VISIR Exposure Time Calculator 


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.
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 uptodate 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 signaltonoise, 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, signaltonoise 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.
From the following, choose the spectrum shape you want for your target.
The flux density is constant at all wavelengths: F(λ) dλ = constant.
The flux density distribution model F(λ) is a blackbody BB(T,λ) defined by its temperature T in unit Kelvin.
(This is identical with a greybody spectrum with exponent α=0).
F(λ) dλ = BB(T,λ) dλ= 2hc dλ λ^{5}[exp(hc/λkT)1]^{1}.
The flux density distribution model F(λ) is a greybody GB(T,λ,α). This is proportional to a blackbody BB(T,λ) multiplied by the wavelength λ to the power of the exponent α. A pure blackbody is thus a greybody with α=0.
F(λ) dλ = GB(T,λ,α) dλ = BB(T,λ) dλ (λ/λ_{o})^{α}, where λ_{o} is a reference wavelength.
This can also be expressed as:
F(λ) dλ = GB(T,λ,α) dλ = constant * 2hc * dλ * λ^{5α}[exp(hc/λkT)1]^{1} , where the constant has the value constant = λ_{o}^{α}.
If the target model is one of those selected above, the flux will be scaled to the specified flux in mJy at the central wavelength of the selected filter (imaging) or spectral setting (spectroscopy).
The input spectrum is a single emission line. It is an analytic Gaussian, centered on the wavelength parameter, defined by its total flux and fullwidth at halfmaximum (FWHM).
The minimal width is internally defined by the ETC and set equal to the model's sampling of the wavelength range for the selected configuration (filter or spectral setting). In the case that the width set by the user is smaller than the minimal width, the minimal width is used
for the calculations and a warning is issued.
When the single line input option is used (in the section Input Flux Distribution), the (doppler shifted, if applied) wavelength of the line is used as reference instead of the central wavelength of the selected spectral setting.
Point Source is a source which is unresolved at the wavelength specified by the user. 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 (the FWHM is limited by the effective seeing in the shorter wavelengths, and the telescope diffraction limit in the longer wavelengths).
The value of the flux specified by the user corresponds to a flux per square arcsecond.
In imaging, the signaltonoise ratio for extended sources is calculated for an area of 1 square arcsec on the detector.
In spectroscopy, the signaltonoise ratio for extended sources is calculated for an area of 1 arcsec in the spatial direction and 1 pixel in the dispersion direction.
The sky background radiance and transmission model in the VISIR ETC is based on the Cerro Paranal advanced sky model, developed by a team at the University of Innsbruck as part of the Austria inkind contribution to ESO. See also the SkyCalc web application.
Seeing is an inherent property of the atmospheric turbulence, which is independent of the telescope that is observing through the atmosphere; Image Quality (IQ), defined as the full width at half maximum (FWHM) of longexposure stellar images, is a property of the images obtained in the focal plane of an instrument mounted on a telescope observing through the atmosphere.
The IQ defines the S/N reference area for point sources in the ETC.
With the seeing now consistently defined as the atmospheric PSF FWHM outside the telescope at zenith at 500 nm, the IQ FWHM is modeled by the ETC considering the transfer functions of the atmosphere, telescope and instrument, with s=seeing, λ=wavelength, x=airmass and D=telescope diameter:
\( { \small \begin{equation} \begin{aligned} \mathit{FWHM}_{\text{IQ}} & = \sqrt{\mathit{FWHM}_{\text{atm}}^2(\mathit{s},x,\lambda)+\mathit{FWHM}_{\text{tel}}^2(\mathit{D},\lambda)+\mathit{FWHM}_{\text{ins}}^2} \\ \end{aligned} \end{equation} } \) .
