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HAWK-I Exposure Time Calculator
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HAWK-I Exposure Time Calculator

Important notes and bug reports

Note: These tools are 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 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 regularly updated to reflect the known performance of ESO instruments.

The exposure time calculator consists of two pages.
Input page: 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.
Output page: 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 in several formats. Finally a summary of the input parameters is appended to 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.

The exposure time calculator models the observation chain which includes the target spectral distribution, atmosphere parameters, instrument configuration, and detector setup. An instrument description for HAWK-I is available on the instrument page.

Input Spectrum

The following options are available to describe the input spectrum of the target.

Uniform

The flux density is constant at all wavelengths (F(λ) = const.) The flux density level is determined from the specified object magnitude.

Blackbody

The target model is a blackbody defined by its temperature, expressed in Kelvin. The intensity distribution is scaled to the object magnitude.

Spectral Type

The target model can be defined by a template spectrum which is scaled to the provided magnitude and filter. The spectrum can be red-shifted.
References: Pickles (1998, PASP 110, 863); Coleman et al.: 1980ApJS; Kinney at al.: 1996ApJ.

Object Magnitude

Enter the V (650 nm), Y (1000 nm), J (1250 nm) , H (1650 nm), or K (2160 nm) magnitude, ideally closest in wavelength to the selected filter. The reference for the zero points used in conversion into photon fluxes:
Vega-system: (B,V,R,I): Bessel, 1979, PASP, 91, 589. (J,H,K): Bessel and Brett, 1998, PASP, 100, 1134.
AB-system:Oke, 1974, ApJS.

Emission Line

The input spectrum is a single emission line. It is an analytic Gaussian, centered on the Wavelength parameter, defined by its total Flux and full-width at half-maximum FWHM. Line flux is given in 10-16 erg.cm-2.s-1.

NB: When requesting a single line as input spectrum, the magnitude parameter is not taken into account. Only the line flux will be used to determine the signal magnitude.

NB: The FWHM of a single line is limited by the sampling. If the requested FWHM is too narrow, it will be replaced by the minimum supported value, and a warning will be issued in the beginning of the result page.

Source Geometry

Atmosphere

Airmass

The airmass of the observed target. The airmass must be ≥ 1.

Seeing and Image Quality

Since version 6.0.0, the definitions of seeing and image quality used in the ETC follow the ones given in Martinez, Kolb, Sarazin, Tokovinin (2010, The Messenger 141, 5) originally provided by Tokovinin (2002, PASP 114, 1156) but corrected by Kolb (ESO Technical Report #12):

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 long-exposure 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 non-AO point sources in the ETC.

With the seeing consistently defined as the atmospheric PSF FWHM outside the telescope at zenith at 500 nm, the ETC models the IQ PSF as a gaussian, considering the gauss-approximated transfer functions of the atmosphere, telescope and instrument, with s=seeing, λ=wavelength, x=airmass and D=telescope diameter:

Image Quality
\( { \begin{equation} \mathit{FWHM}_{\text{IQ}} = \sqrt{\mathit{FWHM}_{\text{atm}}^2(\mathit{s},x,\lambda)+\mathit{FWHM}_{\text{tel}}^2(\mathit{D},\lambda)+\mathit{FWHM}_{\text{ins}}^2(\lambda)} \end{equation} } \)

For fibre-fed instruments, the instrument transfer function is not applied.

