The following gain and read-noise values for the VATT4K CCD were provided by Rolf Jansen and are based on data obtained during several observing runs from Dec 2008 through Dec 2011. They were derived using Janesick's method from clean image sections within pairs of bias frames and dome flat frames at different exposure levels. The analysis provided here supersedes those from May 2008 and Mar 2010.
These results can be summarized to 0-th order as:
| Amplifier 3 | Amplifier 4 | ||
| gain [e-/ADU] | readnoise [e-] | gain [e-/ADU] | readnoise [e-] |
| 1.730 ± 0.020 | 5.080 ± 0.124 | 1.790 ± 0.017 | 5.130 ± 0.119 |
| The gain values of the 2 amplifiers are matched to 1 : 1.035 | |||
The effective gain, G, shows a smooth dependence on the bulk (mean) signal level, I, measured within a frame. To 1-st order, this dependence may be expressed as:
| Amplifier 3 | G = (1.7825 ± 0.0005) − I · (1.454 ± 0.014)×10−6 [e-/ADU] |
| Amplifier 4 | G = (1.8312 ± 0.0006) − I · (1.333 ± 0.017)×10−6 [e-/ADU] |
That dependence is, however, more accurately approximated with a 2nd-order polynomial as function of a reduced signal level I' = (I − 32750.) [ADU], as:
| Amplifier 3 | G = (1.7322 ± 0.0003) − I' · (1.505 ± 0.014)×10−6 + I'2 · (1.37 ± 0.08)×10−11 [e-/ADU] |
| Amplifier 4 | G = (1.7831 ± 0.0003) − I' · (1.366 ± 0.015)×10−6 + I'2 · (2.32 ± 0.09)×10−11 [e-/ADU] |
During the November and December 2011 observing runs (but also during the December 2008 one) the read-noise, R, shows a variable excess noise component that is likely related to the occassional "striping" patterns visible after read-out. Even when such patterns are not evident in the raw data, there is still some excess noise observed over the nominal read-noise.
This striping problem was solved at the end of Dec
2008 by swapping out the fiber-optic pair connection from camera to
computer (R. Boyle, Dec 22 2008). It may be time to take a similar
measure. None of the excesss noise was present during the February 2009
and March 2010 runs (representing ∼2/3 of the smal grey data points
in the graph above).
Last updated: Dec 14, 2011 [RAJ]