Image 5a. Average 120 s dark (average bias subtracted, ak06 seq. 120-124, CCD-TEMP=-40) |
Image 5b. Average 60 s dark (average bias subtracted, ak06 seq. 125-129, CCD-TEMP=-41) |
Image 5c. Average 15 s dark (average bias subtracted, ak06 seq. 130-134, CCD-TEMP=-41) |
Image 6a. ak04 dark. (2003 Jan 13, T=-40) |
Image 6b. ak05 dark. (2003 Jan 14, T=-40) |
Image 6c. ak06 dark. (2003 Jan 21, T=-41) |
Image 6d. ak07 dark. (2003 Jan 22, T=-41) |
Image 6e. ak08 dark. (2003 Feb 26, T=-40) |
Image 7a. ak04 dark divided by ak05 dark. |
Image 7b. ak04 dark divided by ak06 dark. The fuzzy dark streaks on this image correspond to the positions of the brightest stars in the ak06 field, and reveal that "ghost" remnants of the star images from the ak06 field persisted on the chip after readout and show up in the dark frames. This is quite an unpleasant surprise because none of these stars were blooming, and only two (the uppermost streaks) were even saturated. (The shape is a streak rather than a spot because imperfect telescope tracking caused the images of the stars to drift slowly as they were imaged repeatedly during the 90 minutes prior to recording the dark frames.) |
Image 7c. ak05 dark divided by ak06 dark. |
Image 7d. ak07 dark divided by ak06 dark. Light streaks corresponding to persistence of bright stars in the ak07 field, none of which were even saturated, join the dark smears from ak06. (Since ak07 was divided by ak06 the ak07 persistence is bright and the ak06 persistence is dark.) |
Image 8a. Difference between two consecutive 120-s science images (ak06 seq. 118 and 119) from which the average bias frame (but not the dark) was previously subtracted. The arcs don't appear in the difference frame, which confirms that they didn't change between the two science frames. |
Image 8b. Difference between two consecutive 120-s dark images (ak06 seq. 120 and 121) from which the average bias frame was previously subtracted. These darks were recorded immediately after the science images differenced for image 8a. The arcs don't appear in the difference frame, which confirms that they didn't change between the two dark frames. |
Image 8c. Difference between one of the preceding 120-s science images (seq. 119) and the immediately following 120-s dark image (seq. 120). The average bias frame was previously subtracted from both images. The arcs do appear in this difference frame, which demonstrates that the arcs did change between seq. 119 and seq. 120. Seq. 120 was started only 18 seconds after Seq. 119 finished, and the telescope wasn't moved between these two images. This is consistent with our experience that the arcs do not subtract out in calibration with the rest of the dark signal. The arc contrast is low because the image is unflattened; flattening this image gives the same result shown as sample image (4a). We note that the image display contrast of the sky background is much higher in sample images (8a) and (8b), and in those images there's no hint of the arcs at all. |
| Last modified 2003 Apr 12 |