SINFONI Exposure Time Calculator Help Page

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 SINFONI ETC is an exposure time calculator for the ESO Spectrograph for INtegral Field Observations in the Near Infrared, SINFONI, which uses the SINFONI AO module. 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. The ETCs are maintained on the ESO web servers to always provide up-to-date 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 and reference source 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. 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 within Java applets allowing interactive manipulation. The results are also provided in ASCII and GIF formats for further analysis and printing. Finally, a summary of the input parameters is appended to the result page.

Instrument descriptions are available for SINFONI

In the Target Input Flux Distribution field, you can select a spectral type and filter magnitude for the target. Alternatively, you can choose to specify the target with a blackbody temperature (and a filter magnitude). In both cases, the flux will be scaled to the specified magnitude in the selected band.
You can also choose to specify a single emission line instead; an analytic Gaussian, centered on the wavelength parameter, defined by its total flux and full-width at half-maximum (FWHM) width.

• Spectral Type

The target model can be defined by the target's spectral type. It uses a template spectrum, which is scaled to the provided magnitude and filter. The spectral type is used to make the color correction.

• Target Magnitude

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

• Target Spatial Distribution

The geometry of the target will affect the signal to noise, since extended sources will cover a wider area of the detector. You can either select:

• Point Source

If point source is chosen, the target object is assumed to be an emitter with negligible angular size. This can be selected for objects with an angular radius of much less than the sky-projected pixel size. The reference area for the S/N depends on the configuration:


• In the AO/25 mas case, the reference area is the diffraction limited core of the PSF; a disk with radius=1.22 lambda/D, which is 70 mas for an 8 m diameter telescope observing in the K band.
• In the AO/100 mas and AO/250 mas cases, the reference area is to a disk with a radius that encloses 50% of the object's total flux.
• In the non-AO case, the reference area is the seeing disk at the wavelength of observation. This effective seeing is computed from the given seeing value (which refer to FWHM in the V band), using Roddier's formula: FWHM(λ)=FWHM(500nm)*(λ/500nm)^−0.2.

• Extended Source (per pixel)

The target object is assumed to have a uniform intensity and the S/N on the result page is given per 2 pixels of the detector. (We use 2 pixels to obtain a square area on the sky). Note that the magnitude (or the flux for an emission line) is always given per arcsec² for extended sources.

• Extended Source (with a given area)

The source is assumed to have a uniform intensity over the given area (Ω) on the sky. Since we use 2 pixels to obtain a square area on the sky, in this case the number of pixels in the S/N area is 2 × Ω / pixelScale². To obtain the S/N per arcsec², enter Ω=1 here. Note that the magnitude (or the flux for an emission line) is always given per arcsec² for extended sources.

• Target/Reference source separation

The entered separation between target and reference star refers to the value at Zenith. The ETC will scale the given separation d(Zenith) to an effective separation d(X) at the given airmass X, using different formulas for NGS and LGS:

• NGS: d(X) = d(Zenith) * X^8/5 , d(X)≤30"
• LGS: d(X) = d(Zenith) * X^3/2 , d(X)≤60"
Note that the AO-performance model is currently limited to effective separations d(X)≤30" in NGS mode and d(X)≤60" in LGS mode.

• Reference Source Color

• Reference Source Magnitude

Choose a value from the drop-down menu. Stars brighter than 10th mag will be dimmed to 10th mag with a neutral density filter. Stars fainter than 17th mag do not provide significant AO correction.

• Seeing

Seeing conditions. The value refers to the FWHM of the seeing disk in V band, at observed airmass.

• Airmass

The seeing FWHM in V-band.
Seeing(airmass=X) is the seeing at the airmass at which the observation is performed. Use this seeing in the phase I proposal and the OB constraints.
The values in the two seeing fields are mutually updated when one of the values changes. The field "Seeing(at Zenith)" also depends dynamically on the airmass value. The seeing is assumed to scale with the airmass X like this: Seeing(X) = Seeing(Zenith) * X^3/5.

Instrument Setup

• Angular Resolution Scale

Choose one of the three available spatial scales. Note that a pixel projects to a non-square area on the sky, namely for pixelscale x, the size of the projection is x*x/2.

• Grating

This refers to the combination of filter and grating determining the (fixed) wavelength range of observations. The entire J, H, K or H+K band is fit onto the detector, respectively.

You must supply information about the total observation time. This can be done in terms of DIT (Detector Integration Time), which is the duration of individual exposures, and NDIT (Number of DIT's), which is the number of exposures. The total exposure time is the product of DIT times NDIT. This exposure time does not take into account instrument and telescope overheads.
Alternatively, you can specify a signal to noise ratio, in which case the ETC will compute the minimal number of individual exposures (each of duration DIT) required to reach the requested S/N ratio.

• S/N Ratio

The Signal to Noise Ratio (SNR or S/R) is defined for a point-like source at the central observation wavelength. Indicate here a value and choose a DIT, to get an estimate on how many exposures (NDIT) will be needed to achieve it.

• Exposure Time

The Exposure Time is the product of DIT and NDIT.

• DIT is the detector integration time (in seconds)
• NDIT is the number of exposures of duration DIT.
• INT is the total exposure time (excluding overheads). INT = DIT x NDIT

• Resulting spectrum (object+sky)

The sum of the object signal and the sky background spectrum.

• Object spectrum only

The total integrated counts contribution from the object, in e-/pixel/DIT

• Sky spectrum only

The sky contribution on the same area as the one covered by the object, in e-/pixel/DIT.

• 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.

• Input spectrum in physical units

The input flux distribution as a function of wavelength in units of nm is displayed in units of photons/cm²/s/A.

• Encircled Energy on Target:
This is the fraction of the object's total flux contained in those pixels over which the S/N is calculated, i.e. "Number of Pixels in PSF spatial profile" on the results page.

• Strehl Ratio:
This is the peak intensity of the observed PSF to that of a perfect diffraction limited PSF.