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GIRAFFE 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.

See also: Frequently Asked Questions

General description

The GIRAFFE ETC is an exposure time calculator for the ESO optical multiple-target single-order echelle spectrograph.

The input 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 output 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 as images and interactive Java applets as well as ASCII and PDF formats for further analysis and printing.

An instrument description of GIRAFFE is available

Input Parameters

The model includes an input spectrum (e.g. a template star spectrum), atmospheric parameters , optical instrument path and an observation criterium. The model generates output graphs describing the spectral illumination of the CCD, the instrument efficiency or the signal to noise as a function of the exposure time or ImageQualityFWHM.

Source Model: Input Flux Destribution

Source Geometry

Sky Conditions

Instrument Setup

Observation Setup


The result graphs are Java based applications. A static version of the graphs is also provided in GIF and ASCII format.

Possible Outputs

Text Summary Results

Spectroscopy results such as efficiency, signal, signal-to-noise estimates are dependent on the wavelength and given over the wavelength range in graphics form. A summary of results is provided in text form referring to the value at the reference wavelength.

Reference Wavelength: Normally the central wavelength.

Wavelength Range: The respective wavelength associated to the first and last pixel of the detector for the given configuration and dispersion, in nanometers.

Dispersion: The dispersion of the spectrum, in nanometers per pixel.

Plate scale: The plate scale of the system, in arcsecs per pixel.

FWHM of the fiber spatial profile: The full-width at half-maximum of the fiber spatial profile. This value is the fiber size divided by the plate scale.

Efficiency at the reference wavelength: Total efficiency of the system at the reference wavelength, telescope transmission, optics and detector efficiency, in percent.

Fiber injection loss: The percentage of light that is lost at fiber entrance. For point sources, a higher loss will occur at poorer seeing, since only the central part of the PSF will enter into the fiber. For extended sources, the fiber injection loss is independent of the seeing. For Extended sources in the IFU and ARGUS modes, a 5% loss occurs due to the "filling factor" of the microlens arrays.

Object-fiber displacement: In the MEDUSA fiber mode only, the fiber injection loss is partially caused by the possible displacement between fiber and object. In the computed results the entry "Loss due to object-fiber displacement" will be present. This is the fraction of the total fiber injection loss due to decentering, as specified by the input parameter "Object-fiber displacement". In modes other than MEDUSA, the value of this input parameter is irrelevant and has no effect on the computed results.

Total object signal at reference wavelength: The total flux contribution from the object, integrated over all fibres within the extend of the PSF, and expressed in electrons per pixel along the dispersion direction. In Medusa the ETC gives directly the SNR of the extracted spectrum, while in IFU or Argus mode to obtain the SNR of the central extracted spectrum, the SNR given by ETC should be divided by sqrt(Number of Fibers covering the source). The value is given at the central wavelength and corresponds to "object_signal" in the signal-to-noise formula.

Sky background level at reference wavelength: The flux contribution from the sky for one row along the dispersion direction, in electrons per pixel along the dispersion direction. The value is given at the reference wavelength and corresponds to "sky_signal" in the signal to noise formula.

Max. intensity at reference wavelength (object+sky): This value is the sum of the sky background level and the fraction of the object signal falling on one pixel at the center of the slit profile. If there are more than 1 fiber on the source, this value refers to the sum of fibers and must be carefully interpreted (see VIMOS ETC User Manual).

Detector saturation: The detector saturation level. A message will be displayed if the maximum intensity is greater than this limit. Please note that the actual saturation level may depend on the CCD readout-mode, and that the saturation is here tested only for the central wavelength.

Detector read-out noise level: CCD read-out noise in electrons/pixel. This value corresponds to CCDnoise in the signal-to-noise formula.

Detector dark current: CCD dark current in e-/pixel/hour. This value corresponds to DarkCurrent in the signal-to-noise formula.

Number of fibers:The number of fibers covering the source over a circular area with radius=1.5*PSF FWHM, the radius inside which a gaussian profile is at least 1/16 of its central value. If the source is extended, results are given pr. fiber, ie. only one fiber is used in the calculations. MEDUSA mode is always single fiber.

Fiber diameter: The number of detector pixels the fiber projects onto. It is computed for the GIRAFFE dispersion physical model and takes into account the effect of inclined projection. It is approximately twice the FWHM of the fiber spatial profile.

Signal to Noise at reference wavelength: The signal to noise is calculated over 1 pixel along the dispersion and summing the sky signal over (Number of fibers * fiber diameter in pixels) in the spatial direction.

PSF extension: number of pixels over which the signal-to-noise is estimated. This value is computed as twice the ImageQualityFWHM divided by the plate scale. This value corresponds to "Npsf" in the signal-to-noise formula.


Object spectrum only

The total integrated counts contribution from the object, in e-/pixel. The integration is done along the slit. The counts are expressed in electrons per pixel along the dispersion direction.

Sky spectrum only

The sky contribution on each row of the detector, in e-/pixel. This value is not integrated along the slit.

Input spectrum in physical units

The input flux distribution for the selected target is diplayed in units of ergs/cm2/s/A

Signal to Noise as a function of wavelength

Toggling this option will display a curve showing the evolution of Signal to Noise Ratio against wavelength.

Total efficiency and Wavelength range

This option will display a curve showing the total efficiency of the system, and a second graph showing the dispersion relation.

Version Information