Pro­cess­ing

How to use images taken at dif­fer­ent times of Jupiter’s ro­ta­tion to cre­ate a cylin­dri­cal map

Sky at Night Magazine - - CONTENTS - Ethan Chap­pel is a US-based as­tropho­tog­ra­pher who spe­cialises in imag­ing plan­ets. He was short­listed at the IIAPY in 2018 for his ‘Jupiter Un­rav­elled’ 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

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