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VISIR Exposure Time Calculator

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Important notes and bug reports

Note: This Exposure Time Calculator is only provided for the technical assessment of feasibility of the observations. Variations of the atmospheric conditions can strongly affect the required observation time. Calculated exposure times do not take into account instrument and telescope overheads. Users are advised to exert caution in the interpretation of the results and kindly requested to report any result which may appear inconsistent.

General description

The Infrared ETC is an exposure time calculator for the ESO infrared instrument VISIR. The HTML/Java based interface allows to set the simulation parameters and examine interactively the model generated graphs. The ETC programs allow easy comparison of the different options relevant to an observing program, including target information, instrument configuration, variable atmospheric conditions and observing parameters. Being maintained on the ESO Web servers, the ETCs are maintained to always provide up-to-date information reflecting the known performance of ESO instruments.

These programs provide an HTML/Java based interface and consist of two pages. The observation parameters page presents the entry fields and widgets for the target information, expected atmospheric conditions, instrument configuration, observation parameters such as exposure time or signal-to-noise, and results selection. An "Apply" button submits the parameters to the model executed on the ESO Web server. The results page presents the computed results, including number of counts for the object and the sky, signal-to-noise ratios, instrument efficiencies, PSF size etc.. The optional graphs are displayed within Java applets allowing interactive manipulation. The results are also provided in ASCII and GIF formats for further analysis and printing. A summary of the input parameters is included in the result page.

Note: These tools are only provided for technical assessment of observation feasibility. Variations of the atmospheric conditions can strongly affect the required observation time. Calculated exposure time do not take into account instrument and telescope overheads. Users are advised to exert caution in the interpretation of the results and to report any result which may be suspected to be inconsistent.

Instrument description is available for VISIR

Input Spectrum

From the following, choose the spectrum shape you want for your target.


Doppler Shift


Sky Conditions

The sky background radiance and transmission spectra are based on an advanced sky model, developed by a team at the University of Innsbruck as part of the Austria in-kind contribution to ESO.
The radiance model incudes contributions from scattered moon light (Krisciunas et al. 1991), zodiacal light (Leinert et al. 1998), telescope emission (grey body) and, as the major component, emission by the Earth atmosphere. The model for the latter contains both thermal components, derived with the radiative transfer code LNFL/LBLRTM and the HITRAN line database, meteorological data (radiation/transmission, lower atmosphere), and a model of chemi-luminescent airglow (upper atmosphere).
The atmospheric extinction includes Rayleigh scattering, aerosol extinction (Patat et al. 2011, A&A, 527), and molecular absorption.

  • Seeing

  • The seeing parameter is given in arcseconds and corresponds to the full-width at half-maximum (FWHM) of the seeing disk at 500nm.
    The spatial extent of the area over which the S/N is computed is equal to the FWHM of the PSF at the obserserving wavelength. This is modelled as described in the following:

    s_diff = 0.98 (lambda/D) 206265
    where s_diff is the FWHM of the point-spread function of the telescope (in arcsec).
    lambda is the central wavelength (in m).
    D is the diameter of the telesscope (in m).
    206265 is the conversion factor between radian and arcsec.
    (The factor 1.22 usually given for the diffraction limit corresponds to the radius of the 1st Airy ring).

    s_atm = seeing_opt (lambda/0.500)−0.2 airmass0.6 f_corr
    where seeing_opt is the optical seeing (at 0.500 micron), which is the seeing parameter entered on the ETC input page, and f_corr is a correcting factor to take into account the effect of the turbulence outer scale L0. We currently use f_corr = 1.

    The FWHM of the seeing disk in the VISIR ETCs is calculated like this:
    s_visir = max( s_diff(lambda) , s_atm(lambda,airmass) )

  • Sky

  • The program will invoke a flux calibrated sky model, yielding the sky background at the specified wavelength.


    Instrument Setup


    Results

    Set a Signal to Noise Ratio (SNR) to achieve and get an Exposure Time estimation, or the opposite: given an exposure time, estimate the SNR you would get. 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 filter is assigned, and NDIT is computed according to the S/N or INT requested by the user.

    The output form will give an estimate of the SNR or Exposure Time, together with graphs you selected for output. These graphs are interactive Java applets and may require that you setup your browser to enable Java applets.

    To set up the observation, either a SNR or an exposure time must be user-defined. The simulation computes then the associated value.


    Possible Outputs in Imaging

    Possible Outputs in Spectroscopy


    Version Information


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