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

• Continuum

  The target model is a spectral distribution constant with the wavelength.

• Blackbody

  The target model is a blackbody defined by its temperature and monochromatic apparent magnitude at a given wavelength. Temperature is expected in Kelvin and wavelengt in one of the band filters U, B, V, R or I.

• Template spectrum

  The target model can be defined by a template spectrum . As with the blackbody it will be scaled to the provided magnitude and band filter U, B, V, R or I.

• Object Magnitude

  You must select the filter and filter magnitude for proper scaling of the template spectrum. Available filters are V, J, H, K, L and M. For extended sources, the magnitude must be given per square arc second.

Source Geometry

• Point Source

  Point Source are sources whose spatial extent on the sky is much less than the ImageQualityFWHM diameter. The signal to noise is computed over an area of diameter twice the ImageQualityFWHM.

• Extended Source

  The signal to noise for extended sources is given per pixel on the detector. The magnitude is given per square arcsecond.

Sky Conditions

• Seeing and Image Quality

  Turbulence Category (T category)

  With the advent of instruments using new adaptive optics (AO) modes, new turbulence parameters need to be taken into account in order to properly schedule observations and ensure that their science goals are achieved. These parameters include the coherence time and the fraction of turbulence taking place in the atmospheric ground layer, in addition to the seeing. Starting from Period 105, the turbulence constraints are standardised to the turbulence conditions required by all instruments and modes, whether they are seeing-limited or AO-assisted.

  The handling of atmospheric constraints thus changes for both Phase 1 (proposal preparation) and Phase 2 (OB preparation). In Phase 1, the seven current seeing categories are replaced by seven turbulence categories for all instruments. Each category can be defined by other parameters than a pure seeing threshold, depending on the instrument. For all instruments, all categories share the same statistical probability of realisation, which is key for an accurate time allocation process. In Phase 2, the image quality will still be the only applicable constraint for seeing-limited modes, whereas the same turbulence category as for Phase 1 will be used for diffraction-limited modes.

  Users are encouraged to read the general description of these changes for Phase 1 and Phase 2 on the Observing Conditions webpage, as well as instrument User Manuals for specifics per instrument.

  Seeing and Image Quality

  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 \({ L_{0} = 46m}\) , this corresponds to \({ r_{\text{t}} = 5.4m} \). \({ L_{0} }\) is the wave-front outer-scale. We have adopted a value of \({ L_{0} }\)=46m (van den Ancker et al. 2016, Proceedings of the SPIE, Volume 9910, 111). \(F_{\text{Kolb}} \) is the Kolb factor (ESO Technical Report #12): $$F_{\text{Kolb}} = \frac {1}{1+300 {\text{ }} D/L_{0}}-1$$ For the VLT and \({ L_{0} }\)=46m, this corresponds to \(F_{\text{Kolb}} = -\)0.981644. \( {r_0} \) 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.

• Sky Model

The sky background model is based on the Cerro Paranal Advanced Sky Model, also for instruments at la Silla, except for the different altitude above sea level. The observatory coordinates are automatically assigned for a given instrument.

Since version P101, the ETCs include a dynamic almanac widget to facilitate assignment of accurate sky model parameters for a given target position and time of observation. The sky radiation model includes the following components: scattered moonlight, scattered starlight, zodiacal light, thermal emission by telescope and instrument, molecular emission of the lower atmosphere, emission lines of the upper atmosphere and airglow continuum.

Alternatively, the almanac mode can be overridden to allow manual assignment of airmass and moon phase. In that case, the sky model will use fixed typical values for all remaining parameters (which can be seen in the output page by enabling the check box "show skymodel details").

The almanac is updated dynamically by a service on the ETC web server, without the need to manually update the web application.

Notes about the algorithms, resources and references for the almanac are available here

Almanac Usage

Hovering the mouse over an input element in the almanac normally displays a pop-up "tooltip" with a short description.

Time

The upper left part of the almanac box refers to the date and time of observation.
This can be done with a UT time or a MJD. A date/time picker widget will appear when the UT input field is clicked, but the UT can also be assigned manually. In any case, the UT and MJD fields are dynamically coupled to be mutually consistent.

The two +/- buttons can be used to step forward or backward in time by the indicated step and unit per click. The buttons can be held down to step continuously until released.

The third of night corresponding to the currently selected time is indicated. This is an input parameter to the airglow component in the sky model. Twilight levels (civil, nautical and astronomical) referring to the sun altitude ranges are also indicated in the dynamic text. These levels refer to the sun altitude:

• Astronomical Twilight −18^° ≥ alt_☉ < −12^°
• Nautical Twilight −12^° ≥ alt_☉ < −6^°
• Civil Twilight −6^° ≥ alt_☉ < 0^°

Target

The target equatorial coordinates RA and dec can be assigned manually in the two input fields or automatically using the SIMBAD resolver to retrieve the coordinates.
If the lookup is successful, an "info" link will open a window in which the raw SIMBAD response can be inspected.
The units can be toggled between decimal degrees and hh:mm:ss [00:00:00 - 23:59:59.999] for RA and dd:mm:ss (or dd mm ss) for dec. A whitespace can be used as separator instead of a colon.

Output Table

The table dynamically displays the output from the server back-end service, including temporal and spatial coordinates for the target, Moon and Sun. The bold-faced numbers indicate the parameters normally relevant in the phase 1 proposal for optical instruments. The numbers appear in red color if they are out of the range supported by the sky model.

Visiblity Plot

The chart dynamically shows the altitude and equivalent airmass as function of time for the moon and target, centered on midnight for the currently selected date.
The green line, which refers to the currently selected time, can be dragged left and right to change the time, dynamically coupled with the sections in the Time section.

A more advanced version of the almanac is included in our SkyCalc web application, which provides more input and output options.

Instrument Setup

• Fiber Mode The fiber mode is selected from a list with an option menu. The MEDUSA mode uses 132 fibers for observations, plus 5 fibers for calibrations. In the IFU mode, there are 300 fibers for observations, plus five for calibration. Sky sampling is the projected size of a fiber microlens onto the sky.
• Resolution Two different gratings exist for the GIRAFFE echelle spectrograph. The links below will show tables of the spectral resolution for the two modes, and their dependence on fiber mode (due to size of microlens) and central wavelength.
  • High Resolution:HR table
  • Low Resolution:LR table
• Central Wavelength The wavelength for the central pixel, in nanometers.

Observation Setup

• Signal to Noise A curve of signal-to-noise as a function of exposure time is generated. The centre value and a range to both sides of the centre value are provided.
• Exposure Time A curve of signal-to-noise as a function of exposure time is generated. The centre value and a range to both sides of the centre value are provided (in seconds).

Results

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.

Graphs

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

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