Instrument Characteristics

Imaging Performance

The VISIR performance estimates listed on this page are based on observations of mid-infrared calibration standard stars before the upgrade. The post-upgrade sensitivities are still being established, but are expected to be better than those given here. Limiting fluxes were measured in the old VISIR detector small field (SF, Pixel Field Of View=0.075") and intermediate field (IF, PFOV=0.127"). The values for each filter given below correspond to SF and IF, respectively, and refer to the median of more than 600 different measurements taken during September and December 2004. Better sensitivities can be obtained, but depend on atmospheric conditions. 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]

median sensitivity (SF / IF)

[mJy 10σ/h ]

PAH1 8.59 0.42 5    /    8
ArIII 8.99 0.14 6    /    70
SIV_1 9.82 0.18 30    /    60
SIV 10.49 0.16 8    /    13
SIV_2 10.77 0.19 9    /    20
PAH2 11.25 0.59 6    /    9
SiC 11.85 2.34 7    /    18
PAH2_2 11.88 0.37 7    /    15
NeII_1 12.27 0.18 12    /    20
NeII 12.81 0.21 12    /    18
NeII_2 13.04 0.22 15    /    22
Q1 17.65 0.83 50    /    120
Q2 18.72 0.88 50    /    80
Q3 19.50 0.40 100    /    160
B8.7 8.92 0.97 TBD    /    TBD
B9.7 9.88 0.82 9    /    20
B10.7 10.65 1.37 5    /    8
B11.7 11.52 0.85 5    /    6
B12.4 12.47 0.99 8    /    12
J7.9 7.78 0.55 14    /    25
J8.9 8.72 0.73 3    /    5
J9.8 9.59 0.94 7    /    10
J12.2 11.95 0.53 8    /    10

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

Entire N-band


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.