STEP BY STEP
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.