There are two excellent methods for this.
1) Select the area around the arcs widely with either the marquee or the lasso. Then use Select by Color range. You will get what you get. Tune the "Fuzziness" slider to get more or less. It is likely you will not get everything. Use a Wacom tablet and a brush to paint in the rest of the mask.
2) Use the tonality you have. Command-click on the Grey layer in the Channels palette. This will load up the image as a selection. Immediately create a Curve Adjustment layer and you will see the mask. Option Click on the mask to see it in the "black and white" mode. Pull up a curves dialog (not adjustment layer) and increase the contrast of this layer to separate your arcs. (Don't overdo this.) Then use a brush to paint in black everything you won't want, or grab large parts of the image with a lasso and fill it. However, don't let the lasso anywhere near your edges. This is technically called a luminosity mask but its just a selection based on the tonal values of the original.
The hard edge techniques like the lasso will make things look like a cutout. Edge refinement certainly helps, but there is nothing like a brush to refine an edge. Especially one that is pressure sensitive and can spray like an airbrush when you want it to...
I switch back and forth from modifying the "black and white" view of the selection to the marching ants to the Quick Mask and paint what I need. It's very efficient once you get a handle on it.
Also, bear in mind that you can add and subtract masks from each other. That's where it gets really interesting....
Have fun,
Lenny
