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Commissioning of the NOTCam science array

(20-31 Oct-2005)

The NOTCam science array, which has been stored in a bank box in Santa Cruz for 4 years, was installed in NOTCam on October the 20th. The installation was made with the assistance from Preben Nørregaard from CUO, and the procedure is now documented here for future reference.

The first warm tests on the 20/10 showed that we had contact with all four quadrants of the array, and that we could read the detector. Much more could not be tested warm. NOTCam was closed and set to pumping, moved to the dome, more pumping, and finally cooled down according to normal procedures. NB! The array controller was powered off during cooling down!

The first cold tests were made as soon as NOTCam was cold on the 25th. The new array has much better cosmetics than the engineering grade array. Some time was spent adjusting the bias and reset voltages for each quadrant to make sure we have the reset level within an optimum range. If it is too low, we get more zero level pixels, and if it is too high we get a small dynamic range before entering non-linear regime. We selected to use a level of ~ 5000 ADUs.

NOTCam was mounted on the telescope on the 26th, but the weather did not allow opening the dome. We got the data stream for the quality control observations of darks and domeflats. Because of the bad weather and the presence of our detector experts we took the opportunity to study the strange darks more in depth.

We refer to the NOTCam 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 14-15 electrons, a bit higher than the earlier 10-12 electrons. The gain is 2.2e/adu compared to earlier 2.8e/adu.

Linearity

The new array starts to saturate around 54000 adu (before: ~ 40000 adu). The 1% deviation from linearity starts at ~32000 adu (before at 16000 adu). Taking the different gain into account this gives a larger linear regime range than for the old array.

Cosmetics

The science array looks much better cosmetically than the engineering grade array. The number of cold and zero pixels is about 1% alltogether. The number of hot pixels ~1.4% for long dark exposures, but < 0.2% for normal exposures. The bad pixels are practically all located along the edges, but there is one region inside that has an elevated number of individual bad pixels.

Dark level

The behaviour of the dark level with time is similar as for the engineering grade array, but still very poorly understood. Firstly it is too high, and secondly it is highly non-linear and variable. For the engineering grade array it was: ~ 7-9 e/s/pix for short darks, but much smaller (< 1 e/s/pix) for longer darks taken with the reset-read-read mode. For the science array it seems to vary from 2-9 e/s/pix for the ramp-sampling mode, while it is 2-3 e/s/pix for short exposures in reset-read-read mode and < 1 e/s/pix for longer exposures in this mode. The offset level is variable, perhaps depending on the time since last reset. The problem is briefly described under the engineering grader array and the recommendation given in the NOTCam User's Guide was not to do separate dark subtraction. Earlier a dependency of 400 adu per degree change of the detector temperature was found, but the detector temperature is more stable since Apr-03 when we started using the PTR also when mounted on the telescope - although the temperature we actually monitor is at the location of the array only, so we do not monitor the internal array temperature as it might vary depending on its active or dormant state. We suspect the dark level behaviour might be related to the way the on-chip FET has been disabled, and investigations are on-going.

Flat field

The detector flat field is flatter than before. For Ks filter dome flats (lamp ON-OFF) the stddev in 20x20 pixel boxes is ~2% and the deviation across the whole array +-5% for both the HR and WF camera. 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

The memory (or charge persistency) effect is 1% or less, and goes away after one dummy read (e.g. dark 0) for non-saturation. For levels above saturation, however, it is more persistent and seems to produce transient changes of the gain (~ 50% lower) for the affected pixels. The gain recovers after one dummy read, but the memory persists as a positive signal after 6 dummy reads, although at a low level of 0.03%.

Quantum efficiency

The sensitivity of the new array has been tested on calibration lamps only so far. If we can rely on the stability of these, it seems the array is at least 30% more sensitive than the previous one. The old array has a QE of about 55%, so 30% more is > 70%. This will be checked on standard stars as soon as we get clear sky on a NOTCam night.

Alignment

The alignment of the array looks OK. Internal focus has changed by only 50 units, and the array is 1.6 degrees rotated with respect to the old one (unavoidable, as it goes like this into its socket). 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 the old array.

Still to be done:

  1. The quality control pipeline s/w will be adjusted for the new array to have proper boundaries and limits etc.
  2. Sensitivity must be found from standard stars, and the SIGNAL code (exp time calculator) must be updated.
  3. A time-efficient clean-array command should be implemented.
  4. Understand the origin of the strange dark level behaviour and remedy it.


Comments to Anlaug Amanda Djupvik

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