Half HSO, half SHO. (Daniel Meek/RASC Robotic Telescope)
Half HSO, half SHO. (Daniel Meek/RASC Robotic Telescope)
Using filters and the RASC Robotic Telescope

The data used to build each of these images is the same. So what makes them so different? Palettes.

Using the RASC Robotic Telescope, Daniel Meek took this image of IC 1871, a small emission area within the larger IC 1848, the Soul Nebula in Cassiopeia.

The data used to build each of the images is actually the same — captured between October 2019 and February 2020 for a total exposure time of 15 hours. So what makes the pictures so different?

Meek explained that it’s all in how the colours are assigned to filters.

As with many pictures taken of astronomical objects, astrophotographers use filters to allow very specific bands of light, and they take several images allowing in different bands. Colours are assigned to those filters in photo editing software afterward, and then they are put together to create one image.

When choosing colours, an astrophotographer can use a “palette,” which is determined by assigning the narrowband filters (actually the signal or data through the filters) to the red-green-blue (RGB) colour channels.

Red, green and blue light can be added together to make all the colours under the rainbow. When added together fully, they make white.

In the gallery at the start of the article, the photo on the left uses the HSO palette. HSO stands for hydrogen (Ha), sulphur (SII) and oxygen (OIII). Hydrogen (Ha) is assigned to red, sulphur (SII) to green, and oxygen (OIII) to blue.

SHO is another palette, and it is used in the image on the right above. SII is assigned to red, Ha to green and OIII to blue.

There are other permutations. And people have also given names to the various palettes, such as the “Hubble Palette.”

“The [HSO palette] is probably the more common one that you see for this particular image, but I find the SHO palette can sometimes bring out more detail in some images,” Meek said.

“For this image IC 1871, I think that I got more depth in the SHO image, but more detail in the HSO image.

“Lots of experimenting.”

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