First NOTCam Science Grade Array (SWIR2)
Content of this page:
The Science Grade Array was installed in NOTCam on the 20th of
October 2005 and the first cold tests made 25/10 (see the
Science Array Commissioning report ). All NOTCam data
taken before this date has been using the
Engineering Grade Array , for which we refer to that web page.
Also, the Engineering Grade Array was re-installed in NOTCam on the
5th of May 2006 upon malfunctioning of the Science Array.
NOTCam is offered with two possible readout modes: the standard
reset-read-read mode and a ramp-sampling mode
(multiple non-destructive reads during the integration). Read more
about this in NOTCam User's Guide.
Fig 1: Schematic drawing of the detector with the four quadrants and
their numbering marked. The arrows show the direction of the fast
readout and the corners in which it starts. All four quadrants are
read out simultaneously. The lower left corner of quadrant #1 is the
location of pixel (x=1,y=1). Note that this is valid for the image as
displayed on BIAS DS9, while the stored images are flipped in X since
Cosmetics - bad pixels
Fig 2: Two dark images obtained with the reset-read-read mode
(command dark t). Integration time (t) is 0 seconds
(left) and 100 seconds (right). Note the amplifier glow and the higher
number of hot pixels on the 100s long dark integration.
The hot rows seen in the darks of the science
array are differently placed
than for the engineering grade array. But they are stable and subtract out
well. Since they are fixed at a given y-value and equal for each quadrant
they are probably intrinsic to the array.
There is a higher brightness in the first few
rows in each quadrant where
the count level can be more than 3 times the average level in the remaining
part of the quadrant. This seems to subtract out well, although the it
may take time to stabilize sufficiently that the subtraction is perfect.
In dark images of longer integration times there is clear evidence of
amplifier glow along the edges. This effect
also apparently subtracts out well.
There are two dead columns [1,*] and [513,*]
along the whole detector, contributing with 0.2 % zero pixels. This
is inherent to the controller which has problems with the first column
of each quadrant. (Note that since Jan-06 the stored data are flipped
in X, and therefore the dead columns are now [1024,*] and [512,*].)
Pixels are called bad when they deviate by more than 8 sigma from the
mean level. Among the bad pixels we distinguish
between hot and cold. The cold pixels include
also the zero pixels which show no response at all. The
hot pixels may be strongly non-linear. In dark images the amount
of hot pixels is 1.4 % (on 42s darks), while in well exposed images the
number of hot pixels is < 0.2 %. (Note that this high percentage for
the darks is NOT
due to the wrap-around effect of very low value pixels in the darks. That
effect amounts to only 0.1 % in the cases that have been tested. It is
more likely that the hot pixels drown in the higher noise of the exposed
dome flats.) The majority of the hot pixels are found close to the edges.
The number of hot pixels increases with exposure time. The table below is
taken from the Spectroscopic mode commisioning report which has more
information also on clusters of hot pixels.
Percentage of hot pixels (i.e. more than 8 sigma above
background) in the central 250x250 pixels of the top-left quadrant
| Exptime [s]|| 0
|| 3 || 9
|| 27|| 81
| Hot pixels [%] || 0.1
|| 0.2 || 0.3
|| 0.6 || 1.0
|| 1.8 || 2.5 |
There is a small bad pixel group (mostly hot
and non-linear) of about 8 x 9 pixels large, centred at [784,268], or
in images taken after Jan-2006 at [240,268].
The majority of the individual bad pixels
(hot and cold) are found along edges and corners.
There is a region inside the array at [524:674,800:900] which also has an
elevated number of individual bad pixels.
There are also a few individual bad pixels spread over the whole array.
The number of dead pixels was believed to increase with every thermal cycle of the array. This is because of the
different thermal expansion
properties of the layers of an infrared array, which may cause
detachment of the bump bonds upon repeated thermal cycles. However,
for the engineering grade array which has undergone a number of thermal
cycles during the more than 4 years it has been inside NOTCam, we did
not see any increase in the number of zero-level pixels. The apparent
coming and going of zero-level pixels was found to be due to a drift
in the array causing the reset level to vary. This effect is described
under the engineering grade array .
The NOTCam detector quality control is monitoring any change in the
number of bad pixels:
The behaviour of the dark level with exp time is not well understood.
The readout noise is calculated in a representative area within each
The readout noise in [e-].
|| Readout mode
|| Quad 0
|| Quad 1
|| Quad 2
|| Quad 3
Please, check the NOTCam User's Guide
for a description of the two different readout modes available with
The gain in [e-/ADU].
|| Readout mode
|| Quad 0
|| Quad 1
|| Quad 2
|| Quad 3
The gain is calculated in a representative area in each quadrant.
Non-linearity is an inherent feature of infrared arrays which
distinguishes them clearly from the linear CCDs. While the saturation
of the detector starts at 54000 ADUs the array is found to be linear
to 1% accuracy up to about 32000 ADU on the average.
For each readout mode you can check the non-linear behaviour for each
of the four quadrants from the monitoring data:
Detector flat field
Fig 6: Processed flat field obtained from 10 differential dome flats
taken with the WF camera through the Ks band. The differential method
(pair-wise subtraction of "lamp on" minus "lamp off") is used to
eliminate the thermal contribution from the master flat. No bad pixel
correction was attempted, instead the final master flat was median
smothed by a 3 pixel box, which almost eliminates the bad columns.
The detector flat field looks relatively flat and has few disturbing
features. The figure above shows the master flat obtained from 10
dome lamp ON images minus 10 dome lamp OFF images for the WF camera
and the Ks filter. The standard deviation in small boxes of 20 x 20
pixels is about 2%. The deviation over the whole field is ± 5%.
These numbers are the same for the HR camera Ks flat. A worst case
diagonal cut through the flat field above is shown
here (eps file). The above
detector flat field can be compared to the data sheet with the
QE image that came with the array
(flip in y-direction).
Memory effect (charge persistency)
For non-saturated pixels there is memory (or charge persistency)
only in the first of the subsequent exposures. The memory effect
is negative and the level is 1 % or less.
For saturated pixels the memory is more persistent. It is negative
in the first subsequent exposure, but positive thereafter. The level
is 0.2 % in the 2nd exposure and 0.03 % in the 6th exposure.
If you can not avoid saturation, it is recommended to clean the array
with a couple of dark 0 commands between each science exposure.
The behaviour of the memory effect is illustrated by a series of
images shown below.