For fibrefed instruments, the instrument transfer function is not applied. For nonAO instrument modes, the atmospheric PSF FWHM with the given seeing \(s\) (arcsec), airmass \(x\) and wavelength \({\small \lambda }\) (nm) is modeled as a gaussian profile with:
\(
\begin{equation}
\begin{aligned}
& {\small \mathit{FWHM}}_{\text{atm}}(\mathit{s},\mathit{x},\lambda) = \mathit{s} \cdot x^{0.6} \cdot (\lambda/500\text{ nm})^{0.2} \cdot \sqrt{1+F_{\text{Kolb}} \cdot 2.183 \cdot ({r_0}/L_{0})^{0.356}} \text{,} \\
& \small{\text{where}}
\end{aligned}
\end{equation}
\) \( \begin{equation} \begin{aligned} {r_0} & = 0.976 \cdot 500.0 \cdot 10^{9}\text{ nm } / \mathit{s} \cdot (180/\pi \cdot 3600) \cdot (\lambda/500.0\text{ nm})^{1.2} \cdot x^{0.6} \\ F_{\text{Kolb}} & = 1/(1+300 \cdot D/L_{0})1 \end{aligned} \end{equation} \) \({ L_{0} }\) is the wavefront outerscale. We have adopted a value of \({ L_{0} }\)=23 m, which is the generally accepted value for Paranal. D is the telecscope diameter in meters. \( {r_0} \) is the Fried parameter at the requested wavelength and airmass. \(F_{\text{Kolb}} \) is the Kolb factor (ESO Technical Report #12). If the argument of the square root \({\small (1+F_{\text{Kolb}} \cdot 2.183 \cdot ({r_0}/L_{0})^{0.356}) < 0 }\), which happens when the Fried parameter \({\small {r_0} } \) reaches its threshold of \({\small r_{\text{t}} = L_{0} \cdot (1.0/2.183)^{1/0.356} } \), the value of \({\small \mathit{FWHM}}_{\text{atm}}\) is set to \({\small 0.0 }\). 
The Paranal seeing statistics is based on the socalled UT seeing measurements obtained from the UT1 Cassegrain ShackHartmann wavefront sensor used for active optics.
The measurements are deconvolved in order to represent the seeing outside the dome (i.e. they are corrected for the instrument+telescope resolution).
The La Silla seeing statistics is based on the DIMM FWHM measurements corrected for the instrumental resolution.
These data come from http://www.eso.org/genfac/pubs/astclim/paranal/seeing/singcumul.html
The airmass of the observed target. The airmass must be ≥ 1.
The precipitable water vapor PWV is the vertically integrated total mass of water vapor per unit area for a column of atmosphere. PWV=2.5 mm is close to the median value for Paranal. Details about the seeing and PWV at Paranal are available from http://www.eso.org/genfac/pubs/astclim/paranal.
The other setup sections differ whether you are working in imaging or spectroscopy mode.
The filters available for the instrument is listed in the pulldown menu. In the square bracket is listed the position of the filter in the filterwheel, as well as the wavelength range of the filter.
Select the pixel scale.
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 onthefly. 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 userdefined. The simulation computes then the associated value.
Indicate here a value of the Signal to Noise Ratio (SNR) and get an estimation of the exposure time required to achieve it.
Toggling this option will display a curve showing the evolution of Signal to Noise Ratio as a function of Exposure Time.
Toggling this option will display a curve showing the SNR spectrum.
The input flux distribution is displayed in units of ergs/cm^{2}/s/A
The sum of the object signal and the sky background spectrum for the central row of the spectrum.
The total integrated counts contribution from the object, in e/pixel/DIT
The sky contribution on each row of the detector, in e/pixel/DIT.
Toggling this option will display a curve showing the evolution of Signal to Noise Ratio against wavelength.
The input flux distribution as a function of wavelength in units of nm is displayed in units of photons/cm^{2}/s/A.
Adapted to CallforProposals P95.
All ETC version numbers have been aligned at 5.0.0 as the ETC system infrastructure was refactored and installed on a new web server. Please report significant discrepancies to the ESO user support group usdhelp@eso.org
Sky model updated (Austria inkind SM01), now supporting explicit PWV as well as season typical atm. profiles
Cosmetic changes in input and output pages
New prototype version for AQUARIUS detector. All noise factors set to 1.
Using first version of Austria inkind sky background model SM01
