CRIRES 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 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

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).

• 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. The template spectra can currently only be used in JHK bands.

• MARCS Stellar Model

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
Parameter	Number of unique values	Unique Values
model	1	"st"
[Fe/H]	4	-4.00,-2.00,-1.00,0.00
Teff/K	9	4000,4500,5000,5500,6000,6500,7000,7500,8000
log(g)	5	-0.50,0.00,1.00,2.00,3.50
geometry	1	"s"
microturbulence	1	2
mass	2	1,5
total (product)	360 (this is the number of possible combinations, but only 87 models exist)





MARCS subset: Plane Parallel Geometry
Parameter	Number of unique values	Unique Values
model	1	"st"
[Fe/H]	6	-1.00,-2.00,-4.00,0.00,0.50,1.00
Teff/K	9	4000,4500,5000,5500,6000,6500,7000,7500,8000
log(g)	1	4.00
geometry	1	"p"
microturbulence	1	2
total (product)	54 (this is the number of possible combinations, but only 50 models exist)




• Target 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.

• 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 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
   

• Extended Source

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.

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.

• Adaptive Optics

Choose the AO mode. NB: The optical path is the same in all cases.

• Seeing

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

• Airmass

The airmass at which the observation is performed.

• Atmosphere model

The sky flux is based on a model with absolute flux calibration that takes into account thermal background and OH lines.

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.

Instrument Setup

• Slit width

  Specify the width of the slit.

• Wavelengths

  Note that all wavelengths are vacuum vavelengths.

• 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-.

• 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

• 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 Transmission Spectrum

The atmopheric transmission spectrum, in percent.

• Sky Emission Spectrum

The atmopheric emission spectrum, 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.

• Target PSF and Slitwidth

The Point Spread Function for the target. The chosen slitwidth is indicated with two vertical overplotted blue lines.

• Enslitted Energy for the Target

The Enslitted Energy in the PSF for the target, as a function of the slitwidth. The chosen slitwidth is indicated with overplotted blue lines.

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

Version Information

• Version 3.2.19 (October 12, 2012)

  A bug in the sky background model has been corrected. In version 3.2.18, the extinction of the airglow continuum is not correct. However, at low airmasses and high transmission (i.e. within the J and H bands), the systematic errors has not influenced the exposure time calculation in a significant way.

• Version 3.2.18 (September 20, 2012)

  A serious bug was identified in the sky background model of 3.2.17. Results obtained with the previous version could be wrong, in particular for high airmass values.

• Version 3.2.17 (August 19, 2012)

  A bug which affected the max. intensity for target objects with extended geometry has been corrected; the intensity was underestimated by a factor depending on the seeing and slitwidth.

• Version 3.2.16 (June 29, 2012)

  The sky model code and library were updated. New list of PWV values.

  The spectral convolution algorithm of the object and sky spectra has been refined.

• Version 3.2.15 (February 27, 2012)

  Additional options for PWV in the sky model.

• Version 3.2.14 (August 8, 2011)

  Fixed a bug which prevented the Single Line mode to work in the previous version.

• Version 3.2.13 (June, 2011)

  Improved model in convolution kernels applied to target object and sky background spectra

  S/N correction factor changed for λ<1100 nm

  Javascript functionality updated

• Version 3.2.8 (June 29, 2009)

  Spectral configuration tables updated.

  The "Instrument Setup" section in the input page has been re-arranged.

• Version 3.2.6 (June 16, 2008)

  Sky spectra offered for 3 different PWV values (precipitable water vapor).

  Wavelength range in plots can be changed.

  Javascript-driven procedures validate the input in the instrument section of the input page. The default values for wavelength ranges and requested wavelength are automatically set (using AJAX-technology), based on the spectral setting.

• Version 3.2.4 (March 10, 2008)

  Spectral format updated after the intervention Dec.2007.

  Aimass dependency is now included in the atmosphere model, using new HITRAN emissivity and transmissivity spectra for different airmasses.

  AO-performance estimates now takes the airmass into account.

  Added support for MARCS stellar model spectra (full wavelength range coverage).

  Added options to get plots of AO-PSFs and the Enslitted Energy as function of the slit width.

• Version 3.2.1 (Aug 31, 2007)

  Spatial extent of S/N reference area is now always >=2 pixels.

  Added warnings in case of detector non-linearity and/or saturation.

  Graphs improved.

• Version 3.2.0 (June 21, 2007)

  Wavelength range corrected for orders 59-41.

  Optional sky transmission spectrum added.

  Code optimized.

• Version 3.1.4 (February 23, 2007)

  S/N correction factor changed.

  Refined AO-model.

  GS magnitude range extended.

• Version 3.1.3 (January 2, 2007)

  Corrected a bug in the list of ranges for the reference wavelength in the free settings.

• Version 3.1.3 (December 15, 2006)

  New Algorithm for AO-estimation and new layout of AO section.

  Wavelength configuration list updated ("wlen29092006")

  Free Setting for spectral format supported.

  Blaze function (grating efficiency) now applied to efficiency model

  Added display of Strehl Ratio for on-axis K-band (for OB constraint set)

• Version 3.1.2 (September 23, 2006)

  A bug has been found in the ETC only affecting non-AO estimations (corrected in version 3.1.2).
  This bug led to over-estimating the slit transmission by a factor of 2. Therefore the S/N for a given exposure time was overestimated by a factor of 1.4 in the regime where the photon-noise from the target dominates the noise (bright target in the J, H and K bands), and a factor of 2 in the RON-dominated (faint target in the J, H or K bands) or sky noise dominated regimes (L and M bands).

• Version 3.1.1 (August 30, 2006)

  Wavelength configuration list updated (identical with table in sect.11 of CRIRES user manual 14.07.2006).

  A bug in the integration over PSF profile has been corrected. The values of slit transmission factor and the maximum value at the central PSF pixel are affected, as well as the S/N.

  Introduced a correction factor=1/4, adjusting the S/N value to the observed sensitivity.

• Version 3.1.0 (July 25 2006)

  Algorithm for mode selection corrected.

• Version 3.0.8 (July 21, 2006).

  Added options to give target mag. in bands L and M.

• Version 3.0.8 (July 12, 2006).

  Default slitwidth changed to 0.4 arcsec.

• Version 3.0.8 (June 26, 2006).

  Updated prototype for Science Verification.

• Version 3.0.6B (October, 2005).

  Updated prototype.

• Version 3.0.6 (April 28, 2005).

  First prototype for internal evaluation.

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