Astro pixel Processor - How to combine a OSC broadband image and Extracted Ha and OIII from a dualband filter?

Mikołaj Wadowski · Sep 5, 2025, 08:16 PM

You should stack each filter separately, combination usually happens towards the end of processing.

After stacking and extracting Ha and Oiii, you’ll need to remove the continuum features present in these stacks. Then, you’ll need to stretch both the broadband and the narrowband, colorize the narrowband and screen them on top of the broadband (you can use other image blending algos but I find that screening gives the best results).

There’s a good guide that explains continuum subtraction, you can find it here. I think APP should have all the tools necessary to do either method described in that article.

Thanks so much. I took a look at the article but seems i need pixinsight and i use photoshop. I thought APP could do this as well. Can this be done in APP? Also, the BB subs and the NB would need to align perfectly the subs/stacks would always be slightly off . I assumed APP would do the align. How is this done without stars?

You can do this using the Combine RGB tool in APP. It's located under the Tool tab. The tool has many filter combinations you can try, and many settings to experiment with.

Here’s a basic workflow:

For dual-band data (Ha + OIII):

1. Load all your images and calibration files. Before running the integration, go to the "0) RAW/FIT" tab. Under Algorithm, select "Ha-OIII extract Ha", then run the integration as usual. This will extract and stack only the Ha signal. Let’s call this result Ha-stack.

2. Repeat the process for OIII: go back to the "0) RAW/FIT" tab and now select "Ha-OIII extract OIII". Run the integration again. This will extract and stack the OIII signal. Let’s call this one OIII-stack.

For color (RGB) data:
Stack your RGB data as usual. Let’s call this RGB-stack.

Registration:
Now, register Ha-stack and OIII-stack with your RGB-stack. Load all three stacks as lights, and go through the workflow up to the registration step. Once registration is complete, save the registered frames using the "Save Registered Frame" option under the Registration tab. We’ll call them Ha-reg, OIII-reg, and RGB-reg.

Cropping:
Batch crop these three registered frames to remove bad edges or non-overlapping areas. Use the Batch Crop tool under the Tool tab. Lets call the cropped versions as Ha-crop, OIII-crop, and RGB-crop.

Gradient removal:
Run gradient removal on all three cropped images (Ha-crop, OIII-crop, and RGB-crop).

Splitting RGB channels:
To separate the R, G, and B channels from RGB-crop, load RGB-crop, go to the Calibration tab, scroll down to Split Channels, and use it to extract and save the individual R, G, B channels. We’ll call them R-crop, G-crop, and B-crop.

Combining with RGBHOO formula:
Open the Combine RGB tool under the Tool tab. Choose RGBHOO as the combination formula. Click Add Channel, and load the following:

• R-crop as Red

• G-crop as Green

• B-crop as Blue

• Ha-crop as Hydrogen Alpha

• OIII-crop as Oxygen III

Now play around with the parameters until you get a result you like. Save the final result as a FITS file. From here, continue your normal RGB processing workflow (e.g., further gradient removal if needed, background extraction, color calibration, etc.).

Star removal:
You can remove stars from Ha-crop and OIII-crop with starnet++ before using them in the RGB combination tool.

Final touches:
If you’re using GIMP or a similar tool, you can apply final touches—like using the starless Ha-crop image as a mask to further enhance Ha details.

I hope I did not miss anything. Thanks to chatgpt for cleaning my original text lol.

Jon Woodhead · Sep 5, 2025, 11:37 PM

There are a number of videos online showing how to add Ha to RGB in APP such as this:

Ha-RGB

but I’m not sure (may be wrong) that APP has the capability in the RGB combine tool to add a ‘continuum-extracted’ Ha to RGB, which would be beneficial for galaxies such as Andromeda.

You can , however, stack and align everything in APP, export a TIFF for each channel, then do the combination (including the continuum extraction) in Photoshop. Here’s an example:

continuum subtraction

Just a heads up, continuum subtraction must be done in linear data. In the second video you linked, the guy does it on stretched data which is not only less accurate but also more hassle. To give them credit, they do know why and how continuum subtraction must be performed, but choose to ignore it for some reason. This lead to most of the continuum being left in the Ha image as well as the whole core getting nuked despite there being some emission signal.

After some digging I think you’re right, APP does not seem to have a pixelmath feature or any feature allowing for continuum subtraction (very surprising for the price they’re asking imo), so I recommend OP switches to Siril or perhaps a simple python script to perform continuum subtraction and then continues in their software of choice.

Behnam Darvish · Sep 5, 2025, 10:40 PM

You can do this using the Combine RGB tool in APP. It's located under the Tool tab. The tool has many filter combinations you can try, and many settings to experiment with.

Here’s a basic workflow:

For dual-band data (Ha + OIII):

1. Load all your images and calibration files. Before running the integration, go to the "0) RAW/FIT" tab. Under Algorithm, select "Ha-OIII extract Ha", then run the integration as usual. This will extract and stack only the Ha signal. Let’s call this result Ha-stack.

2. Repeat the process for OIII: go back to the "0) RAW/FIT" tab and now select "Ha-OIII extract OIII". Run the integration again. This will extract and stack the OIII signal. Let’s call this one OIII-stack.

For color (RGB) data:
Stack your RGB data as usual. Let’s call this RGB-stack.

Registration:
Now, register Ha-stack and OIII-stack with your RGB-stack. Load all three stacks as lights, and go through the workflow up to the registration step. Once registration is complete, save the registered frames using the "Save Registered Frame" option under the Registration tab. We’ll call them Ha-reg, OIII-reg, and RGB-reg.

Cropping:
Batch crop these three registered frames to remove bad edges or non-overlapping areas. Use the Batch Crop tool under the Tool tab. Lets call the cropped versions as Ha-crop, OIII-crop, and RGB-crop.

Gradient removal:
Run gradient removal on all three cropped images (Ha-crop, OIII-crop, and RGB-crop).

Splitting RGB channels:
To separate the R, G, and B channels from RGB-crop, load RGB-crop, go to the Calibration tab, scroll down to Split Channels, and use it to extract and save the individual R, G, B channels. We’ll call them R-crop, G-crop, and B-crop.

Combining with RGBHOO formula:
Open the Combine RGB tool under the Tool tab. Choose RGBHOO as the combination formula. Click Add Channel, and load the following:

• R-crop as Red

• G-crop as Green

• B-crop as Blue

• Ha-crop as Hydrogen Alpha

• OIII-crop as Oxygen III

Now play around with the parameters until you get a result you like. Save the final result as a FITS file. From here, continue your normal RGB processing workflow (e.g., further gradient removal if needed, background extraction, color calibration, etc.).

Star removal:
You can remove stars from Ha-crop and OIII-crop with starnet++ before using them in the RGB combination tool.

Final touches:
If you’re using GIMP or a similar tool, you can apply final touches—like using the starless Ha-crop image as a mask to further enhance Ha details.

I hope I did not miss anything. Thanks to chatgpt for cleaning my original text lol.

Thanks so much Behnam. This seems to be very straightforward and ill give this a try. Just a question. You mention in the “Star removal” portion of the above to mask out and remove stars before combining. I would like to do this as to not affect my RGB stars. If i mask out the stars from the Ha and OIII crop and combine them with the RGB data in the the RGB combination tool, I dont understand how APP will know to stack them correctly together in the combine RGB tool without stars? I assumed that stars were needed to allow for proper stacking of the 5 Stacks. I really appreciate your help :)