Astropho­tog­ra­phy

An­i­mate the Galilean moons.

Sky at Night Magazine - - CONTENTS -

Jupiter presents a wealth of de­tail but tan­ta­lis­ingly this can be tricky to im­age be­cause of the planet’s fast spin rate. Jupiter has a di­am­e­ter of 139,822km mak­ing it the largest planet in our So­lar Sys­tem. Spin­ning on its axis in less than 10 hours, its gaseous form bulges at the equa­tor pro­duc­ing an oblate disc through the eye­piece. Fierce jet streams rage around the planet, the at­mos­phere ap­pear­ing banded and tur­bu­lent.

The Earth’s at­mos­phere blurs and dis­torts the disc of a planet. The best way to im­age one is by us­ing a high frame rate cam­era: by cap­tur­ing lots of se­quen­tial frames, some of the im­ages will be less dis­torted than oth­ers. Us­ing a reg­is­tra­tion-stack­ing ap­pli­ca­tion such as RegiS­tax or Au­toS­takkert!, it’s then pos­si­ble to cap­i­talise on the bet­ter frames, ex­tract­ing them from the pack, reg­is­ter­ing them to­gether and av­er­ag­ing the re­sult. Cam­eras with frame rates rang­ing up to sev­eral hun­dred frames­per-sec­ond (fps) are com­mon these days.

Hon­ing your plan­e­tary imag­ing skills on a fast-ro­tat­ing planet like Jupiter can take lots of prac­tice and mat­ters are fur­ther com­pli­cated when the Galilean satel­lites be­come in­volved. Io, Europa, Ganymede and, to a lesser ex­tent, Cal­listo, can all ap­pear to in­ter­act with Jupiter’s disc, pass­ing across it as a tran­sit and cast­ing their shad­ows on its sur­face in what’s known as a shadow tran­sit.

The main is­sue with moon and moon shadow tran­sits is that they take place at a dif­fer­ent rate to the ro­ta­tion of the planet be­low. Ad­vanced tech­niques such as disc de-ro­ta­tion, al­low for ex­tended cap­ture times be­yond which ex­ten­sive mo­tion blur would nor­mally oc­cur. They work by ef­fec­tively ro­tat­ing all of the frames back to a com­mon time, un­do­ing the ro­ta­tion of the planet. When a moon tran­sit be­comes in­volved, de-ro­ta­tion doesn’t work as well be­cause of the rel­a­tive mo­tion of the moon.

One way around this prob­lem is to keep your cap­tures short. This pro­duces nois­ier end re­sults but at least ev­ery­thing ap­pears rel­a­tively sharp. Tak­ing lots of short cap­tures will al­low you to pro­duce a se­quence of re­sults which can be added

to­gether as an an­i­ma­tion. The ben­e­fit of this tech­nique is that it cre­ates a dy­namic record of Jupiter’s ro­ta­tion as well as the moon and shadow tran­sit. It also helps dis­guise the ad­di­tional noise in each cap­ture re­sult.

Our step-by-step guide op­po­site shows how to cre­ate a mono­chrome an­i­ma­tion us­ing a mono, high frame rate cam­era fit­ted with an in­frared pass fil­ter. A mono cam­era could be used with in­di­vid­ual RGB fil­ters but this would en­tail a lot of work and open the door to ad­di­tional is­sues be­cause of the rel­a­tive mo­tion of the moons and their shad­ows. One big ad­van­tage of us­ing an in­frared pass fil­ter in con­junc­tion with a mono­cam­era is that it can make moons such as Io and Europa clearer to see as they shine brighter in in­frared than they do in visual wave­lengths.

Al­ter­na­tively, a one-shot, colour, high frame rate cam­era can be used, but with Jupiter now get­ting lower as seen from the UK, the best re­sults re­quire a more ad­vanced ap­proach us­ing an op­ti­cal de­vice called an at­mo­spheric dis­per­sion cor­rec­tor (ADC). This is used to coun­ter­act the ef­fects of at­mo­spheric dis­per­sion that be­come more ev­i­dent for ob­jects lower in the sky.

How­ever you do it, a suc­cess­ful an­i­ma­tion of the Jo­vian sys­tem is a de­light to watch as it re­ally helps em­pha­sise how the Galilean moons move in their or­bit around their host planet.

A great way to ob­serve the re­la­tion­ship be­tween Jupiter and its moons is with an an­i­ma­tion

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