Category: Astrophotography

My current workflow for processing one-shot colour images of deep-space objects, particularly nebulae, heavily relies on the process of reconstruction a foraxx palette outlined in this video by Paulyman Astro (https://youtu.be/Gy42AeZ_XB4?si=lNb4B7a0jVKWi4I1).

The video is very detailed, and it has been exceptionally useful for me, but I found myself scrolling through it and pausing so much that I wanted to have a written summary of the steps that I could quickly refer to. Apologies in advance, if the following steps appear out-of-context – they are really short notes that would make sense to those who have used the Pixinsight software and worked with the foraxx palette.

Before this image processing is even started, I generate an integrated image using the following pre-processing sequence (see this guide by Adam Block for details: https://youtu.be/VKOTCuqD2Qs?si=EdbONwT8GO_DUAkR) :

First of all, all the individual exposures (light frames), which are typically 5-min long each, are calibrated with “flats”, “dark flats” and “darks” using the WeightedBatchPreprocessing script. 

Then, the resulted de-bayered images are aligned using the StarAlignment process.

Finally, the aligned images are integrated using the ImageIntergation process with Winthorised Sigma Clipping background rejection method. This produces the “integration.xisf” image, which is the basis for the nebula processing workflow itself:

1. Right-click on the identifier tab at the top-left of the image frame and set the new identifier to ‘osc’ (for ‘One-Shot Colour’).
2. Use Process>All processes>ScreenTransferFunction to preview a stretched image. I unlock the RGB channels before pressing the “nuke” button to avoid a high colour cast. This is not important though, because nothing is actually being done to the image – this is just a preview.
3. Use Process>All processes>DynamicCrop to crop the image.
4. Use AutomaticBackgroundExrtactor with Function degree (under Interpolation and Output) set to 1 and Target Image Correction to ‘Subtraction’. This works if there is simply some light pollution gradient in the image and little to no vignetting. Otherwise, use the DynamicBackgroundExtraction process.
5. Split the RGB channels using an icon in the toolbar.
6. Set the identifier of the Red channel to ‘ha’ for “Hydrogen alpha”.
7. Combine the Green and the Blue channel using the following Pixelmath expression: 0.5*B+0.5*G. Give the resulting image an identifier ‘oiii’. 
8. Use StarExterminator process to extract the stars. Check “Generate Star Image” and “Unscreen Stars” boxes. Drag the triangle from the process window onto the ha window and then onto the ha window.
9. Use Generalized Hyperbolic Stretch (GHS) script or process to stretch the ‘ha’ and the ‘oiii’ (starless) images.
   • First stretch: Just right of the peak. Local stretch intensity ~10. Stretch factor ~3.
   • Second stretch: Secondary drop-off (log view). Local stretch intensity ~5.
10. Foraxx process: Create the false Green channel (‘ho’) using the Pixelmath expression: (ha*oiii)^~(ha*oiii). Press the square button.
11. Boost the brightness of the ‘ho’ image by doing the first-level stretch in GHS.
12. Create the colour image by using the Pixelmath expression (uncheck ‘Use a single RGB/K expression’, set Color space to ‘RGB color’):
    • R: ha
    • G: ho*ha+~ho*oiii
    • B: oiii
13. Apply CurvesTransformation. Start with saturation, proceed to individual channels.
14. Apply NoiseExterminator with default values or Denoise ~0.9, Detail ~0.55.
15. Stretch ‘ha_stars’ and ‘oiii_stars’ using HistogramTransformation. Use checkmark to track the histogram and use live preview to monitor the stretch amount. Drag middle slider to the left.
16. Apply the Foraxx process (step 10) to ‘ha_stars’ and ‘oiii_stars’.
17. Use CurvesTransformation to boost saturation of ‘foraxx_stars’.
18. Put the stars back using the Pixelmath expression: ~(~foraxx*~foraxx_stars).

And this is it! Here are some examples of my application of this process applied to various emission nebulae.

This emission nebula is one of the youngest star-forming regions in our galaxy. Some components of it are only few million years old. In the cosmic time scale, this is basically star birth happening in front of our eyes. This nebula also contains the hottest star found within 1 kpc of our Sun, the BD+66 1673, which has the surface temperature of 45,000 K and the luminosity 100,000 times that of the Sun. It is primarily responsible for ionizing the gas of the nebula and for compressing it by the strong stellar wind, leading to creation of new stars. 

This light travelled for 3,000 years before reaching my yard in Victoria, BC in June 2024.

NGC 7822 is a large target, filling the full-frame sensor of my camera attached to a 478 mm-long telescope. This is a 3.5 hr-long RGB exposure at f/5.9, processed using a Foraxx palette.

I captured this image of the Croc’s Eye Galaxy (M94, also called the Cat’s Eye Galaxy) yesterday in my yard in Victoria, BC, using a total of 3 hours of exposure. It’s an unusual galaxy – it has a an inner ring with a diameter of 5,400 light-years and an outer one with a diameter of 45,000 light-years. Pressure from the galactic core compresses the gas and dust clouds in the outer ring, where gravity pulls them together to form new stars. These stars pull in more gas and dust, resulting in a relatively empty region separating them from yet another layer of gas at the periphery of the galaxy.

M94 has a remarkably low amount of dark matter for a galaxy – the stars comprise almost all of its mass. Their light travelled for 16 million years before reaching my telescope.

