Turn your camera towards Perseus to find the bejewelled handle of The Hero’s sword

Perseus rides high across the night sky at this time of year. The constellation represents the mythological Greek hero wielding his sword, the handle of which is marked by a rather special pair of deep-sky objects: two open clusters, which together are known as the Double Cluster.

The clusters are visible to the naked eye under dark skies and burst into life through binoculars or a small telescope. They’re individually identified as h Persei and Chi (χ) Persei, or NGC 869 and NGC 884 respectively, and have a number of similarities. They shine with an integrated magnitude of +4.0, are similarly aged at 12.8 million years and are of similar distance: a 2010 study put the clusters at around 7,600 lightyears, although more recent research suggests the clusters lie about 8,100 lightyears from Earth.

Both also contain many massive stars. Within 10 arcminutes of their centres, they contain around 5,000 stars. An extensive outer star halo bumps the figure up to at least 13,000 and possibly as high as 20,000. Visually and photographically the number of stars seen is far lower than these figures suggest, but both remain very rich targets for astrophotography.

Easy to find, easy to see

The Double Cluster is easy to find. First identify the W-shaped constellation of Cassiopeia, which passes virtually overhead close to midnight BST during October. Extending the line from Gamma (γ) Cassiopeiae through Delta (δ) Cassiopeiae for twice the distance again will bring you to the Double Cluster.

A benefit of choosing stars as a target, as opposed to a diffuse, extended object such as a nebula, is that they have a resilience against light pollution. Light pollution may affect the colour of the surrounding sky, but stars are able to punch through.

Photographically, h Persei and Chi Persei make excellent targets for DSLR cameras. A field width of 2° gives good coverage and this is easily achieved using a focal length of 1,000mm with a full-frame camera, or 600mm for an APS-C format sensor. The clusters sit against a portion of sky through which the Milky Way passes. As a consequence, a wider field also works very well and helps to put the pair nicely in context with their surroundings.

Both clusters contain a good number of blue-white supergiant stars. The eastern cluster, Chi Persei, contains five distinctive red supergiant stars that give excellent colour contrast with the other members. When photographing both clusters close up, it’s worth adjusting your camera settings to really bring out these rich colours. Follow the steps opposite and see whether you can bag yourself a stunning photograph of this beautiful deep-sky duo.

Recommended equipment: a DSLR camera or equivalent, telescope or lens (see Step 1), polar-aligned tracking mount

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Step by Step

Step 1

Choose a lens appropriate to the view of the Double Cluster you want to portray. A 35mm format sensor camera coupled to a telescope with a 1,000mm focal length will cover an area 2° by 1.3° giving you a good close-up on the pair. If using a smaller APS-C sensor, a 600mm focal length achieves a similar field of view.

Step 2

A wider view of the Double Cluster field will be less demanding on tracking accuracy and potentially show the area of Milky Way where they sit. A 35mm lens with an APS-C sensor or 60mm lens with a 35mm sensor will give enough of a field of view to include the whole of Cassiopeia as well as the Double Cluster.

Step 3

A tracking mount will give the best results. A polar-aligned tracker can be used for cameras with lenses attached. If you’re attaching your camera to a telescope, an equatorial mount will help carry the extra weight. If you have this setup already, you could piggy-back a camera with a lens fitted on the telescope.

Step 4

A low ISO (200–400) will capture the Double Cluster’s colours in the best way. Increasing the ISO gives an apparent increase in sensitivity but the higher you go, the more noise appears and the colours will start to wash out. Ensure the mount’s polar alignment is as accurate as possible so you can take longer exposures.

Step 5

Open the lens wide, perhaps closing by a stop or two if you see frame edge distortion. Set focus to manual, centre the Double Cluster and use ‘Live View’ focus assist to focus as accurately as possible. If you can’t see stars, try setting the ISO higher. Using maximum zoom, adjust focus so the stars are crisp and sharp.

Step 6

Set your exposure time for 30 seconds and take several shots. If you have a shutter-release cable, set the camera to ‘Bulb’ and take longer shots. Basic tracking mounts are limited to around 90 seconds for a close-up shot. The sweet spot is when you can use the longest exposure without signs of star trailing.

Send your images to: gallery@skyatnightmagazine.com

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ALL PICTURES: PETE LAWRENCE