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| Table 1: Bad Columns | |||
|---|---|---|---|
| CCD | Type | Position | Detector coordinates (CHIPX, CHIPY) |
| 0 | FI | I0 | 71 72 73 |
| 1 | FI | I1 | 303 304 409 |
| 2 | FI | I2 | ... |
| 3 | FI | I3 | ... |
| 4 | FI | S0 | ... |
| 5 | BI | S1 | 233 |
| 6 | FI | S2 | ... |
| 7 | BI | S3 | [496,441:1024] [497,441:1024] [498,441:1024] |
| 8 | FI | S4 | ... |
| 9 | FI | S5 | ... |
| Table 3: Bad Pixels | |||||||
|---|---|---|---|---|---|---|---|
| CCD | Type | Position | Detector coordinates (CHIPX, CHIPY) | ||||
| 0 | FI | I0 | 303, 253 | ||||
| 1 | FI | I1 |
141, 503 599, 740 303, 253 409, 600 | ||||
| 2 | FI | I2 | ... | ||||
| 3 | FI | I3 | ... | ||||
| 4 | FI | S0 | 513, 14 435, 27 337, 242 | ||||
| 5 | BI | S1 |
| ||||
| 6 | FI | S2 | ... | ||||
| 7 | BI | S3 | 335, 412 | ||||
| 8 | FI | S4 | ... | ||||
| 9 | FI | S5 | 766, 221 | ||||
We have completed a search for non-photometric (i.e. bad) pixels and columns in the ACIS camera. This list should be applied to OAC observations with the ACIS focal plane temperature at T=-100c.
The bad pixels were identified by searching multiple ccd frames (obs_ids) in chip coordinates for pixels consistently above a selected threshold. Five to nine frames per device were usually analyzed, although only one or two frames for chips 0, 1, 4, and 9, were available at the time of this analysis.
Frames used in analysis of each chip:
| Chip | OBSIDs | Chip | OBSIDs |
|---|---|---|---|
| 0 | 62743 | 5 | 1051, 1052, 1055, 1056, 112, 163, 1060, 1061, 1190 |
| 1 | 62743 | 6 | 1051, 1052, 1055, 1056 |
| 2 | 1051, 1052, 1055, 1056, 1058, 163, 164 | 7 | 1051, 1052, 1055, 1056, 163, 164 |
| 3 | 1051, 1052, 1055, 1056, 1058 | 8 | 1055, 1056, 1057, 1058, 1059 |
| 4 | 163, 164 | 9 | 62742 |
Our procedure identified all pixels with pulse heights greater than 2 ADU in each science frame. An auxiliary ccd frame was constructed for each science frame, and those pixels above the 2 ADU clip were assigned the value 1 in each auxiliary frame. The auxiliary frames for each device were then added to form a bad pixel frame. The amplitude of the pixels in the bad pixel frame represents the number of science frames in which a pixel's pulse height was larger than 2 ADUs. A pixel was deemed "bad" and included in our table if it appeared in 50% or more science frames (e.g. for a series of 8 science frames, pixels in the corresponding bad pixel frame with amplitude 4 or larger were included in the table). For device 4, two science frames were analyzed, and we required the presence of bad pixels in both frames. None were found. For devices 0, 1, and 9, with one science frame each, we applied respectively 50, 200, and 200 ADU clips, and required bad pixels to lie above these values.
Bad columns were identified by eye and by searching for peaks in pixel histograms as a function of column number.
Several issues surfaced during the course of this analysis.
1) This list is fairly conservative in that only the most obvious bad pixels and columns are included. A more aggressive approach with a lower clipping threshold would certainly identify additional low amplitude or transient warm pixels. However, our primary interest was identifying nonphotometric pixels at all energies. For some applications, particularly low energy extended imaging or background photometry, observers may want to produce a more extensive bad pixel list.
2) The columns adjacent to the node boundaries appear hot in most images. We initially considered including these columns (256,257,512,513,768,769) in the bad pixel list. However, Bev LaMarr's analysis indicates that although these columns suffer unusually high background, they may otherwise be fine X-ray detectors. It may be possible to remove this background using a combination of grade and spatial filtering. We have therefore excluded these columns from the bad pixel list pending further evaluation. Of course, observers can exclude them at their own discretion.
3) Several "dead" columns were noted in our analysis, or were pointed out to us by M. Markevitch. These columns (in chip 7 for example, and pixels labeled "F" for flight) were included in the flight bad pixel map and appear "dead" in processed images. (Flight bad pixel maps are identified in a memo by Fred Baganoff dated 23 July 1997). The columns adjacent to these bad flight columns often appear hot. However, we have not shown these to be bad X-ray detectors; therefore, we have not included them in this list.
4) The surprisingly large numbers of bad pixels, particularly in the S1 device may be related to the warm focal plane temperature, and we hope the numbers will decrease initially when ACIS is cooled to -120c. Observers may want to consider tailoring their own lists to include noisy pixels and/or columns adjacent to nodes etc. (As Maxim and Allyn Tennant have done). We would also consider publishing an expanded list for optional use upon demand. These issues are open for further discussion prior to publishing our revised lists as we swing into the Science Cal, GTO, and GO phases of the Observatory. At Mark Bautz's suggestion, we will in the future monitor bad pixels in the bias frames.
Thanks to Fred Baganoff, Bev LaMarr, Mark Bautz, and Maxim Markevitch, and Allyn Tennant for their help and advice.
Last modified: 11/15/10
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