Infrared Exposure Time Calculator
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 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.
From the following, choose the spectrum shape you want for your target.
The flux density is constant at all wavelengths (F(lambda) = const.) The flux density level is determined from the specified object magnitude.
The target model is a blackbody defined by its temperature, expressed in Kelvin. The intensity distribution is scaled to the 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
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.
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.
The signal to noise for extended sources is given per pixel on the detector. The magnitude is given per square arcsecond.
The airmass can be computed from the Skycalc page: http://imagiware.com/astro/airmass.html .
The seeing parameter is given in arcseconds and corresponds to the full-width at half-maximum of the seeing disk.
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.Instrument Setup
The other setup sections differ whether you are working in imaging or spectroscopy mode.
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.
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
Slits can be selected from the pull-down menu.
Remember to select the correct low or medium resolution option for ISAAC. Please refer to the relevant instrument pages for ISAAC and SOFI
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.
Indicate here a value of the Signal to Noise Ratio (SNR) and get an estimation of the exposure time required to achieve it.
Non-chopping is only offered in the following modes:
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.
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).
The total exposure time is therefore DIT x NDIT x NINT.
Toggling this option will display the object spectrum as seen by the detector.
Toggling this option will display a curve showing the evolution of Signal to Noise Ratio as a function of Exposure Time.
This option will display a curve showing the efficiency in terms of detected photons against wavelength.
Toggling this option will display a curve showing the evolution of Signal to Noise Ratio against seeing in arcseconds.
The input flux distribution is displayed in units of 1e-20 ergs/cm**2/s/A
The sum of the object signal and the sky background spectrum for the central row of the spectrum.
The total integrated counts contribution from the object, in e-/pixel/DIT
The sky contribution on each row of the detector, in e-/pixel/DIT.
Toggling this option will display a curve showing the evolution of Signal to Noise Ratio against wavelength.
This option will display a curve showing the total efficiency of the system, and a second graph showing the dispersion relation.
The input flux distribution is displayed in units of 1e-20 ergs/cm**2/s/A
Bug fixed in ISAAC and SOFI ETCs. By mistake, a wrong version of the sky model was invoked.
ISAAC: Min. DIT reduced to 3.2ms in LW imaging modes.
Imaging: Corrected sky for SZ filter.
Imaging and Spectroscopy: Corrected a bug: the atmospheric extinction was erroneously applied to the sky signal itself.
Added support for SOFI MR spectroscopy.
New layout of plots. No change in numerical results.
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.
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.
Chopping modes fully integrated. Detector readout mode displayed on output page
Sky background in LW spectroscopy updated.
ISAAC zero points and sky backgrounds updated and chopping mode added.
Enabled ISAAC LW L3 objective. New table of OH lines, more complete in the region 650-1100 nm
Graph display options have been fixed.
Sky background model has been updated, as well as detector dark-current values, and sky magnitudes
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