Imaging the Sun with a hydrogen-alpha filter unveils dynamic, astonishing structures within its chromosphere.
Luckily, there are several digital imaging approaches that can mine these intricate details, enhance the contrasts and map changes in colour brightness.
Sharpening and deconvolution filters will also tease out fine structures, with the latest editing software now adding AI-driven image-processing methods too.
However, showcasing prominences on the edge of the chromosphere can still be challenging.
Read our guide on how to safely photograph the Sun
For one, how do we accentuate these outermost structures and filaments?
With this in mind, we can get creative to ensure prominences are highlighted.
One way is to turn a solar image on its head. I channelled Salvador Dalí to create what I call the ‘Ward-Solar-Transform’ effect.
This turns our Sun's disc inside out to place solar prominences centre stage.
Before applying this technique, bear in mind that it will highlight any shortcomings in your raw data, so only use the sharpest, high-resolution data.
Our start image (Before image, above) was a combination of the best of six bursts of 2,000 16-bit frames.
This generated plenty of data for ‘lucky image’ processing, a method that increases our chances of getting frames that are minimally affected by Earth’s atmosphere.
Here, the sharpest 3% was selected, aligned and stacked in AutoStakkert!
Getting started
We first opened our start image in Photoshop and made sure it was in 16-bit RGB format (click Image > Mode > RGB color > 16 Bits/Channel).
We then colourised our grayscale image (although the transform can be performed without this step).
In this example, we created a coloured gradient that included black, dark red, yellow, orange and white tones to reflect our preferred solar colours.
First colour, then flip
We created this gradient using the Gradient Map adjustment.
First, we added a gradient adjustment layer by clicking Layer > New Adjustment Layer > Gradient Map > Gradient Map 1.
An image layer and adjustment layer appeared.
Clicking on the gradient bar in the Gradient Map window (image above) displayed the Gradient Editor menu.
Here, we could create a custom gradient that included colours of our choice by clicking on the anchor points below the gradient bar (image above).
The Color Picker window appeared, which allowed us to select a colour for each anchor point.
We did this for red, then orange and yellow, clicking on a new anchor point each time that we wanted to introduce a new colour to the gradient bar.
Our grayscale image was now colourised (image above).
Once we were happy with our colours, it was time to apply a transform to turn the Sun inside out!
Cropping and filtering
Your image should be perfectly centred on the Sun in a square-shaped format.
If it isn’t, use the Photoshop crop tool by clicking the Crop icon in the adjustment menu (see image above) and dragging the tool to ensure the Sun is placed in the middle.
Any other format will warp your ‘solar horizon’ after you've done the transform.
Next, we clicked on and applied the following filter to the image layer in Photoshop: Filter > Distort > Polar Coordinates > Polar to Rectangular.
This converted our normal circular solar disc into a rectangle shape, with black space positioned below it (image above).
Next, we rotated this image by clicking Image > Image Rotation > 180° so that the ‘space’ part of our image appeared on the top.
We did this so that once we’d completed our transform, the Sun pixels would surround the space pixels in a doughnut shape, placing them at the centre.
To complete this transform, we converted the image back to polar coordinates, clicking Filter > Distort > Polar Coordinates > Rectangular to Polar.
We now had an image with space at the centre and the Sun’s surface at the periphery (image below).
Sharpening
This transform can cause significant edge distortion of the original image – shown by stretched and blurred pixels (image above).
Less-than-sharp data will, therefore, be particularly noticeable at this stage. You can crop the image again to remove these outer distorted pixels.
The better your data, the less you’ll need to crop at this final step.
Rectangular or square crops work equally well at this stage and will depend on the solar features visible.
Flattening the adjustment layers (click Layer > Flatten Image) and saving the result (click File > Save As) completes our process.
You can see my final inside-out Sun at the top of the page.
3 quick tips
- To avoid distorted edge pixels, use only the highest-quality data for the transform.
- Ensure the image is squared and centered on the Sun to create a perfectly circular solar horizon.
- Vary the amount of dark space around the Sun in your start image to change the central disc size after the transform.
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