Tease detail from the ice giants
Use stacking software to bring out surface features and moons
For the ‘Planets, Comets and Asteroids’ section of the 2020 Astronomy Photographer of the Year competition, I was fortunate to secure second place for my image, ‘In the Outer Reaches’ (above). The image shows the distant ice giant, Uranus, as a pale-green disc accompanied by its five brightest moons. In the shot the planet’s lighter northern polar region and subtle single darker belt indicate the ice giant’s extreme axial tilt.
The image was taken on a night of exceptional seeing in December 2019 from my garden in St Albans and captured using my home-built 444mm Dobsonian sitting on an equatorial tracking platform.
Trying to image the ice giants, Uranus and Neptune, is challenging due to their faintness, small size, and subtlety of surface features. Fortunately, the task has become much easier in recent years, due to the improvements in both the sensitivity and noise reduction (reducing unwanted artefacts) of CMOS-based planetary imaging cameras, which allow the use of much shorter frame exposures. Not so long ago, frame exposures of 0.5 seconds or longer would be required for Uranus, leading to high levels of smearing by atmospheric movement and resulting poor definition. With some of the new generation CMOS video cameras, scopes of 150mm aperture or larger should, in good seeing conditions, be able to easily capture the lighter polar region of the planet.
Keeping the image bright
For my image that night I used a 2.7x Barlow lens giving a focal ratio of f/12.7. With my infraredsensitive, monochrome ASI290MM camera, this gave an image scale of 0.11”/pixel. At a relatively high gain of 54dB, the camera’s high sensitivity allowed me to use an exposure of just 8.6 milliseconds and still keep the image bright. Infrared imaging helps steady the
views and is essential for improving the contrast of Uranus’s surface detail. I used a 610nm long-pass filter, which works well – as it passes more light than most other infrared filters operating at longer wavelengths.
With these camera settings, the individual video frames look very noisy (full of unwanted artefacts) due to the lack of photons (see Stage 1, opposite), but when you stack lots and lots of these frames in AutoStakkert! or RegiStax, the long-accumulated exposure time means that the noise levels drop (see Stage 2, left, top). For my image I stacked the best 13,500 frames in AutoStakkert!, out of a total of 90,000 frames gathered over 20 minutes.
Next, the stacked image was wavelet-processed in RegiStax ‘Wavelets’ to draw out the details, using just the ‘Denoise’ and ‘Sharpen’ settings in the first level (as shown in the Stage 3 screenshot, left).
To capture Uranus’s fainter moons, I recorded later videos with a larger region of interest at maximum camera gain (60dB), and a longer exposure of 100 milliseconds. Next, I stacked the best 45 per cent of 2,600 frames taken over five minutes. Minimal sharpening was needed in RegiStax, but the gamma function, at 3.3, brightened the moons with respect to the dark background (see Stage 4, below, left).
The nicely exposed moons (with an overexposed Uranus) then had to be combined with the correctly exposed planet. This was easy as the camera was not rotated between imaging the two. The moons layer was used as the bottom layer of a new image in PaintShop Pro and the good planet layer pasted on top. The top planet layer was then moved until Uranus’s disc exactly covered the over-exposed version in the bottom moon layer. Reducing the ‘Opacity’ of the top layer to 60-70% allowed me to get the alignment spot-on by being able to see both together. The reduced ‘Opacity’ also allowed me to see the moons in the bottom layer – that was important as the next step was to erase holes in the top layer to reveal the moon on the bottom layer. Once all were revealed the ‘Opacity’ was set to 100%.
Before merging the two layers the planet layer was colourised to make a more pleasing pale green. I used RGB colour balance settings of Red: –20%, Green: +14% and Blue: +14% for this. Once colourised and merged, the image was finished; a pleasing capture of one of Earth’s distant neighbours, orbiting in the far regions of our Solar system.