Slooh.com offers members broadband imaging, that is, Slooh.com captures images using the LRGB filters. Lately I have been getting into narrowband imaging using Telescope.Live and iTelescope.net.

Unlike broadband imaging where colors are fixed, you have a lot more creative freedom with narrowband imaging. There are several palettes you can use to create your finished image; however, the most popular palette is the Hubble Palette, popularized by the Hubble Space Telescope team. The Hubble Palette maps SII to red, H-alpha to green, and OIII to blue, producing very pleasing images.

Not all targets respond well to narrowband filters – some objects simply don’t give off the light needed to create a nice-looking narrowband image. Before I image a target in narrowband, I try to find images others have taken, but I’m not always successful in my search.

To address this, I created a lookup table (LUT) that you can use to convert a broadband image into a narrowband image that uses the Hubble Palette (SHO). A LUT is a color and saturation transformation that gets applied to your image.

Here’s a broadband image of NGC 6188 taken using Slooh.com:

And here is the same region, imaged using narrowband and presented using the Hubble Palette:

The subject is the same but the colors and details are very different.

I created a LUT that you can use with Photopea.com or Photoshop that allows you to see what a narrowband image might look like before you go to the trouble of capturing the image. Here’s what the Slooh.com image looks like after applying the LUT:

The image is pretty close to the actual narrowband image, and you can achieve the same result with just a few clicks.

Using a Custom LUT to Convert a Broadband Image to Narrowband/SHO

Use the following steps to convert a broadband image into a narrowband/SHO image:

  1. Point your browser to Photopea.com
  2. Open a broadband image
  3. Download this ZIP file and extract the contents someplace convenient on your computer
  4. From the menu, select Layer – New Adjustment Layer – Color Lookup
  5. Next to LUTs, click the downward-pointing arrow
  6. Click the second downward-pointing arrow that comes up
  7. Select the option Load .icc .cube .look .3dl
  8. Select one of the files you downloaded in step 3

You can adjust the transform’s intensity by adjusting the Opacity of the new layer. If you are satisfied with the transformation, you can flatten the image by making the layer permanent by selecting Layer – Flatten image and from there you can continue making further adjustments.

There are two LUTs in the ZIP file – one has more blue in it than the other; experiment with both as some targets may require more blue than others.

Conclusion

In this article, you learned about a LUT and learned how to apply it to your RGB images to produce a pseudo-narrowband image that uses the Hubble Palette.