2017 Eclipse Photo Processing

I photographed the total eclipse with a 300mm lens, using a Nikon D90 mounted on my telescope to allow tracking the sun.  The D90 has a 1/3 size sensor, giving a 450mm effective focal length.  Here's our setup on the morning of the eclipse.

To capture all the scene in totality (chromosphere, prominences, inner and outer corona) I captured exposures at shutter speeds from 1/1000 to 2 seconds, giving this set of images.

The fast shutter speeds show the inner features, like this image at 1/125 second:

The slow shutter speeds show the outer corona, but block out the inner features.

These can be layered together in photoshop.  The top layer is the slowest shutter speed with a mask applied to the overexposed portion, like this:

The next layers have a smaller overexposed portion and nest inside the mask for the layer above, like this:

This brings in areas closer to the sun disc, but the mask edges are visible.  To fix this, the edges need to be blended to create a seamless layering.  Instead of creating a sharp-edged mask, I followed the process used to cut out a person from a background, including the difficult areas, like hair.  Here's the process:

• Open the set of files with each image on it's own layer, the slowest exposure on top.
• Select a layer and go to Channels.  Select only one of the color channels (I used red). 
• Make a duplicate of the red channel (drag top the new channel icon at the bottom of the channel window) and rename so you can keep track of the layer it goes with.
• Command-L to bring up the levels window and adjust the black input level to shrink down the perimeter of the corona to just a little outside the overexposed area.
• Command-select the channel and the light areas will be selected.
• Go back to layers (the selection will still be active) and create a layer mask from the selection.
• Option select the new layer mask to view it in black/white. Invert so the mask shows the outer fringes and lets lowers layers show through the middle.  Use the brush tool and paint the disc of the sun black to create a mask that looks like this:

• Repeat this for the layers below, which should yield ever smaller masked areas.
• Stacked up, the fuzzy edges of the masks reduce, but don't totally eliminate the rings of the layers from showing. To further blend, feather the masks by 5-10 pixels.