Instrument Characteristics

Imaging Performance

The VISIR performance estimates listed on this page are based on observations of mid-infrared calibration standard stars during the re-commissioning of VISIR. As observations are ubtained under a wider range of weather conditions, these post-upgrade sensitivities will be revised at the end of ESO period 95. Median sensitivities are listed as 10 sigma detection limit on a point-source for 1 hour of integration time (median) and 1 hour of execution time including overheads (effective), The median sensitivities quoted are the reference for classification of VISIR service mode observations. A graphical compilation of all sensitivity measurements is available for the N-band and Q-band imaging filters in comparison with theoretical expectations. The filters are linked to their transmission profiles measured at operating temperatures in ascii format.

Imaging Filter central wavelength

[µm]

half-band width

[µm]

sensitivity (median / effective)

[mJy 10σ/h ]

PAH1 8.59 0.42 5    /    6
ArIII 8.99 0.14 9   /   12
SIV_1 9.82 0.18 17    /    32
SIV 10.49 0.16 9    /    11
SIV_2 10.77 0.19 8    /    11
PAH2 11.25 0.59 TBD   /   TBD
PAH2_2 11.88 0.37 6    /    8
NeII_1 12.27 0.18 TBD    /    TBD
NeII 12.81 0.21 15    /    25
NeII_2 13.04 0.22 23    /    25
Q1 17.65 0.83 36    /    42
Q2 18.72 0.88 67    /    78
Q3 19.50 0.40 65    /    78
B8.7 8.92 0.97 TBD    /    TBD
B9.7 9.88 0.82 TBD    /    TBD
B10.7 10.65 1.37 5    /    6
B11.7 11.52 0.85 TBD    /    TBD
B12.4 12.47 0.99 TBD    /    TBD
J7.9 7.78 0.55 19    /    24
J8.9 8.72 0.73 3    /    4
J9.8 9.59 0.94 7    /    9
J12.2 11.95 0.53 10    /    12

Spectroscopic Performance

The sensitivities given below are estimates from observations of mid-infrared calibration standard stars. Chopping and nodding along the slit are assumed. The approximate limiting surface brightness for high resolution line observations is around 3*1016W/m2/arcsec2. Graphical representations of the measured sensitivities in comparison with theoretical limits are available for different settings. Note that medium resolution spectroscopy in the N and Q band is offered in visitor mode for all central wavelength settings. See the user manuals for detailed lists of offered settings in service mode.

mode  spectral settings

[µm]

median sensitivity

[mJy 10σ/h]

LR

8.0-13.0


50

MR

7.4-8.0

8.0-9.4

10.2-13.0

17.1-19.0

19.9-20.3


1000

200

200

1200

2000

HR longslit

[H2 S4] 8.02

[NeII] 12.81

[H2 S1] 17.03


3000

900

10000

HR cross-dispersed

[ArIII] 8.99

[H2 S3] 9.66

[SIV] 10.51

[CoII] 10.52

[HD(0,0) R(10)] 11.57

[ClIV] 11.76

[H2 S2] 12.27

[HD(0,0) R(9)] 12.47

[NeII] 12.81

[CoIII] 16.39

[CoI] 16.92

[PIII] 17.88

[FeII] 17.93

[NiII] 18.24

[SIII] 18.71

[NaIV] 21.29


4000

5000

4000

4000

2000

3000

1500

2000

2000-5000

12000

8000

4000

4000

8000

4000

9000

Detectors

The VISIR imager and spectrometer are each equipped with a 1k x 1k Raytheon Aquarius IBC detector. These detectors have a switchable pixel ("well") capacity. The detectors are used in large capacity mode (6.0 x 106 e- ) for broad-band imaging and in small capacity mode (0.6 x 106 e-) for narrow band imaging and spectroscopy. To avoid saturation due to the high MIR sky background, detector integration times (DITs) are a few milli–seconds in broad-band imaging and may increase up to 2 s in high resolution spectroscopy. Detectors are read from the center outwards. To avoid artifacts due to this new read-out structure, it is recommended that targets are placed a few arcseconds in y above or below the central position of the detector.