These are magnificent shots Ken. Since I'm not a photography expert, is light painting the same as double exposure? You do a long exposure to get the starts and then use a flash for the arches/trees etc?
What happens if a cloud comes by during the longer exposure (if it's really long)
Light painting has several definitions/methods, but in the photos I am doing, I am using handheld light sources to illuminate foreground objects, even super large ones such as these. They are not done in double exposures, although some photographers may blend more than one picture (one with light painting, one without; two with light painting; one with no mistakes in a certain region to mask one that was otherwise good but had a mistake in one region; and on and on).
The handheld light sources I personally use are either very bright LED flashlights, such as a Dorcy or a Streamlight, or speedlights (flash attachment), although I always use the speedlights off the camera, triggering them manually. I did not use a flash for the red Double Arch photo because it didn't have enough reach (power).
If clouds come by during a long exposure, they become part of the photo. Sometimes, many of us use a technique called "stacking", which basically involves shooting photos in succession. So for example, I might shoot 30 photos that are 2-minute exposures each, and then "stack", or blend them together, using a program such as Photoshop or StarStax. If clouds begin entering the frame, I may choose to keep them in or simply stop using the frames when the clouds enter. Or if I am shooting a single long exposure (not "stacking", I may look and see clouds coming in and make a decision as to whether to stop the exposure or simply embrace the clouds and not care. If the clouds are moving relatively quickly, the stars (or star trails) can still shine through.
When I give photography workshops, I will ask the participants to consider the clouds. How are they moving? Which direction? Are they intermittent? All these effect the long exposure, and can make or break a photo. Do you want clouds? How do you want them to look?
The following example is an admittedly surreal photo of a light painted rock (again, using a handheld flashlight with a red gel in front) to illuminate the rock, and this time, embracing the quick moving clouds fully in the hopes that they add to the photo.
And again, because the clouds were moving quickly, the star trails still managed to shine through. It's a chance you take when you are taking the photos, although certainly experience with clouds in long exposure night sky photography help when making this decision, so it's not a complete crap shoot.
The above shot is done using the "stacking" technique for star trails.
The Mt. Pinos photos are a single exposure.
The first shot of Double Arch was an attempt at a single exposure, but I could not pull it off because of how enormous the rock formation was, and complicated the light painting was, so I had to admit this early on and blend some of the light painted rock formation together. It was a complicated shot that took a long time and a lot of work.