Removing a moiré in Photoshop

Moiré is caused when two patterns with different frequencies are overlaid on each other. The original halftone screen pattern used a series of repeating lines, probably somewhere between 85 and 133 lines per inch. When you scan a halftone, a second pattern results when the image is displayed on your computer screen (at, say, 72 to 120 “lines” per inch) or printed again (at 300 to 600 dots per inch). The two patterns interfere with each other at regular intervals, producing the unpleasant moiré look.