The diffraction limited PSF FWHM for the telescope with diameter D at observing wavelength λ is modeled as:
\( \begin{equation} \begin{aligned} \mathit{FWHM}_{\text{tel}} & = 1.028 \frac{\lambda}{D} \text{, } & \text{ with } \lambda \text{ and D in the same unit}\\ & = 0.000212 \frac{\lambda}{D} \text{arcsec, } & \text{ with } \lambda \text{ in nm and D in m}. \end{aligned} \end{equation} \)
For point sources and non-AO instrument modes, the atmospheric PSF FWHM with the given seeing \(s\) (arcsec), airmass \(x\) and wavelength \({ \lambda }\) (nm) is modeled as a gaussian profile with:
$${\mathit{FWHM}}_{\text{atm}}(\mathit{s},\mathit{x},\lambda) = \mathit{s} \cdot x^{0.6} \cdot (\frac {\lambda} {500})^{-0.2} \cdot \sqrt{[1+F_{\text{Kolb}} \cdot 2.183 \cdot ({r_0}/L_{0})^{0.356})]}$$
Note: The model sets \({ \mathit{FWHM}}_{\text{atm}}\)=0 if the argument of the square root becomes negative \({ [1+F_{\text{Kolb}} \cdot 2.183 \cdot ({r_0}/L_{0})^{0.356}] < 0 }\) , which happens when the Fried parameter \({ {r_0} } \) reaches its threshold of \({ r_{\text{t}} = L_{0} \cdot [1/(2.183 \cdot F_{\text{Kolb}})]^{1/0.356}}\). For the VLT and \({ L_{0} = 23m}\) , this corresponds to \({ r_{\text{t}} = 2.63m} \).
\({ L_{0} }\) is the wave-front outer-scale. We have adopted a value of \({ L_{0} }\)=23m, which is the generally accepted value for Paranal (Dali Ali et al. 2010, A&A 524, A73; Martin et al. 2000, A&AS, 144, 39).

\(F_{\text{Kolb}} \) is the Kolb factor (ESO Technical Report #12):
$$F_{\text{Kolb}} = \frac {1}{1+300 {\text{ }} D/L_{0}}-1$$
For the VLT and \({ L_{0} }\)=23m, this corresponds to \(F_{\text{Kolb}} = -\)0.990737.
\( {r_0} \) is the Fried parameter at the requested seeing \(s\), wavelength \({ \lambda }\) and airmass \(x\):
$$r_0 = 0.100 \cdot s^{-1} \cdot (\frac{\lambda}{500})^{1.2} \cdot x^{-0.6} \text{ m, } \text{ } \text{ } \text{ } \text{ with } s \text{ in arcsec } \text{and } \lambda \text{ in nm.} $$

For AO-modes, a model of the AO-corrected PSF is used instead.

Seeing statistics:

The Paranal seeing statistics is based on the so-called UT seeing measurements obtained from the UT1 Cassegrain Shack-Hartmann 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/gen-fac/pubs/astclim/paranal/seeing/singcumul.html

Sky Model

Since ETC version 6.0.0, the sky background radiance and transmission model in the HAWK-I ETC is based on the Cerro Paranal advanced sky model.

Instrument Setup

Filter

Choosing the instrument filter determines for which band the exposure time will be computed. For information about the filters (incl. transmission curves), please refer to the Instrument Description.

Detector

ModeThe pixel scale is fixed to 0.106 arcsec/pixel. The read-out mode is fixed to Non-Destructive Read-out (NDR) - the detector is continuously read-out in a non-destructive mode. Providing a read-out noise of about 5 electrons.

Results

The DIT (Detector on-chip integration time in seconds) is needed as input for both of the following cases:

In both cases, the total exposure time will be given a DIT*NDIT, with the specified DIT. The output form will give the estimates for SNR or Exposure Time, together with the selected output graphs.

Do not confuse exposure time and total observation time, the latter being a sum of exposure time and overheads in the telescope and instrument. Please consult the user manuals for guidance on the choice of the integration parameters.

Graphical Outputs

Resultant spectrum including sky

The sum of object signal and sky background spectrum for the central pixel, in e-/pixel/DIT.

Object spectrum only

The total integrated counts contribution from the object per pixel as a function of wavelength, in e-/pixel/DIT.

Sky Emission Spectrum

The sky contribution to each pixel as a function of wavelength, in e-/pixel/DIT.

Sky Transmission Spectrum

The sky transmission in percent as a function of wavelength.

S/N as a function of Exposure Time

The S/N as a function of Exposure Time

Total efficiency and Wavelength range

This option will display a curve showing the total efficiency in percent of the system.

Input spectrum in physical units

The input flux distribution is displayed in units of photons/cm^2/s/A

Version Information
Send comments and questions to usd-help@eso.org