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New NOTCam science array commissioning

(13 Dec-2007)

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 detector.

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 readout modes.

Linearity

The new science array starts to saturate around 56000 adu. The 1% deviation from linearity starts at ~20000 adu in the worst quadrant.

Cosmetics

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.

Dark level

The behaviour of the dark level with time is similar as for the engineering grade array, and still very poorly understood.

Flat field

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 corner.

Memory effect

Data was obtained to study the memory effect (or charge persistency) of this array.

Quantum efficiency

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.

Alignment

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:

  1. Sensitivity must be found from standard stars, and the SIGNAL code (exp time calculator) must be updated.
  2. We still need to understand the origin of the strange dark level behaviour.


Comments to Anlaug Amanda Djupvik

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