PRO­JECT 3 Track a ‘wan­der­ing star’

Cap­ture the planet’s me­an­der­ing path across the sky

Sky at Night Magazine - - ADVERTISEM­ENT FEATURE -

If you’re look­ing for a plan­e­tary imag­ing pro­ject that isn’t ad­versely af­fected by the low al­ti­tude of your tar­get then why not try cap­tur­ing a planet’s ap­par­ent mo­tion against a dark, star-filled sky over the course of a few days or weeks. Jupiter, Saturn and Mars are per­haps the best tar­gets for this as they move across the sky much more quickly than dis­tant Uranus and Nep­tune.

Es­sen­tially what you need to do is record an im­age each night show­ing the planet – ob­vi­ously – but cru­cially all the shots need to be cen­tred on the same patch of starry sky ev­ery night. This is be­cause later you’re go­ing to com­pos­ite to­gether all the shots you get so that the only thing that ‘moves’ be­tween them should be the planet.

For this to work you need to have a setup that can record the planet, and the stars around it, well enough that you can eas­ily over­lay and align the re­sul­tant im­ages. In prin­ci­ple, you can achieve this with a DSLR and a fast lens on a static tri­pod, though you may find it eas­ier – if you have the kit avail­able – to mount a cam­era and lens (or wide-field te­le­scope) on a track­ing mount and cap­ture longer ex­po­sures.

If you de­cide to use a longer fo­cal-length lens or scope for this, the nightly change in po­si­tion of the planet will be more ob­vi­ous. But be aware that you might only record the planet at a hand­ful of lo­ca­tions be­fore it moves out of the fixed field of view. You may, there­fore, find it best to use a lens of, say, 50-100mm fo­cal length.

Once you’ve got at least sev­eral nights’ worth of shots, bring them into a lay­ers-based im­age ed­i­tor and load them as sep­a­rate lay­ers within one im­age file. It’s likely you’ll need to tweak each layer’s po­si­tion and ori­en­ta­tion so the star fields are aligned. Once they are, set each layer’s ‘blend mode’ to ‘lighten’, whereby the planet should ap­pear in mul­ti­ple places across the frame – its wan­der­ing across the sky re­vealed.

The ret­ro­grade mo­tion of Mars can be shown in a mon­tage of sev­eral im­ages

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