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.

• Single 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

  • Point Source

    Point Source are sources whose spatial extend on the sky is much less than the seeing diameter. The signal to noise is computed over a circular area with diameter twice the effective seeing at the wavelength of observation. The effective seeing is given by the following formula: eff_seeing(seeing(V), λ) = seeing(V)*(λ/500nm)^-1/5, where fwhm_V is the given FWHM seeing at 500nm (V-band).

  • Extended Source

    For extended sources, the magnitude is given per square arcsecond. The reference area for the signal-to-noise calculation can either be circular with the specified angular diameter, or it can be set to one detector pixel.

• Airmass

The airmass at which the observation is performed. In the current simple sky model, the only effect of the airmass parameter is the reduction of source flux. The airmass can be computed from the Skycalc page: http://imagiware.com/astro/airmass.html.

• Seeing

The seeing parameter is given in arcseconds and corresponds to the full-width at half-maximum (FWHM) of the seeing disk in the V band.

• Sky Model

The sky flux is based on a model with absolute flux calibration that takes into account thermal background and OH lines. Atmospheric emission and absorption do not depend on the airmass in the current simple model.

• 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 Mode

The 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:
Specify a signal-to-noise ratio to be achieved and the ETC will return you the total exposure time to achieve it,
OR
Specify an exposure time, and the ETC will return the achieved signal-to-noise in that time.
In both cases, the total exposure time will be given a DIT*NDIT, with the specified DIT.

The output form will give you estimates for SNR or Exposure Time, together with output graphs you selected.

• 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/cm²/s/A

Version Information

• Version 3.2.9 (December 2, 2009)

  In this helpfile, a reference for the magnitude system definition has been corrected.

• Version 3.2.9 (September 1, 2009)

  The value of the CCD dark current has been reset to 0.01 e^-/s/pixel. The resulting S/N estimates have changed, most prominently in the filters NB1060 and NB1190, where the sky background is relatively low.

• Version 3.2.8 (2008)

  Sky model updated.

• Version 3.2.7 (November 20, 2008)

  The minimum DIT is set to 1.2572 s.

• Version 3.2.7 (September 9, 2008)

  The value of the CCD dark current was raised to 2.0 e^-/s/pixel. The resulting S/N estimates have changed, most prominently in the filters NB1060 and NB1190, where the sky background is relatively low.

• Version 3.2.7 (September 1, 2008)

  Supporting AB magnitudes

  Additional template spectra

  Supporting redshift for template spectra

• Version 3.2.0 (August 1, 2007)

  Public release

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