I looked at the curves screen, started clicking, pulling, stretching and very soon the image look like crap!

I wrote this tutorial on what I have learned about curves to help other astro imagers.

Quick Start:
1. Open Levels, and find black/white points.
2. Use the eyedropper to anchor black/white points.
3. Find the brightest nebulosity you have in your dark image, click and anchor to curves. Use up/down arrow keys to desired levels.
4. Repeat steps on a fainter nebulosity.
5. Use repetitive curves, don’t try to stretch in a single bound!

Let’s get started with the Step By Step.

FIRST STEP.
If you don’t have your mouse wheel button set to ZOOM in and out…do this first! It makes the zooming function so much easier, and you will need to ZOOM in and out for curves.
Go to EDIT-->PREFERENCES-->GENERAL
Click the BOX next to “ZOOM WITH SCROLL WHEEL”

Step 2.
Make a new layer copy of your original image in Photoshop. If you don’t see this screen press F7 to toggle it on/off.

Click, Hold, Drag and Drop on top of “Create New Layer Icon”.

Now we have a “Background Copy” we can play around with, and keep the original image as “Background” on the layers.

Step 3.
Finding the black and white points on your image. This is a very important step for curves. The Black and White points serve as “anchor” points on the curve scale. To find black, and white points you must use LEVELS.
Click on IMAGE-->ADJUSTMENTS-->LEVELS.
You will see the levels box pop up like this:

In Levels you will see a Histogram with three little sliders, black, gray, and white. We will move the BLACK, and WHITE sliders to find the darkest black on our image, and brightest white on our image.

Take the “Black Slider” and start moving it to the right. You will notice the image getting darker, and darker…pretty soon it will be completely black! Now Slide the “Black Slider” back to the left, and the VERY FIRST SPEC of LIGHT you see will be the brightest white point! Make a mental note where that first spec of light you found on the image.

CLICK CANCEL on the LEVELS screen. We don’t want to save the Levels now, all we were doing was finding the “BRIGHTEST WHITE” point on your image!

Next ZOOM IN where you saw the brightest white point on your image. Make it a DEEP ZOOM as far as you can go!

Next, go back to LEVELS, and Slide the “Black Slider” again until you see the very first Spec of Light! Take your mouse over this “Spec of Light” and
HOLD the SHIFT KEY, and LEFT MOUSE CLICK. This will set the White anchor point.
Next CANCEL OUT OF LEVELS, don’t save any Levels now.
This will create a “new color sampler” or an Anchor for the “brightest white point” on your image! We will need to reference this white point later when we use CURVES.

Next we need to get the “Darkest Black Point” on our image. You guessed correctly…repeat the same steps as described above, but instead of moving the Level’s “Black Slider” to the right, take the “White Slider” and move it to the left!

One thing you will need to do before you begin finding the dark point is to ZOOM OUT so you can view the entire image.

After you have successfully marked both Brightest, and Darkest point you should see something similar to the image below.

Your image will have completely different white, and black points, however, most of the time it’s the brightest star, and darkest area in your image.

Now let’s get started with CURVES!
Here is NGC2264 before any curves have been applied:

If you DON’T see the newly created Black, and White Points…Click the “Eye Dropper” tool.

REPEAT MARKER:
Click IMAGE-->ADJUSTMENTS-->CURVES
You will see this Screen:

Next ZOOM in using your SCROLL WHEEL button on your mouse to the White Point. With the Eye Dropper Over the White Point “Hold the Ctrl Key” and “Left Mouse Click” the Whitest point. This will create a Black square on the Curve line.

Next we need to make an Anchor point for the black point.
ZOOM OUT with the Curves Dialog box still open, and find your dark point, and ZOOM in on your dark point.

After you have set your black and white points on the curve line you should see something like this:

Setting your Black, and White points on the curve line PREVENTS you from pulling or stretching the image out of bounds.

With the Black and White Points Set, next take your Eye Dropper Tool, and find the brightest nebula (or faint star dust) you can see. Once you find a part of the image that is BETWEEN the BLACK, and WHITE points Hold “CTRL” and “Left Click” the mouse. Then use the UP ARROW KEY to bring up the details. I typically press the up arrow key FOUR or FIVE times for each curve, and I NEVER use the mouse!

Here is an example of using the UP arrow key to pull the curve up:

Original Image:

After 1st Curve:

You will not see much of a difference from the first iteration of curves. But you will start to see some details appearing.
Now, go back to the REPEAT MARKER: above, and Open up the Curves window, anchor your Black, and White Points on the Curve Line…then Find an area on your image to Pull UP with the Arrow keys.

Second iteration of curves:

Third Iteration of Curves:

Fourth Iteration of Curves:

We started with this:

And after 4 iterations of Curves we ended up with this:

Final processed image with MORE than 4 iterations of curves!

Other Helpful Notes About Curves:
When in the “CURVES” window you can move to each “Black Square” on the “Curve Line” by Pressing “CTRL + TAB”. This will cycle through each “Black Square”.

AVOID USING THE MOUSE ON THE CURVE LINE!
Start using the mouse on the “Curve Line” and your image will get wildly out of control. USE THE ARROW KEYS! MOSTLY USE UP AND DOWN ARROW KEYS, but you can use left and right arrow keys too. This tutorial only used the UP arrow key.

If you don’t like the Curve settings you made while you are still in the “Curves Window” Hold the “ALT” key, and the “CANCEL” button turns into a “RESET” button! Huge help!

SMALL Incremental Steps with Curves! You will need Multiple iterations of curves!

NEVER CLIP YOUR CURVE DATA LIKE THIS:

I hope this tutorial was helpful, and if anyone knows a better way…please share!