Instrument Characteristics

Imaging Performance

The VISIR performance estimates listed on this page are based on observations of mid-infrared calibration standard stars since the re-commissioning of VISIR. As observations are obtained under a wider range of weather conditions, these post-upgrade sensitivities will be revised with the improving statistics of the sensitivity measurements. Median sensitivities are listed as 10 sigma detection limit on a point-source for 1 hour of integration time (median), that is without chopping and nodding overheads. The sensitivities are determined from multiaperture photometry using the curve-of-growth method and are hence the sensitivity with respect to the integrated flux, not the peak above the background. 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)

[mJy 10σ/h ]

PAH1 8.59 0.42 5
ArIII 8.99 0.14 12
SIV_1 9.82 0.18 27
SIV 10.49 0.16 10
SIV_2 10.77 0.19 14
PAH2 11.25 0.59 4
PAH2_2 11.88 0.37 8
NeII_1 12.27 0.18 17
NeII 12.81 0.21 16
NeII_2 13.04 0.22 15
Q1 17.65 0.83 35
Q2 18.72 0.88 24
Q3 19.50 0.40 59
B8.7 8.92 0.97 5
B10.7 10.65 1.37 5
B11.7 11.52 0.85 5
B12.4 12.47 0.99 5
J7.9 7.78 0.55 11
J8.9 8.72 0.73 4
J9.8 9.59 0.94 9
J12.2 11.95 0.53 7
M-Band 4.82 0.35 6

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. Medium resolution spectroscopy in the N and Q band are currently not offered. See the user manuals for detailed lists of offered settings.

mode  spectral settings

[µm]

median sensitivity

[mJy 10σ/h]

LR
 
8.0-13.0
 
 
70
 
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.