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CRIRES 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 CRIRES ETC is an exposure time calculator for the ESO High-Resolution IR Echelle Spectrometer using the AO system MACAO. The ETC 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 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 can be obtained in various formats. A summary of the input parameters is appended to the result page.

Instrument descriptions are available for CRIRES

Target Input Flux Distribution

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 (doppler-shifted, if applied) requested wavelength, defined by its total flux and width (FWHM: full-width at half-maximum).

Target Doppler Shift

To consider the Doppler shift effect, select the radiobutton "Doppler". In the table, specify the coordinates and the radial velocity of the target, relative to the bary-center of the solar system (a negative velocity means an approaching target). Also specify the date and time of the observation. The program use this information (and the geographic coordinates of the Paranal observatory lon=[-70°,24',00''], lat=[-24°,37',30'']) to compute the doppler shift due to the orbital and rotational movement of the Earth (barycentric correction). In the output page, some partial results of the computation is displayed.

Reference Source Parameters

Sky Conditions

Since version 6.x.x, the ETCs offer a dynamic almanac widget to facilitate the assignment of sky model parameters for given target position and time of observation.

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

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

Instrument Setup


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. Note: In some of the results, the unit contains DIT, e.g. "Object signal in reference area per DIT (at requested wavelength) : 1703.813 e-/DIT" , to emphasize that the quantity refers to one single exposure of duration DIT, as opposed to the total integration time INT. In principle, the unit could simply be e-.

Please note!

The formula to calculate the S/N ratio includes a correction factor to adjust the theoretical value to the observed sensitivity. The current value of this factor is:

1/1.56 = 0.64 (for λ ≤ 1.1 microns)
1/1.20 = 0.83 (for λ > 1.1 microns).

As a consequence, the displayed S/N deviates from the expectations based on photon noise, detector characteristics and spatial profile.

Possible Graphs

A cross of light-blue lines will be overplotted, indicating the requested wavelength and the value there, respectively.

Text Summary Results

Version Information

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