Large aper­tures and long cap­ture times re­veal the ice gi­ants’ se­crets

Sky at Night Magazine - - IMAGING FOR SCIENCE -

They may be large bod­ies, but the ice gi­ants Uranus and Nep­tune are also very dis­tant and this makes them sig­nif­i­cantly more dif­fi­cult to im­age use­fully. Large aper­tures work best but the qual­ity of the at­mos­phere you’re imag­ing through also plays a very im­por­tant part. With an ap­par­ent disc of typ­i­cally just 3.7 arc­sec­onds across for Uranus and 2.2 arc­sec­onds for Nep­tune, it doesn’t take much at­mo­spheric un­steadi­ness to hide any de­tail in these far off plan­ets.

Their great dis­tance also makes them fairly dim for the process of high-frame-rate imag­ing. How­ever, if your goal is just to cap­ture any belts or bands vis­i­ble on their globes, long cap­ture times are per­fectly ac­cept­able as long as the tech­nique is doc­u­mented with the im­age. Cap­ture times run­ning to sev­eral tens of min­utes are not un­com­mon. Up­ping the cam­era sen­si­tiv­ity and cap­tur­ing frames at a rel­a­tively slow frame rate is a vi­able tech­nique here. A use­ful tip for fo­cus­ing is to im­age a nearby star be­fore you start your cap­ture run and pre-fo­cus ac­cu­rately on that. This as­sumes you can find the planet again af­ter fo­cus­ing has been com­pleted!

For Uranus, long-pass or in­frared-pass fil­ters tend to be the most use­ful, com­bined with a mono high­frame-rate cam­era or an IR-sen­si­tive colour cam­era. A large aper­ture of at least 10 inches (250mm) is rec­om­mended, to­gether with an R+IR fil­ter. The Baader RG 610 de­liv­ers a rel­a­tively bright im­age for in­stru­ments at the smaller end of the rec­om­mended size range un­der av­er­age see­ing con­di­tions. For larger aper­tures above and in­clud­ing the 12- to 14-inch (300 to 350mm) size range, a Baader RG685 fil­ter can be used un­der ex­cel­lent see­ing con­di­tions.

Bright spots on the globes some­times oc­cur but the small disc sizes and un­steady at­mo­spheric con­di­tions some­times make it hard to de­ter­mine if they’re real. In which case, make sev­eral cap­tures sep­a­rated by, say, 15 minute in­ter­vals. Pro­cess­ing the cap­tures then turn­ing them into an an­i­ma­tion will help de­ter­mine whether the spots are con­sis­tent, and there­fore real.

The free­ware WinJUPOS can be used to pro­vide im­por­tant ephe­meris and anal­y­sis tools when imag­ing both Uranus and Nep­tune

An am­a­teur ob­ser­va­tion of Nep­tune’s moon, Tri­ton, oc­cult­ing a star. Note the char­ac­ter­is­tic cen­tral peak as the star’s light was fo­cussed by the moon’s at­mos­phere

Uranus (left) and Nep­tune

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