New NOTCam science array commissioning
A new NOTCam science array arrived at the NOT in October 2006
after the unexpected death of the first Science Array in
Apr-06. It has been stored in dry and safe conditions while
waiting for an upgrade of the electronic setup of the NOTCam.
On the 7th of December the array was installed by Graham Cox
according to the
procedure documented here .
The first warm test images showed that we had contact with all
four quadrants of the array. NOTCam was closed and pumped on 7/12,
transported to the dome on the 10/12 and reconnected to the pump
for one more day of pumping. On the 11/12 the cooling process started.
Because we currently have no PTR (the rotary valve is in Japan
for maintenance), NOTCam was cooled down directly with LN2. In order
to stay well within the cooling rate recommended by Rockwell (below
1 degree Kelvin per minute), we made a controlled and slow filling
of LN2 and had a maximum cooling rate of 0.7 degree per minute for
The first cold tests were made as soon as NOTCam was cold on the 13th.
Because of the new electronic setup we can now adjust the biasgate
voltages, and selected to use the maximum values after having tested
this with the engineering grade array. We finetuned the voltages for
the dc offset in order to find a good compromize between having few
zero pixels and a large linear range. The values needed to get a reset
level of ~ 5000 ADUs are stored in the script "volt_newpcb_max.script".
We refer to the
New Science Array page for pictures and further information.
Here follows a short summary of the commissioning results.
Read noise and gain
The read noise was found to be ~ 10 electrons and the gain is
2.5 e/adu. These values are highly consistent between the two
The new science array starts to saturate around 56000 adu.
The 1% deviation from linearity starts at ~20000 adu in the
The science array looks much better cosmetically than the
engineering grade array. The number of bad pixels is about
1% alltogether, and most of the are located in the upper left
corner and in a feature of maximum extension about 20x40
pixels and centred at x=150,y=510.
The behaviour of the dark level with time is similar as for the
engineering grade array, and still very poorly understood.
The detector flat field is flatter than before. For Ks filter
differential twilight flats the stddev in 20x20 pixel boxes is
< 1% and the deviation across the whole array +-2-3%.
For the engineering grade array the Ks sky flats had ~3-4%
stddev in 20x20 pixel boxes and a deviation across the whole
array of +20% (in the bright band) and about -15% in the darkest
Data was obtained to study the memory effect (or charge persistency)
of this array.
The sensitivity of the new array was quickly tested using
calibration lamps. If we can rely on the stability of these, it
seems the array is about 25% more sensitive than the previous
Science Array and 70% more sensitive than the Engingeering Grade
Array in the H band (at 1.6 micron). This must be checked with
standard stars. Data has been obtained.
The alignment of the array looks OK. Internal focus has changed by
only 50 units, and the array is 0.8 degrees rotated with respect to
the Engingeering Grade Array. This means that while we used to have
the sky orientations rotated by 0.9 degrees we now have them almost
aligned with the detectory XY, just 0.12 degrees rotated. Judging
from the calibration arc line widths there is no serious tilt of the
array: the fwhm of the lines vary by 0.7 pix over the array in the
y-direction and by 0.2 pix in the x-direction (i.e. along slit). These
are the same values as for both the old arrays.
Still to be done:
- Sensitivity must be found from standard stars, and the SIGNAL
code (exp time calculator) must be updated.
- We still need to understand the origin of the strange dark level
Comments to Anlaug Amanda Djupvik