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.
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).
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.
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.
The template spectra can currently only be used in JHK bands.
The input spectrum can also be chosen from a subset of MARCS stellar model spectra, kindly provided by Bengt Edvardsson at the Uppsala Astronomical Observatory.
The wavelength coverage of these models includes the full wavelength range of CRIRES at a high resolution.
The parameter space of the MARCS subsets are listed the following tables. Note that not all models (referring to all possible combinations of paramaters) actually exist.
|MARCS subset: Spherical Geometry
|| 360 (this is the number of possible combinations, but only 87 models exist)
|MARCS subset: Plane Parallel Geometry
|| 54 (this is the number of possible combinations, but only 50 models exist)
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.
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:
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 computation depends on the configuration, the reference area has a rectangular shape and the
size a*b depends on the configuration. In the direction of dispersion, b = 1 pixel.
- In the AO case, the size in the spatial direction is the width (diameter) of the Airy disc:
a = 2 * 1.22*λ/D, where D=8.2m is the diameter of the telescope. If this value is smaller than 2 pixels,
then the size in the spatial direction is taken to be 2 pixels to account for the fact that a spectrum is
rarely centered on only 1 pixel.
- In the non-AO (seeing limited) case, the size in the spatial direction is
mainly given by the width of the seeing disk at the wavelength of observation, but also
the diffraction limit is considered (it mostly plays a role at longer wavelengths).
- The effective seeing is computed from the given seeing value (which refer to FWHM in the V band),
using Roddier's formula: effective_seeing = FWHM(λ)=FWHM(500nm)*(λ/500nm)−0.2.
- The diffraction limit enters as the width (diameter) of the Airy disc:
2 * 1.22*λ/D, where D=8.2m is the diameter of the telescope.
- The width (considering both the effects above) is then computed like this:
a = (effective_seeing2 + (2 * 1.22*λ/D)2 )1/2
If an extended source is chosen, the S/N reference area is per pixel in the
spectral direction and integrated over 1 arcsec in the spatial direction.
The source is assumed to have a uniform intensity and the magnitude is given per square arcsec.
Reference Source Parameters
- Target/Reference source separation: Enter the separation between the target and the reference source here. The closer the target and reference source the better the correction.
- B-R Color
The (approximate) B-R color of the AO or TipTilt guide star. It is used with the R magnitude to compute the flux on the wave-front sensor / tip-tilt sensor. The input is only critical for extreme colors.
- Reference Source Magnitude
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.
The sky background radiance and transmission are based on the Cerro Paranal advanced sky model, developed by a team at the University of Innsbruck as part of the Austria in-kind contribution to ESO.
Seeing conditions. The value refers to the FWHM of the seeing disk in V band, at observed airmass.
The airmass at which the observation is performed.
The precipitable water vapor PWV is the vertically integrated total mass of water vapor per unit area for a column of atmosphere. PWV=2.5 mm is close to the median value for Paranal.
- Slit width
Specify the width of the slit.
- Requested wavelength
This is the the wavelength of interest. The wavelength-dependent numeric results (the signals from object and background, S/N, ...)
will refer to this particular wavelength. If any of the graphs at the bottom of the input page are selected, a cross of light-blue lines will
indicate the requested wavelength and the value of the spectrum there. When a new spectral setting is selected, the value
in the Requested Wavelength field is automatically updated to get the same value as the Reference Wavelength for the newly selected standard or free setting.
This is just a convenience feature; you can change the Requested Wavelengh to any valid value as you please.
- Wavelength range to plot
The λ(min) and λ(max) values are automatically updated to the full range when a Standard Setting is being changed.
For a Free Setting, the λ(min) and λ(max) values will be set to the corresponding full range by pressing the "set plot range" button.
In both cases, you can change the plot range if you wish to "zoom into" a subrange. Only the plots are affected; if a pixel saturates somewhere in the full spectrum (for the selected spectral setting), a warning will still be issued.
- Reference Wavelength
This wavelength refers to the center (pixel 512) of detector 3. You have the choice between selecting a standard setting or a free setting:
- Standard Setting: When this option is chosen, you must select one of the standard modes in the drop-down list.
The reference wavelength (wref) will then be assigned to the fixed value indicated in the drop-down list.
- Free Setting: When this option is chosen, you must select one of the orders in the associated drop-down list,
and enter a reference wavelength in the wref field. The entered reference wavelength must be within the range indicated
for the chosen order in the drop-down list. Don't forget to pres the "set plot range" button or you will get a reminder when you submit.
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.
- S/N Ratio
The Signal to Noise Ratio (SNR or S/N) is defined for a point-like source at the observation wavelength
in one spectral dispersion element (1 pixel). It is obtained by integrating the spectrum profile along the spatial
direction. 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
A cross of light-blue lines will be
overplotted, indicating the requested wavelength and the value there, respectively.
Text Summary Results
- Strehl Ratio:
This is the peak intensity of the observed PSF to that of a perfect diffraction limited PSF.
- Version 5.0.1 (Dec. 19, 2013)
- Added support for FowlerNsampGrStWin windowed detector read-mode
- Supporting variable RON depending on DIT
- Version 5.0.0 (July 1, 2013)
- The Sky model in the CRIRES ETC has been updated.
- All ETC version numbers have been aligned at 5.0.0 as the ETC system infrastructure was re-factored and
installed on a new web server. Please report significant discrepancies to the ESO user support group firstname.lastname@example.org