Almost all “Reader gallery” pictures seem to be four or more long exposures. Are these pictures combined, and if so, how?

Yes, and in “Reader gallery” captions, we list the type of image and what exposures the imager combined to arrive at the final picture. Take, for example, “LRGB image with exposures of 80, 20, 20, and 30 minutes, respectively.” The published image contains 4 exposures: an 80-minute unfiltered luminance (L) image, 20-minute photos through red (R) and green (G) filters, and a 30-minute exposure through a blue (B) filter.

Depending on which filters you use, only light associated with specific wavelengths — red, green, blue, or even infrared — will hit the photographic plate. When you add the images together, associating each filtered image with either R, G, or B, you get a full-color image.

Some imagers use small, modified video cameras — or webcams — to acquire data. So a caption that reads “best 50 of 600 images” means the imager used software to select the best 50 images out of a continuous batch of 600 obtained with a webcam. One frame differs from another in clarity because our atmosphere is in constant motion.

For LRGB (and similar) images, each exposure is saved as a separate file that software such as Adobe Photoshop can interpret. Software such as Registax combines multiple webcam exposures into a final image. Each imager has his or her own way of processing exposures to highlight different aspects of the photographed object. MICHAEL BAKICH, PHOTO EDITOR