Introduction

The LW arm of ISAAC uses a Aladdin 1024x1024 array. When reducing data from the LW arm, you should keep in mind the following points.

• The array is simultaneously read out from 32 ports. There are 8 ports per quadrant and the readout is done along detector rows.

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

• The array is intrinsically more non-linear that the Hawaii array, and it is common to work in the regime where corrections of around 0-10% are required.

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. We offer three readout modes: UCR (Uncorrelated Read - High Bias), DCR-HB (Double Correlated Read - High Bias), DCR-LB (Double Correlated Read - Low Bias).

UCR first resets the array and then performs a single read. The minimum integration time of this mode is 0.1073 seconds. Before March 2001, the minimum integration time of this mode was 0.0733 seconds.

DCR performs two reads and the difference between these two reads is the image.

DCR-HB and DCR-LB differ in the way array voltages are set. DCR-LB has a slower readout and a smaller well depth, but far fewer hot pixels. The minimum integration time for DCR-HB is 0.2771 seconds and the minimum integration time for DCR-LB is 0.3451 seconds. Before March 2001, the minimum integration time for DCR-HB was 0.1419 seconds and DCR-LB was not offered.

The readout modes with a high bias setting are used with chopping. They have a very large number of hot pixels. The readout modes with a low bias setting are used without chopping. They have much fewer hot pixels, but they have a smaller well depth.

In addition to these modes, there are three modes that are used for a small number of calibration frames. The modes, their names, the minimum integration time, the recommended flux range and the saturation (both in ADU) are listed in the following table.

 

Table: Current LW detector settings. Prior to March 2001, only the first two modes were available.

Detector Mode Name	Current MINDIT	Recommended Range	Saturation
UnCorrHighBias (UCR)	0.1073	0-28000	37000
DoubleCorrHighBias (DCR-HB)	0.2771	0-28000	37000
DoubleCorrLowBias (DCR-LB)	0.3451	0-16000	21000
UnCorrHighBiasCal	0.0733	0-28000	37000
DoubleCorrHighBiasCal	0.1419	0-28000	37000
DoubleCorrLowBiasCal	0.1419	0-16000	21000