PROCESSING
Handling problem greens in your astrophotos
Using a Photoshop plug-in to easily balance colours around deep-sky targets
BEFORE
AFTER
Effective colour management can really boost your astro photos. Without it, deep-sky objects can get lost against a noisy background. This is not to say you should aim for flat, black backgrounds with white stars. That can look unrealistic and unrepresentative of a deep-space environment.
Colours in space are a tricky thing to replicate. Our cameras pick up details that our eyes simply cannot. However, their sensors – which vary greatly depending on camera type, model and age – all pick up colour differently, so the same deep-sky image can look very different from camera to camera. Some produce more colour (or chromatic) noise than others, resulting in a noisy or off-colour image to process.
One channel that creates chromatic noise problems is green. Typically, unless imaging comets or the Northern Lights, this is one colour that astrophotographers wish to reduce or eradicate. Given that most astronomical objects do not emit visible green light, it is associated with noise, and can subdue nebulous details or discolour stars. Anything green in a deep-sky image is therefore usually an unwanted artefact we want to edit out.
Hasta La Vista, Green
Here we will take you through the steps to use Hasta La Vista, Green (HLVG) to deal with this troublesome colour. HLVG is a Photoshop plug-in developed by Rogelio Bernal Andreo that has been around for over a decade. It is compatible with recent versions of Photoshop and it is free.
Start by downloading the plug-in from www.deepskycolors.com/archive/2010/04/26/hasta-La-Vista-Green.html.
Locate the folder on your computer (usually in the ‘Downloads’ folder). This will be in a compressed format. Right-click the folder and click ‘Extract All’. To move the uncompressed file into Photoshop, navigate to the Photoshop plug-in folder. This is usually somewhere like C:\Program Files\ Adobe\AdobePhotoshop2022\ Plug-ins. Drag the HLVG file into this folder. HLVG can now be found in Photoshop by clicking Filter > DeepSkyColours > HLVG.
We applied HLVG to our stacked, widefield image of the Orion Nebula, captured using an astro-modified Canon DSLR. Cameras that have been modified to allow in extra ‘red’ infrared light can complicate the colour-balance process because our red channel has a lot more data compared to the green and blue channels. HLVG therefore comes in handy.
Before applying HLVG, we balanced the colours using the ‘Levels’ function, found at Image > Adjustments > Levels (see Screenshot 1). We ensured the histogram was set to ‘Colours’. In the Levels window, we dragged each of the anchor points in the red, green and blue channels so that they touched the edge of the curves. We checked the histogram each time to make sure the red, green and blue curves were roughly the same size and sitting neatly on top of each other. We repeated this process three times by right-clicking on the Levels layer and selecting ‘Merge down’ so that it merged into Layer 1. We then repeated the colour balance steps again. After the third time, we started to see noise in the image, so we stopped.
Check your balance
We could see that despite balancing the colours as much as possible, our image had a distinctly green tinge to it which suppressed both the Horsehead and the Orion Nebulae. Now was the time to use HLVG. We clicked Filter > DeepSkyColours > HLVG. This brought up the plug-in menu (see Screenshot 2). Because there was still a significant amount of green, we selected ‘Strong’ before clicking ‘OK’ (‘Medium’ and ‘Weak’ options are available if you don’t want to remove all the green). The green removal command was applied to our image and we could see it created a more realistic background (see Screenshot 3).
With the green now dealt with, to finish processing we made a series of final adjustments to the image’s brightness and contrast (Image > Adjustments > Brightness/Contrast), vibrance (Adjustments > Vibrance & Saturation) and selective colour (Adjustments > Selective colour). To further reduce noise, we also applied Photoshop’s median filter (Filter > Noise > Median) and noise reduction functions (Noise > Reduce Noise). The result, a final, naturally coloured image is shown above.
3 QUICK TIPS
1. Consider using HLVG’s ‘Strong’ setting for deep-sky nebula and ‘Medium’ or ‘Weak’ for comets or planetary nebulae.
2. Use hide all/reveal all masks to apply HLVG to the background or object only.
3. To maximise HLVG’s impact, try applying it after every colour balance in ‘Levels’, after merging down the layer.
Charlotte Daniels is an amateur astronomer, astrophotographer and journalist