Noel - That's a superb tutorial! Many thanks for taking the time to do it 8-). I'll certainly be using it!

Have also made this topic sticky...

Offline ianporter13

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Re: Smoothing Deep Space Backgrounds, Partial Black Point Clipping
« Reply #5 on: 10:14:23, 08 June, 2006 »
Thanks Noel,

worked a treat on my M57 shot last night.

will this be making its way into your actions sometime ;)

Ian
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Offline davenash

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Re: Smoothing Deep Space Backgrounds, Partial Black Point Clipping
« Reply #6 on: 11:32:27, 08 June, 2006 »
Yes, exactly.  In this case, the top layer would be 75% opaque.  It's essentially applying the Curves operation 75% to the image, allowing a small amount of the uneven background to remain, to keep things looking natural.

Thanks for the clarification. That's a technique I often use anyway with sharpening or blurring.

Dave
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Offline dciobota

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Re: Smoothing Deep Space Backgrounds, Partial Black Point Clipping
« Reply #7 on: 22:48:49, 08 June, 2006 »
 Excellent tutorial Noel!  Is this a sticky yet?  I hope it it, it should be.  :-)
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Offline GJBC

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Re: Smoothing Deep Space Backgrounds, Partial Black Point Clipping
« Reply #8 on: 00:39:51, 09 June, 2006 »
Nice post Noel

Thanks for the insight


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Offline the fordster

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Re: Smoothing Deep Space Backgrounds, Partial Black Point Clipping
« Reply #9 on: 09:58:45, 09 June, 2006 »
Excellent tutorial Noel!  Is this a sticky yet?  I hope it it, it should be.  :-)


Sure is, Dan! 8-)

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Re: Smoothing Deep Space Backgrounds, Partial Black Point Clipping
« Reply #10 on: 06:59:31, 13 June, 2006 »
Please, will somebody here tell me what to do :bang_head: I've been trying to get my head around all of this but Failed. I get as far as Adjustment Layer to turn up the lights then get Stuck.

Please what do you do  :cry:
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NoelC

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Re: Smoothing Deep Space Backgrounds, Partial Black Point Clipping
« Reply #11 on: 07:10:10, 13 June, 2006 »
From the menus, choose Layer - New Adjustment Layer - Curves.  This adds a curves layer above the image, and if you pull up on the curves it will lighten up the image.

-Noel

Offline kendru

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Re: Smoothing Deep Space Backgrounds, Partial Black Point Clipping
« Reply #12 on: 11:58:32, 13 June, 2006 »
Thanks very much for this Noel - this is something I have particular difficulties with.

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Offline ChrisLX200

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Re: Smoothing Deep Space Backgrounds, Partial Black Point Clipping
« Reply #13 on: 12:46:37, 13 June, 2006 »
From the menus, choose Layer - New Adjustment Layer - Curves.  This adds a curves layer above the image, and if you pull up on the curves it will lighten up the image.

-Noel

It seems Adobe have a habit of moving menu commands around - the step 8. Edit/Fade Curves does not appear under any heading in Version7, in version 5.5 it appears under 'Filter'. I eventually twigged what you were trying to achieve and could duplicate with layer blending options. It does make it difficult to author a tutorial when the commands keep changing!
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Online PaulB

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Re: Smoothing Deep Space Backgrounds, Partial Black Point Clipping
« Reply #14 on: 16:49:13, 13 June, 2006 »
From the menus, choose Layer - New Adjustment Layer - Curves.  This adds a curves layer above the image, and if you pull up on the curves it will lighten up the image.

-Noel

I keep doing that. This bit I understand. It's the rest of the tutorial which I find confusing. What do I do after I have brightend the image?
Paul.
"Whirlpool Observatory"
Losmandy G11-Gemini L4.
Celestron C9.25.
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Orion OMC 140
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Atik 428EX
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QHY5L-II.
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CCD Images
http://www.flickr.com/photos/paulabrierley/

 

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