The Great Globular in Hercules.
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