Narrow band photography was developed initially for scientific purposes. The first uses were designed for the Hubble telescope. The combination of filters served to bring out in the images the chemical distribution of the main elements of emission nebulae. The association is as follows; SII for the red layer, Ha for the green layer and OIII for the blue layer. The Ha filter being the most dynamic layer, green will dominate in the image. The red layer (SII) and the blue layer (OIII) will be used to give relief to the photo by revealing the regions of the nebula rich in sulfur and oxygen. We now denote this association of filters by la hubble pallet. The image produced will therefore be in false colors.
This talk will present the technique I use (with Photoshop) for processing images produced with the Hubble palette (SII-Ha-OIII). I will also explain how to convert the green cast of images to "copper" appearing colors. The presentation will take place in four stages:
- The balance of the three images SII, Ha and OIII
- Assembly of the three layers of colors (SII-Ha-OIII)
- Converting green color to copper color
- Using a luminance image
1. Balance of the three images SII, Ha and OIII
To bring out the nuances and colors of each of the three images SII, Ha and OIII, it is very important to treat these images individually. The most dynamic image (offering the most details) being the image Ha, we will develop the histogram of the layers SII and OIII to make them correspond in intensity with the histogram of the image Ha. The success of processing Hubble Palette images will largely depend on this initial processing.
To perform this task properly, we will use the first three processes of the Image processing for each of the images SII, Ha and OIII:
- The withdrawal of the gradient. To be done as needed (training video, duration duration 11:59 minutes)
- The balance of the images with the technique of black and white points (training video, duration 15:59 minutes)
- The use of curves for the development (stretching) of each of the images (training video duration 12:48 minutes)
Here is what I got for my image of the NGC2237 Rosette:
We can observe the following on the three images:
- There is a good signal on all three images. SII, Ha and OIII images are therefore eligible for the Hubble palette. For example, if the signal from the OIII or SII image is very weak or nonexistent, you cannot use the Hubble palette for that image.
- The histogram of the three images has the same intensity. This will produce all varieties of colors.
- The Ha image is the most dynamic, revealing all the beautiful nuances of the nebula. It is also noted that the image Ha contains the information of the images SII and OIII. In this case, the image Ha will therefore be used as the luminance image. It is by looking at each individual image that the composition of the luminance image will be determined. For example, if the information of the image OIII contains shades that the image Ha does not have, the images Ha and OIII will be used as the luminance image.
2. Assembly of the three layers of colors (SII-Ha-OIII)
Open files your image SII.tif, Ha.tif and OIIl.tif
We have to choose an image as a basis for our work. Let us use the SII image considering that the Ha or OIII image could also be used. If not already done, convert the SII image to color (Image | Fashion | RGB color)
Click on the picture Ha.tif to make it active. Selection | Select all and then Edition | To copy
Click on the picture SII.tif to make it active
Open the window Layers and click on the Green Layer to make it active only:
Paste the image Ha.tif in the green layer by selecting Edition | To stick on
Repeat procedures 3 to 6 for the image OIII.tif to paste this image into the Blue layer.
Make all layers active by activating the eye for each one. From this moment we have a composite image SII-Ha-OIII
Save SII.tif in .tif format and give it the name Your name SII-Ha-OIII.tif
Close images Ha.tif et OII.tif
Rebalance the images with the technique of black and white points
We notice that the stars have a magenta color. To reduce it, we use a first selective color correction:
- Image | Adjustment | Selective correction ...
- Colors: Magenta
- Bring the Magenta slider to -100
Click on this link for a training video (8:16 minutes).
Here is what I got on my image of the Rosette:
NGC2237 - The rosette - SII-Ha-OIII
We notice the following in the image:
- There remains a red-magenta halo around the stars. We will make it disappear when superimposing the Ha luminance image on the SII-Ha-OIII image in process # 4 below
- Green dominates. We will convert it to copper colors in the next processing
3. Converting the color green to copper color
We are going to use Photoshop's selective color correction: Image | Adjustment | Selective correction ...
- For green color: decrease cyan and magenta
- For yellow color: decrease cyan and slightly increase magenta
- For the cyan color: decrease the yellow.
The intensity of the corrections varies from image to image. If necessary, more than one pass can be made.
Click on this link for a training video (duration 7:24 minutes).
Here is the result on my image of the Rosette:
NGC2237 - The rosette - SII-Ha-OIII with selective color correction
The green color has been converted to copper undertones
4. Using a luminance image
For my Rosette image, I used the Ha image as the luminance image, as it contains all the shades of the SII and OIII images. It is she who has the best Signal / Noise ratio. Using only the SII-Ha-OIII image, without a luminance image, will give a more noisy image with less depth. For all the technical details of assembling the Luminance image with the SII-Ha-OIII color image above, click this link. Also we can see here a training video (duration 13:27 minutes).
The rest of the treatments will be done with Photoshop and PixInsight. See processes 3 to 18 of the Image processing and if necessary the Advanced treatments. Use the treatments relevant to your image.
Here is the end result on my image of the Rosette:
The red-magenta halo remaining around the stars has completely disappeared. It is by putting a Gaussian blur on the SII-Ha-OIII color image that I succeeded in making it disappear, the luminance image retaining all the nuances and sharpness of the image. Also, I changed the colors of the nebula slightly to remove some of the green colors and bring out the copper colors in the image even more. Of course, the Hubble palette is in false colors, so it's all about personal taste in color rendering. Looking at the final image in full screen, one can appreciate the great variety of colors, thus helping to bring out even more the beautiful shades and contrasts of the nebula. We can also observe a 3D effect in the image. It is due to the enhancement of shades and contrasts by the varied colors of the image.
Sky Astro - CCD
Revise on 2021/01/15