Infrared 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 Infrared ETC is an exposure time calculator for the ESO infrared instruments SOFI and ISAAC. 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. Being maintained on the ESO Web servers, the ETCs are maintained 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 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.

Note: These tools are only provided for technical assessment of observation feasibility. Variations of the atmospheric conditions can strongly affect the required observation time. Calculated exposure time do not take into account instrument and telescope overheads. Users are advised to exert caution in the interpretation of the results and to report any result which may be suspected to be inconsistent.

The exposure time calculator models the observation chain which includes the target spectral distribution, atmosphere parameters, instrument configuration, and detector setup. The following diagram illustrates the observation chain for imaging.

Instrument descriptions are available for SOFI and ISAAC

From the following, choose the spectrum shape you want for your target.

• Continuum

The flux density is constant at all wavelengths (F(lambda) = const.) The flux density level is determined from the specified object magnitude.

• Blackbody

The target model is a blackbody defined by its temperature, expressed in Kelvin. The intensity distribution is scaled to the object magnitude.

• Object Magnitude

Enter the J (1250 nm) , H (1650 nm), or Ks (2160 nm) magnitude closest in wavelength to the selected filter in imaging or the central wavelength of the defined spectrum (e.g. J if you choose J or NB-1.187, H if you choose H or NB-1.644, Ks if you choose Ks of NB-2.122). In SOFI spectroscopy the blue grism is associated to J filter, and the red grism to the Ks filter. Zero points are taken from the reference: Wilson, 1972, ApJ, 177, 533

• Single Emission Line

The input spectrum is a single emission line. It is an analytic Gaussian, centered on the Wavelength parameter, defined by its total Flux and full-width at half-maximum Width. Line flux is given in 10^-16 erg.cm^-2.s^-1.

NB: When requesting a single line as input spectrum, the magnitude parameter is not taken into account. Only the line flux will be used to determine the signal magnitude.

NB: the fwhm of a single line shall not be smaller than 0.5nm. User-provided values smaller than this will be clipped to 0.5nm.

• Source Geometry

  • Point Source

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

  • Extended Source

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

• Airmass

The airmass can be computed from the Skycalc page: http://imagiware.com/astro/airmass.html .

• Seeing

The seeing parameter is given in arcseconds and corresponds to the full-width at half-maximum of the seeing disk.

Sky

The program will determine automatically from the closest broad band filter the magnitude of the sky. The values currently set in the simulation are the following:

    Broad Band Filter        Sky magnitude  (mag/arcsec2)  
    
    J                        16.0             
    H                        14.0             
    Ks                       13.0             
	

Note: Sky brightness can vary substantially during the night and with season.

In imaging the program automatically computes S/N and respective exposure times for the above sky magnitudes and for a brighter and a fainter sky. In spectroscopy both the OH emission lines and thermal continuum are modeled and scaled to the above values. The list of OH lines covers the range 624-2624 nm. These data have been computed by Philippe Rousselot (Observatory of Besancon, France). The calculations are based on the energy levels published by Abrams et al., 1994 (Ap.J. Suppl. Seris 93, 351-395) and on the transition probabilities published by Mies, 1974 (J. of Mol. Spec. 53, 150-188). The upper relative populations have been computed by assuming a Boltzmann distribution with a rotational temperature equal to 190 K and a vibrational temperature equal to 9000 K. These values are approximates and the resulting relative intensities should be regarded as indicative. The accuracy of the wavelengths is of the order of a few hundredth of Angstroems.

The other setup sections differ whether you are working in imaging or spectroscopy mode.

Imaging ETC

• Filter

Choosing the instrument filter determines in which band the observation will be performed. The magnitude parameters for object and sky are defined in the observation band you choose by choosing an instrument filter. It is not possible to define an input spectrum in J and observe it in Ks, for example. For information about the filters, please refer to the links ISAAC and SOFI.

• Objective

The list of available objectives can be selected from the pull-down menu.

    Objective                Pixel Scale

    SOFI Small field              0.144 arcsec/pixel
    SOFI Large field              0.292 arcsec/pixel
    ISAAC S2                      0.147 arcsec/pixel
    ISAAC L2		          0.162 arcsec/pixel
    ISAAC L3		          0.078 arcsec/pixel
    

Spectroscopy ETC

• Slit

Slits can be selected from the pull-down menu. 

• Grisms and Gratings

Remember to select the correct low or medium resolution option for ISAAC.
Please refer to the relevant instrument pages for ISAAC  and SOFI

• Grating

Remember to select the correct low or medium resolution option for ISAAC.

The Exposure Time Calculator is made of 2 interactive Web forms. Fill in the input form to define the kind of object you want to observe, the atmospheric conditions, instrument and detector setups. Set a Signal to Noise Ratio (SNR) to achieve and get an Exposure Time estimation, or the opposite: given a DIT, NDIT and NINT (i.e. exposure time), estimate the SNR you would get.

