PixInsight: Bi-Color Combination - Tutorial
Updated: Jun 6, 2023
Bi-Color imaging is often use on deep sky objects that are rich in two specific narrowband channels but lack one of the three. For example, the Pacman Nebula contains a lot of Hydrogen Alpha and Oxygen III gas, but lacks Sulfur II. Amateur astrophotographers sometimes decided to completely skip one of the three gases and use the limited time they have focused on the two richest ones.
Combining two master files from a monochrome Astrophotography camera into one color image can be tricky because, as you probably know, a color image is made of three channels (Red, Green and Blue).
This process is indeed a little bit difficult to get used to, because the settings we'll use are going to depend on the target, but we will do our best to explain how to do it in a quick and easy way. You can also watch our commented video walkthrough if you'd like to see us combine them in a more... dynamic format!
Looking for a tutorial about regular Narrowband combination? You can read our post or watch our video about combining three narrowband channels on PixInsight.
In this tutorial, we will show you how to combine two narrowband files from a cooled monochrome Astrophotography camera into one color file. We will use the Pelican Nebula as our example for this post, and will also include an example using the Cone Nebula and the Helix Nebula at the end of this post. The Helix Nebula was our first attempt at bicolor imaging and it turned out pretty good!
All were shot with our cooled astronomy camera, in Ha and Oiii.
Once again, we'd like to mention LightVortexAstronomy.com which is where we learned most of what we know about PixInsight.
Pre-Requisites
This tutorial is only for combining two narrowband images together. Make sure to first calibrate, register, and stack your files before proceeding. You can watch our video about basic PixInsight processing if you're not sure about how to do this:
Stack your files using Batch Pre-Processing
This is a little bit different than if you were to stack files from your DSLR camera. Make sure to uncheck "CFA", and use "Add Custom" instead of "Add Lights" when importing your Lights. When the "Add Custom" window pops up, import files from each filter, and name the group accordingly (example: H or Ha for Hydrogen Alpha files).
Note: If you know for sure that your FITS Header is correct for each file and includes the filter used, then adding all the files at once through the "Add Lights" button should work just fine.
Open up all the masters (two in our case, Ha and Oiii)
Perform a background Extraction using either Dynamic (DBE) or Automatic (ABE) Background Extraction.
Do some noise reduction using ATrousWaveletTransform (ATWT)
Lastly, you need to ensure all images have the exact same size. This can easily be done using Dynamic Crop. You can see us do this in the video.
Then, we can proceed to the actual processes we will use to combine these masters into one color image:
Stretch to Non-Linear
The first step is to stretch our image to non-linear.
Use STF to quickly stretch the images and see what you are dealing with (you've probably already done this in the previous steps)
Make a tiny preview reference window on the HA image on a part of the sky that has no stars, just plain dark background.
Then, clone this preview to the second image so that you have the same preview everywhere.
If you're unsure about how to clone the preview, all you have to do is slide it to the other two files.
You can now reset the STF and close it entirely.
Next, open up the process "Masked Stretched".
You will need to apply the process to both images with their OWN preview reference. Here is how:
Select the HA preview from the dropdown menu in Masked Stretched and apply it to the HA image.
Select the SII or OIII preview from the dropdown menu in Masked Stretched and apply it to the corresponding image.
Once this is done, close the process and feel free to delete all previews.
Making Sure We Have the Same Background Brightness
The second step is to ensure that the brightness of each background is the same for all, that way it will not look odd when they are combined.
This is very quick and will take you less than a minute.
Open the process "Linear Fit".
Select the HA file as the reference image in the dropdown menu.
Apply the process to your second image.
Combining the Channels
Combining our two images will be done using one of the most powerful processes PixInsight has to offer: Pixel Math.
Unlike with narrowband combination techniques, we will not be using Channel Combination and we also do not necessarily need to apply a SCNR or play with Curves like we did in the other tutorial.
Open "PixelMath"
In the dropdown menus, match these settings:
Uncheck "Use a single RGB/K expression"
Select "Create a new image" instead of "Replace target image"
In "Color space", select "RGB Color"
The goal is now to populate the three color fields (R/K, G, and B) with our two grayscale channels (Ha and Oiii). There are hundreds of ways to do this, but below are a couple of options that are popular among amateur astrophotographers:
1) Regular RGB style
Red: HA
Green: OIII
Blue: OIII
By combining the Hydrogen Alpha channel to Red and the Oxygen III channel to both Green and Blue, you will be able to get an image that is close to "true colors".
2) Hubble Style
Red: HA
Green: (0.6*HA)+(0.4*OIII)
Blue: OIII
If you are trying to combine two channels into a color image that looks similar to the Hubble Palette, try including both Ha and Oiii into the Green channel at the same time. You might want to mess around with the numbers there until getting something you are satisfied with, but you should be able to get pretty close to the Hubble Palette style of combination.
*Note: Make sure the name for each channel match your actual file (example: If your Hydrogen Alpha file is named "H", then type "H" into the PixelMath fields instead of HA).
To apply the process to the image and see what these will actually look like, slide the blue triangle to either the Ha or the Oiii file.
After applying the process, a new image should pop up. This is your combined color image!
You should now have a single color image on your PixInsight dashboard that you are satisfied with!
This image should be treated as if you just turned a linear image into non-linear, so you still have a lot of processing to do! We recommend you do a Background Neutralization followed by a Color Calibration right away, as shown in the video. These two processes will reveal what the real colors in the image are as the colors shown after applying PixelMath might be a little off.
We will not be showing the rest of the processing workflow here as this tutorial only covers how to combine the monochromes images into one. If you feel a little lost as to what to do next, you can watch our 3-part video about our PixInsight workflow on our YouTube channel.
Playing around with different combination options on PixelMath
As we said earlier, there are hundreds and hundreds of different possible outcomes when combining channels using PixelMath. We thought we'd show you a few of them so that you can really see what kinds of colors you can get from the same channels.
Below are four bicolor combinations on the Christmas Tree cluster and Cone Nebula, obtained by randomly playing with the numbers in the PixelMath combination fields. Which one do you prefer?
BiColor Combination of the Helix Nebula
Although the Helix Nebula (NGC 7293) looks great when photographed in RGB, it can also be imaged in Ha and Oiii Narrowband and combined pretty easily! Below is our shot of the Helix Nebula using our Hydrogen Alpha filter (left), Oxygen III filter (middle), and combined (right).
The combination settings we used were:
Red: HA
Green: (0.4*HA)+(0.6*OIII)
Blue: OIII
Make sure to bookmark this page for a quick reminder on how to combine your images anytime you capture a new object with your monochrome camera!
Let us know in the comments if you have any questions.
Tutorial Video
Clear Skies,
Galactic Hunter
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