Sky at Night Magazine - - THE SKY GUIDE -


We’d rec­om­mend try­ing to achieve an im­age size of at least 45-60 ar­cmin­utes across for the long di­men­sion of the im­age frame. This will give M13 a good scale on frame and at the low­est scale in­clude the two 7th-mag­ni­tude stars that ap­pear ei­ther side of the clus­ter, so pro­vid­ing a sense of scale. Line up on the clus­ter and fo­cus as ac­cu­rately as pos­si­ble.


In­crease the ex­po­sure length to record more de­tail; try to dou­ble M13’s size. Keep stars sharp up to the limit of what your mount al­lows. Take mul­ti­ple im­ages and process into a fi­nal re­sult. Ex­pect the core to over­ex­pose. It’s worth grab­bing more sub-frames for the longer ex­po­sures as this will help re­duce noise in the fi­nal stack­ing process.


Hide all lay­ers ex­cept A and B. On B, draw a se­lec­tion to con­tain the area of over­ex­po­sure. Copy the se­lec­tion and paste it as a new layer mask. Se­lect the layer mask. Ap­ply a fairly ag­gres­sive Gaus­sian blur so the sharp mask edge dis­ap­pears. Avoid in­tro­duc­ing a dark ‘moat’ ring around the core.


The core ex­po­sure can be kept rel­a­tively short. You’re look­ing to in­clude the core out to a fea­ture known as the ‘pro­pel­ler’, a trio of slightly darker lanes that to­gether look like the pro­pel­ler’s blades. Mul­ti­ple shots cal­i­brated and av­er­aged will give the smoothest re­sult. Once col­lected, process your re­sults and put to one side ready for fi­nal assem­bly.


Load your pro­cessed re­sults in or­der of ex­po­sure length as sep­a­rate lay­ers in a layer-based editor, short­est ex­po­sure at the bot­tom. Tweak the Lev­els and Curves for each layer, bring­ing out de­tail with­out adding ex­ces­sive noise. Try to keep the re­sults as nat­u­ral look­ing as pos­si­ble. We’ll re­fer to the short­est ex­po­sure layer as A, next long­est as B, etc.


Once you’re happy with how A and B are work­ing to­gether, du­pli­cate them and merge. If you have other ex­po­sures, make the first one (C) vis­i­ble above the AB-merged re­sult and re­peat the process in Step 5. It’s eas­ier to prac­tice get­ting just two lay­ers right be­fore mov­ing on to a third. At the end you should have a large and ex­ten­sive im­age of M13.

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