Narrow Band Imaging Processing Work Flow

1. Attempt to perform as much of your work as 32 bit real images prior to importing into Photoshop.  The use of 32 bit images greatly improves the “smoothness” of the result.
2. Prepare master calibration frames (Bias, Dark, and Flats for each color).  Master calibration frames should be saved as 32 bit real images in fit format.  I perform this activity with CCDStack.
3. Calibrate NBI light frames and debloom if necessary.  Save the calibrated/debloomed light frames as 32 bit real images in fit format.  I perform this activity with CCDStack.
4. Register the images with either Registar or CCDStack.  Save the registered images as 32 bit real images in fit format.
5. Normalize the images.
6. In CCDStack, perform a data rejection.  Typically, I use a Poison Distribution with a 2% rejection because the number of 15 minute sub frames that I obtain are less than 20 for any one NBI filter.
7. Combine the images as a Mean.  Save the combined NBI frames as 32 bit real in fit format.
8. Register the combined NBI frames with either Registar or CCDStack.  Save as 32 bit real in fit format.
9. Open the individual NBI frames with Photoshop CS and the Fits Liberator Plug‑in.  Typically, I select the ArcSinh(x) function to stretch.  Select white and black points that do not clip the image.
10. Select the Ha image and change the mode to RGB.  Open the channel pallet.
11. Select the entire SII image and copy.
12. Paste the SII image into the red channel of the Ha image.
13. Select the entire OIII image and copy.
14. Paste the entire OIII image into the blue channel of the Ha image.
15. Save the composite three filter NBI image under a new name.
16. Select the Layer Pallet of the three filter NBI image.
17. Add a new Levels layer.  Modify the black point on each of the three channels so they all begin in the same place.  Usually I keep open the histogram pallet so that I can see each channel's histogram at the same time to make this adjustment.  Be careful not to clip any portions of the histograms.
18. Add a new Curves layer.  Adjust the curves of the red and blue channels until the histogram approaches that of the green channel.
19. Repeat steps 16 and 17 until the red and blue channels are "similar" to that of the green (Ha) channel.
20. Once the histograms for all three channels are about the same.  Open a new curves layer.  This time stretch all three channels at the same time to obtain a pleasing result.
21. Perform additional adjustments to improve the image as needed.  Here are some of the additional adjustments that I perform.
    1. Deconvolution.  I have found that the SII (red) channel results in many stars having red halos.  Deconvolution of the SII channel prior to combining into the RGB helps reduce this effect.
    2. Large Radius Unsharp Masks.  Unsharp masks with radiuses between 50 -250 can improve the sharpness and contrast of the image.  I often perform this activity after step 20.  This action can” burn-out” the highlights so it may be necessary to deselect the highlights prior to using this technique.
    3. Local Unsharp Mask.  Often I sharpen portions of the image to improve the detail.
    4. Gradient Removal.  Gradients may occur in your images.  Russell Croman’s Gradient Exterminator is a good investment.
    5. Noise Elimination.  Some noise will be visible as a result of stretching these images—particularly in the SII and OIII channels.  Noise Ninja works great for reducing this noise.
    6. Colored Halo Elimination.  Halo reduction is just part of the game.  I find the following technique works well:

       1. Magnify to about 300% to get up close to a star with a halo.

       2. Select the Magic Wand tool with a setting of about 10 and continuous selected.

       3. Locate the Magic Wand and left click so that only the star and not the halo is selected.  You may have to alter the selection setting to select only the star.

       4.  From the menu bar, select Select, Modify, Expand.  Enter a setting that results in the increasing the selection so that the halo of the star is included.

       5. Deselect the star, by holding down the “alt” key at the same time you click on the star with the Magic Wand tool.  After this only the halo is selected.

       6. Click on foreground box.  The Color Picker pop-up box appears.

       7. Locate a portion of the image just outside the halo.  Select this color by clicking on it.  Select OK on the Color Picker pop-up.

       8. Select Edit, Fill.

       9. From the dialogue box, select Hue and 100%.  Click OK.

       10. Select Edit, Fill again.  This time select saturation and a quantity of 75%.  Select OK.

       11. It may be necessary to alter the saturation % to obtain a pleasing result.

7. Lighten Red Stars.  The SII channel will produce more stars than the Ha or OIII channel.  These stars will appear red.  To change the color of these stars so that they appear lighter use the following technique.

   1. Magnify the image.

   2. Select Image, and then Replace Color.

   3. Select the Eyedropper tool and then select one red star.

   4. Next select the Add to Sample tool.  Now select more red stars.

   5. After selecting a number of red stars, adjust the “fuzziness” so that only the red stars and not the nebula are selected.

   6. Reduce the saturation so that the once red colored stars now appear lighter or white.

   7. Color selection, fuzziness, and saturation technique require practice to reduce the number of artifacts that are introduced.

8. Repair Light, Dark, and Colored Pixels.  I use the following technique.

   1. Flatten the image.

   2. Select and copy the image into a new layer.

   3. Rename the new layer Dust and Scratch Layer.

   4. Select noise, Dust and Scratches.

   5. Modify the radius so that dark, light, or colored pixels disappear.  I find a radius of 1 or 2 usually works well.

   6. From the Layer Menu, select Add Layer Mask and then Hide All.

   7. On the tools menu, select the little black/white icon to adjust the foreground and background colors to black and white.

   8. If the foreground color is not white, the switch background/foreground icon should be selected.

   9. Select the brush tool with a brush size of about 9 pixels.

   10. At high magnification, find light and dark pixels.  On each light/dark pixel, locate the cursor and click.  The mis-colored pixels will disappear.

   11. Flatten the image.