Instrument Characteristics

Imaging Performance

The VISIR performance estimates listed on this page are based on observations of mid-infrared calibration standard stars. Sensitivities have not changed much with the relocation of VISIR from UT3 (until 2019) to UT2 (since 2021). Values are regularly adjusted when changes are detected. The last update has been made for the P115 Call for Proposals.

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 imaging filters.

The filters are linked to their transmission profiles measured at operating temperatures in ascii format. Filter names with an asterisk are transmission curves obtained with a Fourier Transform Spectrometer and are absolute. The remaining ones are relative transmission curves obtained with a monochromator.

Imaging Filter central wavelength

[µm]

 filter width (FWHM)

[µm]

 sensitivity (median)

[mJy 10σ/h ]
M-Band 4.67 0.52 4
J7.9 7.72 0.56 10
PAH1* 8.58 0.41 4
J8.9 8.70 0.74 4
B8.7 8.91
 0.92 4
ArIII* 8.98 0.14 10
SIV_1 9.81 0.19 25
B9.7 9.82 0.82 7
SIV* 10.47 0.17 10
B10.7 10.64 1.34 4
SIV_2 10.71 0.20 11
PAH2* 11.24 0.54 5
B11.7 11.51 0.90 5
PAH2_2 11.68 0.37 6
J12.2
 11.93 0.52 7
NeII_1 12.22 0.18 13
B12.4 12.45 1.03
 8
NeII* 12.80 0.20 15
NeII_2* 13.04 0.22 16
Q1* 17.65 0.83 37
Q2* 18.72 0.88 36
Q3* 19.50 0.40 80
SAM Filters central wavelength

[µm]

 filter width (FWHM)

[µm]

 sensitivity (median)

[mJy 10σ/h ]
10_5_SAM 10.56 0.37 -
11_3_SAM 11.23 0.55 -

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*10-16W/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
 
 
62
 
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. To avoid saturation due to the high MIR sky background, detector integration times (DITs) are a few milliseconds in broad-band imaging and may increase up to 2s 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.