Astropho­tog­ra­phy

The Great Glob­u­lar in Her­cules.

Sky at Night Magazine - - CONTENTS -

Deep-sky ob­jects come in all shapes, bright­ness and sizes. How­ever, as far as astropho­tog­ra­phy is con­cerned, they fall into one of two cat­e­gories – dif­fuse and fuzzy, or star-like. The dis­tinc­tion is im­por­tant be­cause it will in­flu­ence how suc­cess­ful you are at tak­ing pho­tographs ac­cord­ing to the qual­ity of your sky.

Light-pol­luted or hazy skies are, un­for­tu­nately, rather good at fil­ter­ing out dif­fuse ob­jects such as neb­u­lae and gal­ax­ies. How­ever, such skies are less ob­struc­tive with stars. Granted the back­ground sky colour may suf­fer, but this can be ad­dressed in post cap­ture pro­cess­ing. The bright, pin-point na­ture of a star will still record through light pol­lu­tion, and this makes ob­jects such as open and glob­u­lar clus­ters much bet­ter tar­gets for low- to medium-qual­ity skies.

The Great Glob­u­lar in Her­cules, M13, falls into this cat­e­gory and is an ex­cel­lent tar­get to ap­proach dur­ing the spring and early sum­mer months. The pe­riod sur­round­ing the north­ern hemi­sphere’s sum­mer sol­stice, which this year oc­curs on 21 June, is renowned for hav­ing skies that never get truly dark. This is es­pe­cially true if you live in the north of the UK. Even if your lo­ca­tion doesn’t ex­pe­ri­ence as­tro­nom­i­cal night, a pe­riod de­fined by the Sun be­ing greater than 18º be­low the hori­zon, you can still im­age M13 as long as your skies reach a deep twi­light.

M13 is listed as hav­ing a mag­ni­tude of +5.8 and, in­deed, un­der clear dark skies it can just be seen with the naked eye. Pho­to­graph it through a tele­scope with a cam­era at­tached and the core will record fairly quickly. Per­haps it’s the eu­pho­ria of achiev­ing any re­sult at all, but many new­com­ers stop when they reach this stage. In re­al­ity, core pho­tos ac­count for around 15-20 per cent of M13’s ap­par­ent di­am­e­ter; with a bit of ad­di­tional work much of its oth­er­wise hid­den charms can be brought back into frame.

The core is rel­a­tively bright and rep­re­sents the part of the glob­u­lar that can be seen with the naked eye. It oc­cu­pies a cir­cle ap­prox­i­mately 5 ar­cmin­utes across. Longer ex­po­sures start to re­veal the fil­i­gree star halo that sur­rounds this re­gion and here is where M13 can re­ally de­liver a sur­prise.

One of the char­ac­ter­is­tics of the area around the clus­ter is two rel­a­tively bright stars ei­ther side of M13. They are 30.3 ar­cmin­utes apart (roughly the same size as the ap­par­ent di­am­e­ter of the full Moon). In­crease the ex­po­sure time to in­clude the faint out­liers and you’ll dis­cover that M13 vir­tu­ally fills this space.

How­ever, here lies the prob­lem, be­cause an ex­po­sure which is long enough to re­veal the fainter outer re­gions of M13 will un­doubt­edly over­ex­pose the core. Here we need to cre­ate a high dy­namic range im­age com­pos­ite of the clus­ter, show­ing the core and out­ly­ing re­gions off as a sin­gle ob­ject.

To do this you need to cap­ture an im­age of the core and an­other that de­lib­er­ately over­ex­poses this re­gion to record the fainter outer stars. Us­ing a layer-based im­age editor, you can com­bine these im­ages us­ing a layer mask to cre­ate an im­age of M13 which shows the ob­ject in its full nat­u­ral glory.

M13’s true size only be­comes ap­par­ent in high dy­namic range shots; ex­pos­ing for the core only (in­set) pre­vents you from cap­tur­ing some of its far­ther reaches

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