W5 is a large emission nebula located in the Cassiopeia constellation, close to the Heart Nebula (IC 1805). There are several open star clusters inside the Soul Nebula, including IC 1848, which is often used to identify it.

The Soul Nebula is a star nursery, where new stars are created practically before our eyes. In fact, most stars have been born in regions like W5, where hundreds or thousands stars form at the same time.

The Soul spans 300 light-years, which is about 100 times the distance from the Sun to the nearest star.

About a dozen of giant O-type stars are primarily responsible for creating this emission nebula. These giant stars are approximately 30 times heavier and 10,000 times more luminous than our Sun. This intense luminocity is mostly in the form of ultraviolet radiation. It forms a powerful stellar wind that ionizes gas molecules and drives them away from the giant stars, creating the bubble structure in the middle of the nebula. 

This structure contains gas pillars that point towards the stars that created them. The pillars form because the denser gas areas take longer to clear, while the material around them is swept away by the stellar wind. The compression of the gas molecules in the pillars accelerates their gravitational collapse and leads to formation of new stars.

In the Soul Nebula, there are at least three generations of star formation. Their light travelled for 6,500 years before reaching my camera in Victoria, BC in June of 2024.

IC 1805 is a faint, but huge emission nebula in the Cassiopeia constellation. It is about 330 light-years in diameter and has an angular size of 2 degrees – 4 diameters of the full Moon! The open star cluster in the centre of the heart (Collinder 26) creates intense stellar wind that drives the shape of the gas cloud and causes it to emit the intense red colour. Some of these stars are 50 times more massive than the Sun.

This light travelled for 7,500 years before reaching my camera in Victoria, BC in the early October.

M31 is the deep-sky object that made me want to do astrophography. This light travelled for 2.5 million years before reaching my yard in Victoria, BC in the early September.

The Andromeda is closest galaxy to our Milky Way Galaxy. It is 152,000 light-years in diameter, contains approximately 1 trillion stars and is moving towards us at 110 kilometres per second. Our galaxies will collide in about 4.5 billion years, eventually forming a single giant elliptical galaxy.

The chance of any stars colliding is actually negligibly small, because despite their great numbers, the distances between the stars are tremendous. The two galaxies will simply pass through each other, as they commonly do. In fact, the M31 itself merged with another galaxy 3 billion years ago.

However, both the Milky Way and the Andromeda galaxies have supermassive black holes in their centres, and when they eventually merge, they would form a quasar and release as much energy as about 100 million supernova explosions. According to the current models, there is a 12% chance that our Sun would get ejected from the new galaxy during the collision, in which case the star itself, as well as its planets would be undisturbed. However, if the Sun comes close to the new black hole, it would be torn apart by its gravity.

This will be of little consequence to the life on our planet, though, because much earlier than that, about 1 billion years from now, the Sun’s luminosity will increase by approximately 40%, and there will be no way for liquid water and terrestrial life to exist on Earth.

Time to introduce planetary engineering into our curriculum? 

NGC 7635 is a unique-looking emission nebula in the Cassiopeia constellation. It looks like a glowing bubble, and in some sense it is. It’s a shell of ionized H II gas blown out by a massive, hot young star SAO 20575. This central star has a visual magnitude of 8.7 and has a mass of 44 Suns. Its light travelled for 11,000 years before it hit my yard in Victoria, BC on June 7, 2024.

The Wizard Nebula (NGC 7380) is considered a challenging photo target, because it is very dim. It’s impossible to see visually, and even photographically it typically requires hours of exposure time to bring out the details. I was lucky to have about 3.5 hours of cloudless skies to take this shot a couple of nights ago. The dual-narrowband (Ha + OIII) filter is tremendously helpful in cutting through light pollution. 

NGC 7380 is actually an open cluster of very young stars in the constellation Cepheus that formed about 5 million years ago – practically a moment ago in cosmic terms.  The nebula itself (S 142) is a cloud of ionized gas approximately 110 light-years long. It took 7000 years for its light to reach my yard in Victoria, BC.

The North America Nebula (NGC 7000) and the Pelican Nebula (IC 5070) in the Swan (Cygnus) constellation barely fit together in the field of view of my full-frame camera on a 478mm-long telescope. The North America nebula, named because it resembles the shape of the continent, is more than four time the size of the full moon! Its most famous feature, the Cygnus Wall lies where the Mexican West Coast would be on the map.

The Pelican Nebula sits just off the “East Coast” of the North America and is separated from it by the dark cloud of dust.

Despite the large size, these emission nebulae are very faint, so I was glad to have about 3.5 hours last night without clouds or moonlight to collect their light, which travelled for 2,600 years before reaching Victoria, BC.

This globular cluster is located in the torso of the Hercules constellation, about third of the way between Vega and Arcturus. It is close to my heart too, because it was the first Messier object that I saw in a telescope, and it’s beauty attracted me to astrophotography many years ago.

M13 is 145 light-years in diameter and contains over 500,000 stars. It is more than a hundred times more densely packed with stars than the neighbourhood of our Sun. The stars are so close that their collisions occasionally produce new stars.

Fun fact: In 1974, a coded Arecibo message about the human race was transmitted towards M13 to potentially contact extraterrestrial civilizations. Ironically, when the radio waves arrive there, in about 22,000 light-years, the cluster will likely have moved to a different position.

In the meantime, I was able to catch some of its photons flying the other way in my yard in Victoria.