ISAAC
VLT Infrared Spectrometer And Array Camera

Built by:	ESO
P.I.:	Alan Moorwood
Instrument Scientists:	Andreas O. Jaunsen & Elena Mason
Location:	Nasmyth B, UT1 - Antu
Status:	Operations

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 arcmin field.
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 top and 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 short wave camera is equipped with a Rockwell Hawaii 1024x1024 pixel Hg:Cd:Te array 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
LW - Aladdin, SBRC	1024 x 1024	27 microns	8.7 electrons/ADU (DCR-LB)	40 electrons	0.80

Given the extreme range of backgrounds, from 0.1 electrons/sec/pixel to 3,000,000 electrons/second/pixel, the LW and SW detectors are used with different 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.

Recent News

2004-11-01

In the end of October 2004 it was discovered that the collimator was no longer performing reliably and the function has therefore been placed in a fixed position. This has implications on the image quality in certain modes. In SWI it is good, SWS-LR is acceptable, and SWS-MR is mostly acceptable, but there may be certain wavelength bands in which the IQ is less than acceptable. LWI and LWS is being investigated.

2004-08-25

There is a dust patch on the Hawaii detector at pixels (275:295,610:635). This may be a problem for spectroscopic observations if the target(s) land on the lines 610-635 (ie 15-18 arcsec from slit center). Therefore, the observers should ensure that their targets are place further than 20" from the center of the slit. This is done by adjusting the NODTHROW and JITTER box parameters.

Old News

Documentation and Tools

Documentation for P75 Phase II & P74 operations

User Manual.

P75 Phase II	version 2, December 2004	online html PDF (1596 kB) \| Compressed postscript (gz) (440 kB)
P73 Operations	v1.13.1 July 2004	online html Compressed postscript (gz) (393 kB)

Observers preparing Aladdin SW imaging OBs should read these exposure time notes.
LW spectroscopy without chopping - recommended min DITs
Exposure Time Calculator.

Calibration

Photometric Standards and Atmospheric Extinction
Telluric (Spectroscopic) Standards and the Hipparcos Telluric standard finder
Polarimetric Standard Stars
Library of stellar spectra
Atlas of OH lines. Also available in compressed postscript (gz) (434 kB) and from A & A, Volume 354, Issue 3, pp 1134-1150.
Atlas of Xenon and Argon lines. Also available in Compressed postscript (gz) (700kB).

Data Reduction

ISAAC Data Reduction Guide,, also available as a PostScript file.
Updated list of pipeline recipes
Slit positions in the ISAAC focal plane
Effect of non-optimal collimator position 2002-08-02 till 2003-04-01. Also available as a Postscript file.
Odd-even column effect
Known problems and observing/data reduction tips

Other Information

Isaac Press Releases

ISAAC Imaging Modes

SW Imaging (SWI1) - 0.98-2.5 µm

The pixel scale is 0.1484 arcsec/pixel and the field 2.5 arcmin x 2.5 arcmin.

Click on the filter name to download the transmission curve.

SW Filters (Imaging)

Name	Line	Central wavelength (µm)	Width (µm)	Width (%)
SZ		1.06	0.13	13
Js		1.24	0.16	13
J		1.25	0.29	23
H		1.65	0.3	18
Ks		2.16	0.27	13
NB 1.06		1.06	0.01	1.0
NB 1.08	He I	1.08	0.016	1.5
NB 1.19		1.19	0.01	0.8
NB 1.21		1.21	0.018	1.5
NB 1.26	Fe II	1.26	0.019	1.5
NB 1.28	P beta	1.28	0.019	1.5
NB 1.64	Fe II	1.64	0.025	1.5
NB 1.71		1.71	0.026	1.5
NB 2.07	He I	2.07	0.026	1.5
NB 2.09		2.09	0.02	1.0
NB 2.13	H2 S1	2.13	0.028	1.3
NB 2.17	Br gamma	2.17	0.028	1.3
NB 2.19		2.19	0.03	1.3
NB 2.25		2.25	0.03	1.3
NB 2.29		2.29	0.03	1.3
NB 2.34	CO	2.34	0.03	1.3

Use the Exposure Time Calculator (ETC) for performance estimates.

