BA­SICS

Essen­tials for imag­ing a fast-ro­tat­ing, low-al­ti­tude planet

Sky at Night Magazine - - IMAGING FOR SCIENCE -

Suc­cess­ful plan­e­tary imag­ing re­quires unique as­tro pho­to­graphic skills and fast-ro­tat­ing plan­ets such as Jupiter, Mars and Saturn add ex­tra pres­sure be­cause mo­tion blur can take place if the cap­ture time is too long.

On top of this, the ef­fect of high speed lo­cal at­mo­spheric wob­bles (see­ing) de­te­ri­o­rates re­sults. High-frame-rate cam­eras lessen the im­pact to a de­gree by tak­ing many still images in rapid suc­ces­sion, which are then pro­cessed by regis­tra­tion stack­ing soft­ware like RegiS­tax or Au­toS­takkert. The au­to­matic process in­volves sort­ing by frame qual­ity, reg­is­ter­ing the best frames to one an­other and av­er­ag­ing these into a sin­gle image. Sen­si­tive and fast cam­eras are es­sen­tial be­cause what you’re at­tempt­ing to do is sam­ple across the fleet­ing pe­ri­ods of good see­ing which should oc­cur dur­ing the cap­ture ses­sion. Max­i­mum record­ing times shorter than 60s or even 30s are rec­om­mended so high cap­ture frame rates are very de­sir­able. Soft­ware such as WinJUPOS of­fers the fa­cil­ity to undo many min­utes of ro­ta­tion via a de-ro­ta­tion func­tion.

Jupiter’s vis­i­bil­ity from 2018-2023 is fur­ther com­pli­cated by its low al­ti­tude as viewed from the UK. The planet’s de­creas­ing height above the hori­zon makes it more likely that images will ex­hibit colour fring­ing from at­mo­spheric dis­per­sion. This can be coun­tered rea­son­ably ef­fec­tively by us­ing an op­ti­cal de­vice known as an at­mo­spheric dis­per­sion cor­rec­tor (ADC) fit­ted be­tween the cam­era and the tele­scope.

An ADC will al­low a colour cam­era to be used on Jupiter at low al­ti­tude. Mono cam­eras will need an RGB fil­ter set to pro­duce colour images. Us­ing an in­frared pass fil­ter as well of­ten helps ob­tain higher con­trast re­sults, while tak­ing ad­van­tage of the fact that longer wave­lengths are less af­fected by poor see­ing. A meth­ane (CH4, 890nm) fil­ter can also be used to good ef­fect on meth­ane-rich Jupiter. This re­quires a cam­era with a good re­sponse in the near in­frared part of the spec­trum.

The movie se­quences recorded by high frame rate cam­eras are good can­di­dates on which to carry out im­pact flash searches. Soft­ware such as De­TeCt (www.as­tro­surf.com// plan­etes­saf/ doc/dtc/doc/dtc_ tu­to_en.htm) looks for the bright tran­sient flashes that may ap­pear on Jupiter’s disc as signs of me­te­ors va­por­is­ing in its at­mos­phere.

Soft­ware such as De­TeCt can an­a­lyse your cap­ture files, look­ing for im­pact flashes – ev­i­dence of rare, me­te­oric events within Jupiter’s at­mos­phere

A high-frame-rate cam­era is es­sen­tial for mod­ern high-res­o­lu­tion plan­e­tary imag­ing

A typ­i­cal at­mo­spheric dis­per­sion cor­rec­tor (ADC) con­tains two counter ro­tat­ing prisms

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