How To...

Im­age struc­tural de­tail on the ISS.

Sky at Night Magazine - - CONTENTS - With Thierry Le­gault

If you own a te­le­scope and a cam­era, you may have al­ready taken de­tailed pic­tures of the Moon and the plan­ets. But have you ever imag­ined chas­ing the In­ter­na­tional Space Sta­tion and tak­ing pic­tures of it that re­veal its shape and its main struc­tures?

There is a kind of magic to the silent and ma­jes­tic move­ment of this huge struc­ture as you watch it sail across

the sky. At the in­cred­i­ble speed of al­most 8km/s, the ISS ac­com­plishes one or­bit around Earth in only 90 min­utes. It is often vis­i­ble over the UK at dawn or dusk.

For­tu­nately, you don’t need to cal­cu­late the favourable pas­sages by your­self: a va­ri­ety of ap­pli­ca­tions and web­sites can do it for you – try Heav­ens Above (www.heav­ens-above.com) or CalSky (www.calsky.com). Ide­ally you are look­ing for a pas­sage with a cul­mi­na­tion of 30° above the horizon as the ISS will ap­pear larger and will be less dis­turbed by at­mo­spheric tur­bu­lence.

Given its bright­ness and its ap­par­ent size – the lat­ter is com­pa­ra­ble to Jupiter – the ISS would be an easy tar­get for a plan­e­tary im­ager if it re­mained still in sky. The down­side of its low al­ti­tude (400km) – the very rea­son it is so bright – is that it has an ap­par­ent speed of more than 1° per sec­ond when over­head.

Even equipped with mo­tors and com­put­erised Go-To sys­tems, most com­mer­cial mounts do not pos­sess the tim­ing pre­ci­sion, re­ac­tiv­ity or fine range of speeds needed to fol­low the ISS by pro­grammed track­ing: you’ll have to do it vis­ually through your find­er­scope. You can use any kind of mount – Dob­so­nian, al­taz­imuth or equa­to­rial – so long as it of­fers smooth and well-bal­anced man­ual movements.

One trick that can be em­ployed is to de­lib­er­ately not po­lar align an

equa­to­rial mount as usual. In­stead, align it with the tra­jec­tory of the ISS dur­ing its tar­geted pas­sage. To do this, aim the po­lar axis of the mount op­po­site the di­rec­tion the ISS cul­mi­nates in: for ex­am­ple, if the ISS cul­mi­nates in the south-south­west, align the po­lar axis north-north­east. Ad­just the lat­i­tude of the mount to 90° mi­nus the cul­mi­na­tion al­ti­tude of the ISS: for ex­am­ple, if the ISS cul­mi­nates at 60°, set the lat­i­tude to 30°. This will make track­ing much eas­ier. The move­ment in dec­li­na­tion will be (al­most) noth­ing, and the track­ing in right as­cen­sion will be the slow­est pos­si­ble.

Any type of te­le­scope can be used. As in plan­e­tary imag­ing, long fo­cal lengths may lead to more de­tailed pic­tures – or to more vis­i­ble track­ing dif­fi­cul­ties and er­rors. Use a re­li­able and sta­ble crosshair find­er­scope with pre­cise align­ment screws. A right-an­gled erect­ing model may be more con­ve­nient, es­pe­cially if the ISS passes close to the zenith.

Choose wisely

The most ca­pa­ble cam­eras for ISS imag­ing are the ones used in plan­e­tary, lu­nar and so­lar imag­ing. A large sen­sor may help to keep the ISS within its field of view. You can also try a DSLR in video mode, al­though the ISS will look smaller and will there­fore need a longer fo­cal length. An­other op­tion, with a DSLR, is the con­tin­u­ous shoot­ing mode (RAW or JPEG).

There is no need to wait for the next pas­sage of the ISS to test and im­prove your track­ing abil­ity: prac­tise on aero­planes, day or night. At high al­ti­tude, their ap­par­ent speed is sim­i­lar to the space sta­tion’s. If you can, im­age the ISS with a friend: one of you can track it, the other can check the po­si­tion of the ISS on the cam­era or com­puter screen and ad­just the shoot­ing pa­ram­e­ters.

Be­gin with your short­est pos­si­ble fo­cal length; the ex­po­sure time will be shorter and keep­ing the ISS within the sen­sor will be eas­ier. In­crease your fo­cal length with a Barlow lens only once you have re­fined your track­ing skills. As in plan­e­tary imag­ing, the best re­sults will be ob­tained in favourable see­ing con­di­tions: per­se­ver­ance an im­por­tant part of suc­cess­ful ISS imag­ing.

Thierry Le­gault is a world-renowned as­tropho­tog­ra­pher whose pic­tures have been pub­lished and broad­cast all over the world.

Space Shut­tle Dis­cov­ery can be seen docked to the ISS (on the left) in this stereo im­age from 2011

The level of de­tail you can ex­pect to see with 4-inch (top) and 10-inch (bot­tom) tele­scopes

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