PixInsight offers users a lot of options for processing images. With all of the available options, it can be difficult to figure out which processes to use and in which order to apply them.

This is an index article for processing narrowband images using PixInsight. A narrowband image is one where you have used filters like H-alpha, SII and OIII. This article assumes you have at least H-alpha and OIII images.

You can use files from one or more observations with this process; this process handles the details of creating master/primary files for you.

The processing workflow relies on scripts like AutoIntegrate and the EZ Processing Suite to simplify the process yet still take advantage of PixInsight’s processing capabilities. Each of the links in this article lead to articles that discuss the processing step in detail to help you learn how to use PixInsihght to process your narrowband images.

Sample Download

If you don’t happen to  have any narrowband images, I have made available a narrowband observation of NGC 2070  – the Tarantula Nebula – that I took using Telescope.live. The observation has H-alpha, SII and OIII images.

Download the sample observation of NGC 2070 here (94Mb)

This image of NGC 2070 is a result you can achieve using the sample download along with the directions in this article.

Directions For Processing A Narrowband Image Using PixInsight 

1. Use Jarmo Ruuth’s AutoIntegrate script, adding your SII, Ha, and OIII narrowband FITS files, and use the default settings 

2. Evaluate your image:

When evaluating your image:

a. Evaluate if the color palette is suitable for what you want to show in your image.

By default, AutoIntegrate uses the SHO palette, but you can choose from several options. You could try the alternate palettes offered under the Color palette dropdown. Before you try those color palettes, rename the AutoRGB image that AutoIntegrate produces and use Close all to avoid problems; use a name that reminds you of which color palette you used. Leave the Integration_H, Integration_S, Integration_O open and use the AutoContinue option to experiment with the different settings and palettes.

b. Try the options in the Extra processing section of AutoIntegrate to further enhance the image

c. Try the extra processing options in the Narrowband section of AutoIntegrate to further enhance the image

3. Perform a histogram or curves adjustment and stop your processing here if you are satisfied with the result.

The rest of this guide walks you through dealing with faint or bright areas, or dealing with a range of other issues.

4. On your PixInsight desktop are several images and one of them is called Integration_RGB; double-click this image to make it the active image. This is your combined master/primary image – it is made up of all of your other images, aligned and stacked. Note that, by default, this is an SHO image; you can change the palette using AutoIntegrate.js.

5. If the image has dark borders or areas at the edges that could be cropped, use DynamicCrop to remove the undesirable parts of the image

6. Use DynamicBackgroundExtraction to even out the background on the luminance image; alternately, for effective gradient removal, consider GraXpert.

7. Use EZ Denoise or use NoiseXTerminator, recommended, to reduce the noise in your image

Using EZ Denoise or NoiseXTermaintor on the whole image is likely to take a long time. You can shorten this time by creating a Preview of an area with high and low signal, then select the Preview in EZ Denoise, and run the EZ Denoise script. Once you are satisfied with the results, note the settings and record them, then rerun EZ Denoise on the whole image using the settings you recorded

8. Use BackgroundNeutralization to neutralize the background of your image

9. Use EZ Decon or use BlurXTerminator to deconvolve your image, if necessary

11. Transfer the STF settings to HistogramTransformation on the image to stretch it

Note: you could use a number of other methods to stretch your image.

You could use:

12. Evaluate the area with the greatest signal – you may have to adjust this region to bring out the details by performing a Curves transformation with an object mask in place using these steps:

a. Create a mask by extracting the luminance component and then adjust the histogram

b. Adjust the image using the mask; alternately, use a simpler approach that doesn’t require a mask

13. Review your image and determine if the stars in your image are magenta colored or have a magenta color cast, as this is a common problem with narrowband images. If you see a magenta color cast, use SCNR with an inverted image to reduce or eliminate the effect

14. Save your image, naming it ‘RGB_HT’

15. Run AutoIntegrate and select the Close all option to close all the windows on your desktop

16. Review your image and determine if you want to use SCNR to reduce the green color cast on your image.

17. Click the AutoContinue button in the Narrowband processing section of AutoIntegrate and allow the process to finish

Note the extra processing options that you can select

18. Use the HDRMultiscaleTransform process on the brightest parts of your image using a mask

19. Use LocalHistogramEqualization with the mask from step 12 in place

20. Use the ColorMask script to isolate the colors in your image through a mask and adjust areas as necessary (for example, to brighten certain areas, or boost the saturation)

21. Perform a Curves adjustment. You may wish to adjust the Curve of the object and background separately and for this, use the mask you created in step 12

22. Adjust the ColorSaturation

23. Although AutoIntegrate has already sharpened your image, determine if you wish to apply more sharpening. Sharpen the image using MultiscaleLinearTransform using the Bias setting (note: you do not need to do this if you already used BlurXTerminator)

24. Determine if you need to use SCNR; you may have to adjust the Amount to achieve your desired effect

25. Use the EZ Star Reduction script to slightly reduce stars if necessary (this is not necessary if you used BlurXTerminator)

26. Rotate your image using FastRotate if necessary

27. Use the DarkStructureEnhance script to enhance darker regions

28. Use the ICCProfileTransformation process to ensure your image is rendered as you intended

You have your final image at this point. Save your final image or, optionally, save as a project to keep all history states. Export your image to TIF or another format to add a logo or copyright information using Photopea.com.

Conclusion

In this article, I listed the steps I typically take for processing a narrowband image using PixInsight. This article focused on narrowband processing; refer to this article for LRGB processing.