The output form will give you estimates for SNR or Exposure Time, together with graphs you selected for output. These graphs are interactive Java applets which may require that you setup your browser to activate them. Help can be found from the output page about how to use the Java applets.

To set up the observation, either a SNR or an exposure time must be user-defined. The simulation computes then the associated value.

• S/N Ratio

Indicate here a value of the Signal to Noise Ratio (SNR) and get an estimation of the exposure time required to achieve it.

• Exposure Time

• Chopping / Non-chopping modes

  Non-chopping is only offered in the following modes:

  • Imaging mode LWI3: J+Block, H, Ks, NB_3.21, NB_3.80.
  • Medium resolution spectroscopy.
  Please see the ISAAC user manual for an explanation of the different chopping modes.

• Total exposure time

  For the chopping mode of ISAAC, the user selects the total exposure time, in minutes. The DIT (Detector Integration Time) depends of the instrument configuration. The NDIT (Number of Detector InTegrations) is then found from the total exposure time, by dividing with the DIT.

• DIT

  Detector on-chip integration time (in seconds).

Exposure Time for an infra-red detector is the product of DIT and NDIT and NINT. For chopping modes, the DIT is fixed and NDIT is found from the total exposure time (NINT is 1).

• DIT is the detector integration time (in seconds)
• NDIT is the number of DIT exposures averaged out within the same frame
• NINT is the number of frames.

The total exposure time is therefore DIT x NDIT x NINT.

• Plotting Range

The Range box sets up the computation interval for the output curve of S/N versus exposure time.

• Detector Illumination

Toggling this option will display the object spectrum as seen by the detector.

• S/N as a function of Exposure Time

Toggling this option will display a curve showing the evolution of Signal to Noise Ratio as a function of Exposure Time.

• Total efficiency

This option will display a curve showing the efficiency in terms of detected photons against wavelength.

• Signal to Noise vs Seeing

Toggling this option will display a curve showing the evolution of Signal to Noise Ratio against seeing in arcseconds.

• Input spectrum in physical units

The input flux distribution is displayed in units of 1e-20 ergs/cm**2/s/A

• Resultant spectrum including OH lines

The sum of the object signal and the sky background spectrum for the central row of the spectrum.

• Object spectrum only

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

• Sky spectrum only

The sky contribution on each row of the detector, 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, and a second graph showing the dispersion relation.

• Input spectrum in physical units

The input flux distribution is displayed in units of 1e-20 ergs/cm**2/s/A

Version Information

Version 3.2.1 (Aug 31, 2007)

ISAAC: Min. DIT reduced to 3.2ms in LW imaging modes.

• Version 3.2.0 (June 21, 2007)

  Imaging: Corrected sky for SZ filter.

  Imaging and Spectroscopy: Corrected a bug: the atmospheric extinction was erroneously applied to the sky signal itself.

  Code optimized.

• Version 3.1.1 (March 7, 2007)

  Added support for SOFI MR spectroscopy.

• Version 3.1.0 (July 25 2006)

  New layout of plots. No change in numerical results.

• Helpfile updated. (March 16, 2006)

  The values listed for J and H sky magnitudes were updated to the current ones.

• Bux fix for ISAAC (imaging), Version 3.0.4, (March 16, 2005)

  The efficiency file for the ISAAC narrow-band filter isaac_1.06 was wrong. It has now been replaced by the correct one.

• Version 3.0.4 (January 7, 2005)

  Code updated. No changes expected for imaging. For Spectroscopy, results referring to the central wavelength differ with respect to previous versions, because an index error has been corrected.

• Version 2.9.0 (August 4, 2003)

  Sky backgrounds in bands J,H adjusted. Values of typical variablity adjusted in all bands.
  LW imaging: LWI3 mode added (objective L2, non-chopping).
  SW imaging: SZ filter added.
  LW,SW spectroscopy: Adding slits 0".8 and 1".5.
  

• Version 2.8.2 (February 26, 2002)

  Chopping modes fully integrated. Detector readout mode displayed on output page

• Version 2.8.1 (November 9, 2001)

  Sky background in LW spectroscopy updated.

• Version 2.8 (September 1, 2001)

  ISAAC zero points and sky backgrounds updated and chopping mode added.

• Version 2.4.1 (September 4, 1999)

  Enabled ISAAC LW L3 objective. New table of OH lines, more complete in the region 650-1100 nm

• Version 2.4 (August 5, 1999)

  Graph display options have been fixed.

• Version 2.3 (March 1, 1999)

  Sky background model has been updated, as well as detector dark-current values, and sky magnitudes

• Version 2.2 (Aug. 7, 1998)

  Released spectroscopy and upgraded imaging models. Sky background model has been updated, as well as detector read-out noise. Note: Single emission line option removed from the spectroscopy interface on 21/09/98

• Version 2.1 (Aug. 1, 1998)

  Public release

Send comments on this document to User Support Department/DMD<usd-help@eso.org> Last modified: March 16, 2005.