Updated: Apr 10, 2020
Combining Narrowband files from a monochrome Astrophotography camera into one color image may seem difficult if you are not familiar with the process, but it is actually very quick and simple! In this tutorial, we will show you how to merge black and grey masters and create a color file ready to be processed, using the Hubble Palette mapping. You can also watch our video walkthrough which might be more helpful!
Looking for a tutorial about bi-color combination? We got you! Read our post or watch our video about bicolor image combination on PixInsight.
If you recently purchased a monochrome camera for Astrophotography, chances are you aren't really sure how to combine your Black & Grey files into a color image. We've been doing DSLR Astrophotography for several years, and were used to how easy it was to just stack all the raw files and see the colorful master light appear on our screen, ready to process.
When we got our ZWO ASI 1600MM camera, we did not really think about processing until it was time to import our first files into PixInsight. Our first light with this dedicated astrophotography camera was Thor's Helmet, which we imaged in Narrowband. As soon as we launched PixInsight, we were not prepared and lost a lot of time trying to find tutorials on how to combine narrowband files into one color image.
We eventually found a great page on LightVortexAstronomy.com explaining how to achieve this in depth. The article is wonderful, but can be a bit tedious and very long for a complete beginner looking for a quick answer. We decided to make a short video tutorial about it, as well as this text version to show you guys how to combine your files for a Hubble Palette processing.
We'd like to thank Cayron Mercieca from Light Vortex Astronomy for teaching US how to do it in the first place. We strongly suggest you check out his website if you want to learn more about PixInsight.
This tutorial is focused on the combination of your images. Before being able to do this, make sure to do the following (watch our video about basic PixInsight processing if you're unsure about how to):
Stack your files using Batch Pre-Processing
This is a little bit different than if you were to stack files from your DSLR camera. This time, 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).
Open up all the masters (three in our case, Ha, Sii, 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 image 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 other two images 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 each image 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 preview from the dropdown menu in Masked Stretched and apply it to the SII image.
Select the OIII preview from the dropdown menu in Masked Stretched and apply it to the OIII 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 Sii and Oiii images.
It is now time to actually combine our images into one. For this we will be using the process Channel Combination. Note that the example below is for a Hubble Palette combination, but you can choose your own color mapping.
We actually recommend that you mess around with all the possibilities so that you can fully understand how this works.
Open "Channel Combination"
In the dropdown menus, match these settings:
Red = SII
Green = Ha
Blue = OIII
This specific color mapping is for the Hubble Palette style. Feel free to switch it around and see the results!
Apply the process globally.
After applying the process, a new image should pop up. This is your combined color image!
REMOVING EXCESS GREEN
If doing the Hubble Palette, you might notice that there is a lot of green. This depends on the target but in our example, the Bubble Nebula contains a ton of Hydrogen Alpha, which translates into green color everywhere. You can start your regular processing workflow now if you’d like, or you could take off the green and/or change the Hue of the image.
For the quick and easy option, you can do as follow:
Open up the SCNR process
Apply your desired amount of SCNR to the image
If you are still not satisfied, do the following instead:
Open Curves Transformation
Go to the "H" tab
Use a couple of points to play with the hue. You should aim to either transform most of the green into orange or blue depending on what you preferApply the process, then open SCNR
Apply your desired amount of SCNR to the 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!
This tutorial only covers how to combine the monochromes images into one, but you can watch our 3-part video about our PixInsight workflow if you do not know what to do next!
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.
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