How to cap­ture the en­tire majesty of the night sky us­ing mul­ti­ple shots stitched to­gether.

Sky at Night Magazine - - CONTENTS -

Cre­at­ing a panorama can be a bit of an art. Many as­pects need to be con­sid­ered, and in this ar­ti­cle we’re go­ing to ex­plain how to cap­ture the images to form a 360° pic­ture, as well as the equip­ment needed to do it. You’ll find it a very re­ward­ing ex­pe­ri­ence once you start to see the im­age come to­gether.

So what is a panorama? Put sim­ply, it’s a 360° pic­ture made up of mul­ti­ple images taken from one lo­ca­tion. They can’t be any old images, though: to get the best re­sults you need to take non-par­al­lax images, where there’s no per­spec­tive change as your cam­era is moved. It’s also best not to spend more than 20 min­utes tak­ing the images that will be com­bined: any longer than that and the Earth’s ro­ta­tion will cause the stars to shift po­si­tions. To min­imise this star shift, it’s also bet­ter to shoot in rows first, then columns so you can pho­to­graph the sky faster. You also want the images to over­lap so that they can be eas­ily com­bined later.

Lens choice plays an im­por­tant role in the cap­ture tech­nique. I typ­i­cally like to work at f/2.8; it lets plenty of light into the im­age but also al­lows for a

pleas­ing depth of field. The other ad­van­tage of us­ing a wide aper­ture is that as the cam­era moves, the edge of one im­age can be the cen­tre of the next. Wide aper­tures will gen­er­ally pro­duce more vi­gnetting, how­ever, and while this can be cor­rected after cap­ture, there’s a risk of noise en­ter­ing the im­age and cre­at­ing a ‘blotchy’ fi­nal pic­ture. I’ve shot at f/2.0 but wouldn’t rec­om­mend go­ing wider than that, as coma (blur­ring) and other lens aber­ra­tions can be­come ev­i­dent at the edges of an im­age.

Ex­po­sure time is cru­cial too and the ‘500 Rule’ is in­valu­able here. Ac­cord­ing to this, you di­vide 500 by the cam­era’s fo­cal length to get the num­ber of sec­onds you can ex­pose for be­fore star trail­ing be­comes an is­sue. So with a 24mm lens, 500÷24mm = 20.8 sec­onds, which means the stars will start to trail with any ex­po­sures longer than that. Most cam­eras have a long-ex­po­sure noise-re­duc­tion op­tion; this makes the cam­era take a sec­ond im­age im­me­di­ately after the first to record the noise of the sen­sor. The cam­era will then au­to­mat­i­cally sub­tract this noise from the photo, ef­fec­tively dou­bling the ex­po­sure time. If ex­po­sure time is an is­sue or your cam­era doesn’t have this func­tion, it’s pos­si­ble to take a sin­gle dark frame im­age with the lens cap on and the eyepiece cov­ered, which can then be sub­tracted from the photo man­u­ally. Do­ing this can be very time con­sum­ing, though and in re­al­ity, it doesn’t save as much time as mov­ing the cam­era to the next po­si­tion while the dark frame is ex­pos­ing.

The the­ory

As pre­vi­ously men­tioned, the images need to over­lap. Gen­er­ally I have a 30 per cent over­lap, which makes stitch­ing the frames to­gether rel­a­tively easy. It also means the high­est res­o­lu­tion part of the lens is used to form the ma­jor­ity of the com­pleted im­age and if there’s a bad frame you can al­most com­pletely re­move it by mask­ing in the neigh­bour­ing frames.

Armed with the fo­cal length and over­lap fac­tor, you can cal­cu­late how many de­grees to turn the cam­era. The first step to do­ing that is to cal­cu­late the field of view us­ing the fol­low­ing for­mula: In this, _ is the an­gle of the field of view, d is the sen­sor size (re­mem­ber to cal­cu­late hor­i­zon­tal and ver­ti­cal sep­a­rately) and f is the fo­cal length of the lens. The ta­ble (above right) shows the field of view that dif­fer­ent fo­cal lengths pro­vide with a 35mm sen­sor, cal­cu­lated us­ing the for­mula. Fi­nally, to cal­cu­late the an­gle to turn the cam­era be­tween shots, you’ll need to take the amount of each im­age that isn’t an over­lap and mul­ti­ply that by the field of view. For a 24mm lens with a 30 per cent over­lap,

the amount of each im­age that isn’t over­lapped is 70 per cent, so (1 – 0.3) x 74 = 51.8° for the hor­i­zon­tal, and that (1 – 0.3) x 53 = 37.1° for the ver­ti­cal.

To make sure that you can cap­ture all your images within 20 min­utes, you now need to cal­cu­late the num­ber of images in rows and columns. If you have an au­to­mated panoramic head, your cam­era will be in land­scape mode, so: 360÷51.8 = 7 frames for the hor­i­zon­tal. Then for the ver­ti­cal, it’s: 180÷37.1 = 4.8 frames, which you can round up to five. That’s a to­tal of 7 x 5 frames, which is 35 pho­to­graphs. Us­ing the 500 Rule you can work out you want a 20-sec­ond ex­po­sure from the 24mm

Con­struct­ing a panoramic shot lets you see all the vis­i­ble ob­jects in the sky in re­la­tion to each other

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