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VISIR Exposure Time Calculator

Infrared Long Wavelength Spectroscopy Mode Version 5.1.0
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Target Flux Distribution

Uniform (constant with wavelength)
Blackbody: Temperature: K
Greybody: Temperature: K, Exponent:
Object Flux scaled to: mJy   at the central wavelength of the selected configuration
Single line: Wavelength: The emission line is centered on the (doppler-shifted, if applied) central wavelength specified below.
  Flux: 10-16 W/m2
  FWHM: nm   (the minimal width is set by the sampling)

Target Geometry

Spatial Distribution: Point Source
Extended Source   Magnitudes and line flux are given per arcsec2 for extended sources

Target Doppler Shift

No Doppler Shift
Target coordinates (J2000) and radial velocity
Coordinates (J2000) alpha:  hr min sec
delta:   deg min sec
Radial velocity
(relative to the barycenter of the Solar System)
km/s (Note: The velocity is negative for an approaching target)
Date of observation:       This information is used with the target coordinates to compute the doppler shift due to the orbital and rotational movement of the Earth (barycentric correction).
Time of observation UT:

Sky Conditions

Altitude of Target above Horizon Alt= [19.5, 90] °       Alt and X are coupled through the
plane parallel approximation X=sec(z),
z being the zenith distance z=90°−Alt
Airmass X = [1, 3]
Seeing : arcsec   (FWHM at 500 nm)
Precipitable Water Vapor PWV Typical Season:
Explicit PWV = mm  (Paranal median ≈ 2.5mm)

Instrument Setup

Low res N band, [ 9700 - 13300 ] nm, center=10500 nm
High res, long slit central wavelength: nm
High res, cross-dispersed
      Offset from main order:

Slit width: arcsec


S/N ratio: S/N = Total integration time (without overheads) INT=NDIT*DIT. In reality, the optimal DIT depends on the atmospheric conditions at the time of observation - the actual partition of NDIT is done on-the-fly. In this model, a DIT typical for the chosen setting is assigned, and NDIT is computed according to the S/N or INT requested by the user
Exposure Time: INT = s

In spectroscopy, the calculations correspond to the sensitivity obtained with chopping and nodding in parallel direction, such that all 3 beams appear on the detector

Plots: Toggle All / No Plots

Resulting Observed spectrum (object + sky)
Resulting Object Spectrum only
Resulting Sky Emission Spectrum only
Sky Transmission Spectrum ()
Sky Radiance Spectrum in physical units (ph/s/m2/micron/arcsec2)
S/N as a function of wavelength
Total Efficiency and Wavelength Range
Input spectrum in physical units
Target PSF and Slitwidth (point source only)
Enslitted Energy for the Target (point source only)
Detailed S/N info

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