How to use images taken at different times of Jupiter’s rotation to create a cylindrical map
When it comes to planetary imaging, there is relatively little room for artistic interpretation when you are creating an image. Unlike nebulas and galaxies – which appear as dim and grey blobs to the naked eye, but as beautifully coloured in photos – the planets you see in a telescope are similar to those you often see in images. Despite this, there is still an opportunity to create an image portraying something unique about a planet.
Occasionally, planetary imagers with multiple images taken on a beautiful night or two as the target rotated, will create a map of their target. But incomplete maps often have artefacts on the edges, since pixels near the planet’s limb in the original photo are stretched on the map to compensate for the planet’s surface that is barely facing us, resulting
in an uneven resolution. A way to deal with this problem is to replace the low-resolution edges with parts of normal images that curve away naturally, to give the map a capsule shape.
To get started, we’ll need a set of multiple images, captured under excellent conditions within a few nights. The images of the planet Jupiter used here were taken on 18 and 19 March 2017 in Cibolo, Texas with a Celestron C11 EdgeHD telescope and a ZWO ASI290MM camera. We follow the typical workfow for planetary imaging: stacking the highest quality frames from videos using AutoStakkert!, sharpening those images using wavelets in a program like RegiStax, and de-rotating them in WinJUPOS to reduce graininess and to transform monochrome images into colour if necessary.
Time for a map
With our normal images fnished, we’re nearly ready to generate maps from them, which is also done in WinJUPOS. Incidentally, the de-rotation process makes this easier by automatically saving the images it creates with measurement fles, which can be used for map generation. We generate maps using the tool under Analysis > Map computation. While there are many options to fddle with, most of the defaults work fne for our purpose. WinJUPOS recommends a width for the maps that also matches the height with the original image. Taking advantage of this will be convenient later. While we can generate maps from
multiple images at once here, the results would contain abrupt seams between images. Alternatively, we can generate maps one-by- one and seamlessly blend them together ourselves.
Once the maps are generated, they can be loaded into Adobe Photoshop with File > Scripts > Load Files into Stack, which loads each as a layer. Only the top map will be initially visible. We’ll reveal the maps hidden underneath by adding masks to each layer. Brighter areas on a mask correspond to more opaque areas on the layer and darker areas are more transparent. Removing areas from the top map by painting the corresponding area on the mask black reveals the next map below. Repeating this process for lower maps, until the bottom layer is reached, reveals the entire area photographed between the two nights.
Sometimes there’s a dark seam caused by uneven lighting as a result of the planet rotating too far between images. This can be fxed by creating a new layer on top and setting the blending mode to overlay. Carefully painting shades of grey on the new layer over the seam allows it to be hidden. The border and extra information can be hidden by selecting it with the marquee tool and colouring the area black.
With the newly merged and cleaned map, it’s time to insert the unmapped images to cap both ends of the map. After opening them with File > Open and copying them over to the canvas with our maps, we hide half of each photo with layer masks and move them to align the planet’s poles with the top edges of the map.
The perimeter should line up nicely, but surface features are likely to be misaligned due to Jupiter’s slight axial tilt, which is present in the normal photos but removed from the maps. To correct this, we need to adjust the map in such a way that preserves the perimeter while warping the area. We can achieve this by fattening the map into a single layer and warping it with Edit > Transform > Warp and moving the central region of the map vertically. Patience with this fnal step is crucial, as warping an area of the map to align it with one end can misalign it from the other.
Once everything is nicely lined up, our capsuleshaped map is complete. It can help to add a new layer with overlay blending, to make subtle brightness corrections with shades of grey for the fnal result if needed.
In Adobe Photoshop each map is loaded as a stack of layers. Notice how the layer masks on the right are coloured to reveal lower layers
To give the map of Jupiter its distinctive capsule shape, two normal images of the planet are blended to both ends of the map
The fnal capsule-shaped map of Jupiter is complete, with subtle corrections to brightness
To compensate for Jupiter’s axial tilt, the planet’s surface features need to be ‘warped’ by fattening the map into a single layer and moving the central region vertically