Instrument Description

ISAAC covers the 1-5µm wavelength range and is capable of:

  • 1-2.5 µm imaging over a 2.5 arcmin x 2.5 arcmin field.
  • 1-2.5 µm imaging polarimetry over a 2.5 arcmin x 2.5 arcminfield.
  • 1-2.5 µm long slit low (Rs ~ 500) and medium resolution (Rs~ 3000) spectroscopy
  • 2.5-5 µm imaging over a 1.25 arcmin x 1.25 arcmin field.
  • 2.5-5 µm long slit low (Rs ~ 500) and medium resolution (Rs~ 3000) spectroscopy

Fig. 1 shows the optical layout of ISAAC. At the topand bottom are two cameras which are optimized for the 1-2.5µm and 2-5µm ranges and used to either re-image the telescope focal plane or the intermediate spectrum produced by the grating spectrometer. The shortwave camera is equipped with a Rockwell Hawaii 1024x1024 pixel Hg:Cd:Tearray and the long wave camera with an SBRC Aladdin 1024x1024 pixel InSb array.The characteristics of the detectors are given in the following table.

ISAAC detector array characteristics
Detector Format Pixel size Gain Read Noise Q.E
SW - Rockwell 1024 x 1024 18.5 microns 4.5 electrons/ADU 11 electrons 0.65
LW - Aladdin, SBRC 1024 x 1024 27 microns 7.8 electrons/ADU (DCR-HB, UCR) 40 electrons 0.80
8.7 electrons/ADU (DCR-LB)

Given the extreme range of backgrounds, from 0.1 electrons/sec/pixel to3,000,000 electrons/second/pixel, the LW and SW detectors are used withdifferent readout modes.


ISAAC detector readout modes
Detector Readout Mode Corresponding instrument modes Linearity Saturation* Minimum DIT
Hawaii Double Correlated Read All SW imaging 99% @ 10,000 ADU 40,000 ADU 1.77s or 3.5451s
Hawaii Non Destructive Read All SW spectroscopy 99% @ 10,000 ADU 40,000 ADU 1.77s or 3.5451s
Aladdin Uncorrelated Read High Bias L, nb_M imaging and LR M band spectroscopy 90% @28,000 ADU 37,000 ADU 0.1073s
Aladdin Double Correlated Read High Bias All other modes that use chopping, and non-chopped imaging with NB_3.21 and NB_3.28 90% @28,000 ADU 37,000 ADU 0.2771s
Aladdin Double Correlated Read Low Bias All modes that do not use chopping except NB_3.21 and NB_3.28 non-chopped imaging 90% @16,000 ADU 21,000 ADU 0.3447s

* Saturation is the number of counts above the bias.