SW Polarimetry (SWP1) - 0.9-2.5 µm

The Wollaston prism for polarimetry has the same scale as for imaging and gives a beam separation of ~ 22 arcsec. It is used with a special mask which has 3 slots of 20 arcsec wide and can be combined with all the broad band filters plus the narrow band 1.06, 1.187, 2.09 and 2.195 filters.

LW imaging (LWI3)- 3-5 µm

The scale is 0.071 arcsec / pixel. The maximum field of view is 72 arcsec x 72 arcsec.

Click on the filter name to download the transmission curve.

LW Filters (Imaging)

Name	Line	Central wavelength (µm)	Width (µm)	Width (%)
L		3.78	0.58	15
NB M		4.66	0.10	2
NB 3.21		3.21	0.05	1.6
NB 3.28	PAH	3.28	0.05	1.6
NB 3.80		3.80	0.06	1.6
NB 4.07	Br alpha	4.07	0.07	1.7

Use the Exposure Time Calculator (ETC) for performance estimates.

ISAAC Spectroscopic Modes

ISAAC is equipped with 2 gratings, for low (Rs~500) and medium (Rs ~ 3000) resolution spectroscopy in both the SW and LW arms (s is the slit width in arcsec).

SW Spectroscopy - Low Resolution (SWS1-LR)

The pixel scale is 0.147 arcsec/pixel. The slit length is 120 arcsec.

SW Spectroscopy - Low resolution

Spectral domain (µm)	Order	Detector spectral coverage (µm)	Number of grating positions	R(2")	R(1")	R(0.6")	R(0.3")
0.98 - 1.1	5	0.29	1	270	550	900	1800
1.1 - 1.4	4	0.36	1	250	500	860	1700
1.42 - 1.82	3	0.48	1	250	500	840	1600
1.82 - 2.5	2	0.72	1	200	450	750	1500

Use the Exposure Time Calculator (ETC) for performance estimates.

SW Spectroscopy - Medium Resolution (SWS1-MR)

The pixel scale is 0.146 arcsec/pixel. The slit length is 120 arcsec.

SW Spectroscopy - Medium resolution

Spectral domain (µm)	Order	Detector spectral coverage (µm)	Number of grating positions	R(2")	R(1")	R(0.6")	R(0.3")
0.98 - 1.1	5	0.046	3-4	1700	3400	5700	11500
1.1 - 1.4	4	0.059	5	1500	3100	5200	10500
1.4 - 1.82	3	0.079	5	1500	3000	5100	10000
1.82 - 2.5	2	0.122	6	1300	2600	4400	8900

Use the Exposure Time Calculator (ETC) for performance estimates.

LW Spectroscopy - Low Resolution (LWS3-LR)

The pixel scale is 0.146 arcsec/pixel. The slit length is 120 arcsec.

LW Spectroscopy - Low resolution

Spectral domain (µm)	Order	Detector spectral coverage (µm)	Number of grating positions	R(2")	R(1")	R(0.6")	R(0.3")
2.55 - 4.2	1	1.4	1	180	360	600	1200
4.45 - 5.1	1	1.5	1	250	500	800	1600

Use the Exposure Time Calculator (ETC) for performance estimates.

LW Spectroscopy - Medium Resolution (LWS3-MR)

The pixel scale is 0.146 arcsec/pixel. The slit length is 120 arcsec.

LW Spectroscopy - Medium resolution

Spectral domain (µm)	Order	Detector spectral coverage (µm)	Number of grating positions	R(2")	R(1")	R(0.6")	R(0.3")
2.55 - 4.2	1	0.255	6(5)	1000	2000	3300	6700
4.45 - 5.1	1	0.237	3	1500	3000	5000	10000