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 significantly changed with the between VISIR at UT3 (until 2019) and at UT2 (since 2021). Values may be adjusted when significant changes are detected.
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 | 6 |
J7.9 | 7.72 | 0.56 | 13 |
PAH1* | 8.58 | 0.41 | 5 |
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 | 5 |
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 | 7 |
J12.2 |
11.93 | 0.52 | 8 |
NeII_1 | 12.22 | 0.18 | 14 |
B12.4 | 12.45 | 1.03 |
6 |
NeII* | 12.80 | 0.20 | 15 |
NeII_2* | 13.04 | 0.22 | 15 |
Q1* | 17.65 | 0.83 | 32 |
Q2* | 18.72 | 0.88 | 34 |
Q3* | 19.50 | 0.40 | 70 |
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 | ||
MR | ||
HR longslit | ||
HR cross-dispersed |
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 milli–seconds 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.