Asteroid Zoo Talk

Black Artifacts

  • by

    I just started doing this want to make sure I'm understanding correctly. When you see black objects that radically move from frame to frame is that considered an Artifact? What causes this to happen?



  • AstroTinker by AstroTinker in response to's comment.

    Yes, Bill, these are the type of Artifact known as a Flat-field Defect ( tag= #ffd ). The apparent motion relative to the background of stars is due to camera motion between frames. The CSS image sets are roughly 30 sec exposures, about 10 min apart. . . . Imagine taking a panoramic series of photos of a mural on a building, and want to show the shadows moving across it every 10 min. So you take one panoramic set, move back to the start, take another set, back, another set, back, and the last set. Each time you go back to the start, your camera does not get back to exactly the same place, so the frame is shifted slightly between each different set. . . . When you match up the images exactly, the outside frame is just a little off in each one. If there was a spot on the lens, the spot would also be in just a little different position relative to the background of the image. . . . Why is it a relatively regular pattern (inverted T, inverted Y, or something similar)? . . Realize the stars are 'moving' across the sky relative to the horizon during this entire process, and the automated mechanical gearing of the telescope has some digital or 'stepping' component of motion. Thus, when the telescope shifts back to the starting point, it may be 'between' the 'steps' in either the x or y dimension, and while getting as close as can be calculated, is still just a touch off. Regular repeated mechanical motion yields repeated regular patterns from one frame to the next in our 4-image sets. . . . You can see the same pattern not only with all types of ffd's (black, white, fuzzy, points, hatted, sharper hazes, etc.), but also with bad borders, hot-pixel, hot-pixel-columns, and even in some lens flares. . . . . . All of these are Artifacts of the image taking process, not something in space. A cosmic-ray (bright pixel in one frame only) or star-bleed (vertical and horizontal 'rays' extending from a bright star)are also something just on the telescope imager, not in space. Image Artifacts in space(or atmosphere) include anything not a star or asteroid: a streak(aka fast moving object or #fmo) could be a meteor, satellite, or airplane; galaxy, globular cluster, comet(very rare)