Introduction

The SW arm of ISAAC uses a Hawaii 1024x1024 array from Rockwell. When reducing data from the SW detector you should keep in mind the following points.

• The array is read out simultaneously in four quadrants.

• Within each quadrant, the read starts in the bottom left hand corner, reads along a single row before moving to the next row and then ends up in the top right hand corner.

• For data taken before 24/07/2001, the detector read out time is 1.7728 seconds. This also corresponds to the minimum integration time.

• For data taken after 24/07/2001, the detector read out time is 3.5451 seconds. This also corresponds to the minimum integration time.

• There is no shutter, so the array is continuously exposed to light.

The ISAAC IRACE controller and the associated DCS software control the readout of the array. They are extremely flexible and allow many different readout schemes to be used. Currently, only two readout schemes are supported: DCR (Double Correlated Read) and NDR (Non-Destructive Read). In the near future we may also offer other readout modes such as FDCR (Fast Double Correlated Read).

DCR first resets the array and then performs two reads, one at the beginning of the integration and one at the end. The difference between these two reads is the image.

NDR first resets the array and then reads the array N+1 times, where N depends on the DIT. Longer DITs will result in more reads. A line is fit to N reads (the first read is discarded) and the slope of the fit gives the image.

For a given DIT, DCR is faster, but NDR results in better noise performance. For these reasons, DCR is used when the background is high (imaging) and NDR is used when the background is low (spectroscopy).

It is important to appreciate that different parts of the detector are read out at different times. If we take DCR as an example, set the DIT to the minimum, i.e. 1.77 seconds^4.1, and set the clock at 0 seconds when the first row is read out, then row 1 is the difference of two reads that occur at 0 and approximately 1.77 seconds and row 512 is the difference between two reads that occur at approximately 1.77 and 3.54 seconds. During the first 1.77 seconds, row 512 is still collecting photons. In other words, different parts of the detector sample different parts of the integration ramp, and because the integration ramp is non-linear, a source at row 500 will always have fewer counts than the same source at row 100. If the peak flux of source and sky is less than 10,000 ADUs, then you do not need to worry about these effects, unless you wish to obtain photometry which is better than 1%. If it is greater then 10,000 ADUs and DITs are small, then these effects become important.