Space Telescope Science Institute   4.5.2 MAMA Overflow of the 16 Bit Buffer  4.5.4 SBC Signal-to-Noise Ratio Limitations

4.5.3 MAMA Darks


MAMA detectors have intrinsically low dark currents. Ground test measurements of the ACS MAMA showed count rates in the range of 10-5 to 10-4 counts per pixel per second as the temperature varied from 28 ºC to 35 ºC degrees. The count rate increased by about 30% for one degree increase in temperature. In-flight measurements, taken weekly throughout June and July 2002, show count rates between 8x10-6 and 10-5. These measurements were taken as soon as the MAMA was turned on and were therefore at the lower end of the temperature range. A 10 hour observation in SMOV, long enough for nominal temperatures to be reached, yielded a dark current of 1.2 x 10-5 counts per second per pixel. Monthly monitoring shows the in-flight dark current to be about 9x10-6 counts per second per pixel. For typical SBC operations in which the detector is turned on for less than two hours, a dark image collected at lower temperatures is more suitable and replaces the current calibration image. This has a mean dark rate of 10-5 counts per second per pixel.

The ACS MAMA has a broken anode which disables rows 599 to 605. There are three dark spots smaller than 50 microns at positions (334,977), (578,964), and (960,851), as well as two bright spots at (55,281) and (645,102) with fluctuating rates that are always less than 3 counts per second. The reference pixel has been moved to (512,400) to avoid these areas (see Table 7.9)

An example of the dark current variation across the detector can be seen in Figure 4.16 below.

Figure 4.16: MAMA dark image.

 

 4.5.2 MAMA Overflow of the 16 Bit Buffer  4.5.4 SBC Signal-to-Noise Ratio Limitations
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