Instrument Characteristics

Imaging Performance

VISIR performance estimates are based on observations of mid-infrared calibration standard stars. Limiting fluxes were measured in the 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

When the imager is used in burst readout mode, achievable sensitivities can be calculated with the ETC, applying for each filter the DIT setting corresponding to SF / IF (assuming the default weather flag = Good).

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

median sensitivity

[mJy 10σ/h]

LR

8.8, 9.8, 11.4, 12.4


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 DRS (former Boing) 256x256 BIB detector. The quantum efficiency of the detectors reaches up to 65% in the N-band and has a sharp absorption feature at 8.8µm.