As is usual in imaging systems that employ charge-coupled devices (CCDs) and
analogue-to-digital converters (ADCs), the input to the initial amplification stage of the
latter is offset by a small, constant voltage to prevent thermal noise at low signal levels from
causing wrap-around across zero digitised units. The Gaia CCDs and associated electronic
controllers and amplifiers are described in detail in Kohley et al. (2012). The readout
registers of each Gaia CCD incorporate 14 prescan pixels (i.e., those having no corresponding
columns of pixels in the main light-sensitive array; Figure 1.3). These enable monitoring
of the prescan levels, and the video chain noise fluctuations for zero photo-electric signal,
at a configurable frequency and for configurable across-scan (AC)
hardware sampling. In practice, the acquisition of prescan data is limited to the
standard un-binned (1 pixel AC) and fully binned (2, 10, or 12 pixel AC depending on
instrument and mode) and to a burst of 1024 one-millisecond samples each once every 70 minutes
in order that the volume of prescan data handled on board and telemetred to the ground does not
impact significantly on the science data telemetry budget.
Video chain offset levels and total detection noise
The read noise (or, more correctly, the video chain total detection noise including
noise contributions from CCD readout noise, ADC and quantisation noise, etc.) can be
assessed from the short timescale fluctuations measured in the prescan levels.
Figure 2.4 shows the offset levels and measured total detection noise
for the FPA science devices.
Table 2.1 gives a summary of the required and measured noise properties
of the various instrument video chains. From the sample-to-sample fluctuations measured in the 1 second
prescan bursts, all devices are operating well within the requirements.
All devices are operating nominally as regards their offset and read noise properties.
Table 2.1: Required and measured total detection noise properties for the various Gaia instruments established early on during commissioning. RVS–LR mode is not being used for science data taking. This table appears as Table 1 in Hambly et al. (2018). Measurements are given in analogue–to–digital units (ADU) and electrons using the gain measurements quoted in the second column.
Total detection noise per sample
ADU / e
Required / e
Measured / e
Measured / ADU
The approximately hourly monitoring of the prescan pixels is suitable for
characterising any longer timescale drifts in the offsets characteristics. For example,
Figure 2.5 shows the total video chain detection noise as measured
from prescan fluctuations over an extended period of over 1000 revolutions (corresponding
to more than 250 days). Over this period (July 2014 to May 2015) there is no discernible
degradation in the video chain performance. This remains true up to the day of writing (February 2018).
Figure 2.6 shows the long timescale stability
of one device in the Gaia focal plane. In this case (device AF2 on row 4 of the FPA),
the long term drift over more than 100 days is ADU, apart from the electronic
disturbance near OBMT revolution 1320 (this was caused by payload module heaters
being activated during a ‘de-contamination’ period in September 2014; Table 1.6). The roughly hourly
monitoring of the offsets via the prescan data allows the calibration of the additive
signal bias early in the daily processing chain, including the effects of long timescale
drift and any electronic disturbances of the kind illustrated in
The ground segment receives the bursts of prescan data
for all devices and distils them into ‘bias records’ containing one or more bursts
per device. These provide robustly estimated mean levels along with dispersion statistics for noise performance
monitoring. Spline interpolation amongst these values is used to provide an offset
model at arbitrary times within a processing period. Figure 2.7 shows a detailed
example around the large excursion seen in Figure 2.6.
Figure 2.6 illustrates the small offset difference
between the un-binned and fully binned sample modes for the device in question. In
fact, there are various subtle features in the behaviour of the offsets for each Gaia CCD associated with the operational mode and electronic environment. These manifest
themselves as small (typically a few ADU for non-RVS video chains, but up to
100 ADU in the worst-case RVS devices), very short timescale (10 s)
perturbations to the otherwise highly stable offsets. The features are known
collectively as ‘offset non-uniformities’ and, because the effect presumably originates
somewhere in the CCD–PEM coupling, it is also known as the ‘PEM–CCD offset anomaly’.
The effect requires a separate calibration
process and a correction procedure that involves the on-ground reconstruction of
the readout timing of every sample read by the CCDs since they are a complex function
of the sample readout sequencing. This procedure is beyond the
time-limited resources of the near real-time daily processing chain and is left to the
offline cyclic data reductions at the Data Processing Centres associated with each
of the three main Gaia instruments. For a detailed description of the characteristics
of these non-uniformities along with their calibration and correction, see
Hambly et al. (2018).